forecast discussions are the basis for defining science objectives during an IOP.
3 Feb-2011 5:00PM
A building ridge of high pressure centered off the California coast has been responsible for the rapid strengthening of the current inversion. Warm air advection aloft associated with the ridge has brought rapid warming above the modified remaining cold low-level airmass left over from the recent arctic blast. The clear skies of Thursday will be replaced by varying amounts of high clouds until thicker clouds arrive with precipitation and a chance of mix-out on Saturday.
T+0->T+24 (Thursday noon-Friday noon)
Increasing high clouds will keep the surface radiation inverson Thursday night relatively weak. Sunshine Friday will be filtered by continued high clouds. 700 mb temperatures near -4 degrees C and northwest winds at 25 kts will remain relatively constant through this period. Numerical guidance predicts the inversion between 800 and 725 mb will lower somewhat during this period.
T+24->T+48 (Friday noon-Saturday noon)
The cold air pool will weaken during this period due to thick clouds Friday night and cooling aloft (only a few degrees C at 700 mb but more below this level). 700 mb winds will increase to northerly at about 35 kts.
T+48->T+72 (Saturday noon-Sunday noon)
A weak shortwave trough will skirt northern Utah in moist northwest flow during Saturday afternoon and evening. Precipitation will be largely orographic in moist NW flow although there may be a period of steadier rain/snow mix showers in the Salt Lake Valley Saturday afternoon. The 700 mb temperatures are only progged to cool to about -9 C by late Saturday night, but given the modification of the low-level air mass (snow-free ground, higher sun angle, precipitation) models are predicting mix-out lake Saturday along a moist adiabat. We will have to wait and see. If this occurs, this is effectively the end of PCAPS as moist disturbed northwest flow is forecast for Sunday and Monday.
28 Jan-2011 12:00PM
A stratus filled strong inversion is place this morning. An extensive region of fog and stratus reside across the lake, west desert, and really all of NW Utah. We'll probably dissipate some of the clouds today do to heating, but may set up for another surge of moist air in the NW flow this afternoon. This could bring a quick return of fog by sundown.
On the larger scale a shortwave trough will move onshore in the pacific northwest today and help to diminish the amplitude of the resident ridge. This will then also open the door for a deeper trough moving into the region late on Saturday. The ridge flattening today will be associated with some cooling at 700 hpa with temperatures dropping from around +2C back to -2C by Saturday morning. Some high clouds may spill across the region.
T+24->T+48 (Saturday AM-Sunday AM)
A pacific trough will move onshore in California prompting increasing WSW flow across northern Utah. High clouds will begin to spread over the region in the afternoon, and thicken and lower by Sunday morning. Some further cooling, down to -4 C is likely at 700 hPa. Radiation cooling Saturday night into Sunday morning will be diminished do to the cloud cover. Increasing southerly flow may prompt some spatial variability in the CAP structure.
T+48->T+72 (Sunday AM- Monday AM)
Trough moves into the region, then slows down as a partially cutoff low. Much colder air will reside north of the region and begin spilling down the east side of the continental divide. The day will be mostly cloudy with a chance for a wintry mix of precipitation, perhaps with much variability within the valley. SW winds will increase in advance of a deformation zone (near surface confluence). Temperature will continue to cool aloft, dropping to aroun -7->-8 C. However no distinct frontal boundary is currently indicated (unless diabatically generated due to precip).
Extended.
As the trough cuts off a surge of cold air from the north joins the more southern system on Monday. Some of the colder air (nearly -20 C) will spill in from the east across the high planes of wyoming. The regional pressure gradient and downslope flows associated with this push of cold air will likely cause a strong canyon and possibly a downslope wind event. It may also be a moist downslope event with snow fall across the region possible.
25 Jan-2011 3:00PM
T-0->T+24 (Tuesday morning - Wednesday morning)
A weak shortwave trough is focusing much of its precipitation in the Salt Lake Valley, resulting in much higher snow accumulations than originally expected. By Wednesday morning the trough should be leaving Utah, and a strong ridge building in its place. Temperatures aloft warm from -8C to -6C by Wednesday morning as well.
T+24->T+48 (Wednesday morning – Thursday morning)
A ridge continues to build over the entire western US. Temperatures at 700mb warm from -6C on Wednesday morning to -2C by Wednesday night. The skies should be clear as the remnants of the shortwave trough that passed through Tuesday move off by the early afternoon Wednesday. A nocturnal inversion could also develop.
T+48->T+72 (Thursday morning – Friday morning)
The ridge looks to maintain its strength or even build slightly on Thursday. Temperatures aloft warm to +1C and winds remain calm. With this warming aloft, as well as a strong chance for deep nocturnal inversions, expect any cool pooling to be persistent.
Extended:
A small shortwave trough may bring some precipitation over the weekend as well as a chance to mix out the cold pool. However, after the trough, the ridge looks to restrengthen and another persistent cold pool could develop.
Zach
T-0->T+24 (Friday noon - Saturday noon)
A quick-hitting short-wave trough will cross the region overnight Friday into Saturday morning with a rain/snow mix in the Salt Lake Valley (precip type early on will depend on how rapidly low-level inversion mixes out). QPF in the Salt Lake Valley look marginal at this time, with only a dusting of snow in the lowest elevations and up to 2" on the benches. Mountaintop wind speeds will increase to 30-40 kts and temperatures will fall to around -10 C by middday Saturday.
T+24->T+48 (Saturday noon - Sunday noon)
Skies will begin clearing during the afternoon on Saturday with brisk NW winds and cool temperatures aloft (~-12 at 700 mb). The combination of cool air aloft, sunshine, and moderate winds will likely mix out any remnants of the IOP7 cold air pool by evening Saturday if current model forecasts verify. Temperatures at 700 mb will moderate slightly and winds decrease to around 25 kts in NW flow overnight into Sunday morning.
T+48->T+72 (Sunday noon - Monday noon)
There is low confidence in the placement of a southeastward-moving short-wave trough Sunday night through Monday morning. Current GFS run has trough getting shunted further east than the NAM runs. If the GFS verifies we may begin IOP8 on Sunday.
Extended: Long-range forecast models are in fairly good agreement that a longwave ridge will build over the region beginning Monday and last most if not all of next week. However, there have been day-to-day differences in the placement of the ridge axis and the strength of the ridge. We will have to wait and see.
--Erik
19 Jan-2011 1:00PM
T-0->T+24 (Wednesday AM - Thursday AM)
An amplifying ridge of high pressure will slowly build over the Eastern Pacific. Utah will remain in cool NW flow on the eastern periphery of the ridge with 700 mb temps remaining near -10 C and 20 to 30 kt northwest flow. Clouds should clear overnight Wednesday and Thursday should remain mostly sunny, allowing for some radiational cooling to begin late Wednesday night when the large-scale flow begins to weaken.
T+24->T+48 (Thursday AM - Friday AM)
IOP7 will begin with a sunny day with warming temperatures aloft during the day Thursday into Thursday night (-10 C to -2 C) as the ridge to our west slowly builds east. NW flow will remain brisk but a decent radiation inversion is likely to form early Friday morning.
T+48->T+72 (Friday AM - Saturday AM)
Clouds will overspread the region ahead of a short-wave trough on Friday morning, with showers or snow possible Friday night into Saturday. Temperatures aloft will cool to around -8 C by Saturday morning. The precipitation type during this event will depend on how deep on surface-based inversion forms and how quickly the cooling aloft moves in. Current forecast soundings indicate that an inversion will remain during the night on Friday with snow in the SL Valley. However, if mix-out were to occur earlier then the precipitation would initially occur as rain. In any case, the end of IOP7 looks to happen during the morning on Saturday at this point with a clear mix-out.
Extended: The short-wave trough clears the region by Sunday morning with cool air in place as a longwave ridge builds over the Western US. Forecast models have yet to reach a consensus on the strength, placement, or duration of this ridge. More details on this in tomorrow's discussion.
-Erik
17 Jan-2011 1:30PM
T-0->T+24 (Monday AM - Tuesday AM)
IOP6 ended around noon Monday with deep mixing associated with cooling aloft combined with brisk NW flow. A continuation of moist northwest flow of 30-40 kts with temperatures at 700 mb remaining near -6 degrees Celsius through the period. Clouds will likely limit surface cooling Monday night.
T+24->T+72 (Tuesday AM - Thursday AM)
A weather system will approach Northern Utah on Tuesday night with a pacific cold front passing during the morning on Wednesday. A burst of snowfall of several inches is possible on Wednesday along with much colder temperatures (-12 C at 700 mb). Cold northwest flow will continue Wednesday night.
Extended: Dirty ridging will regain control during the extended period. With some warming aloft and cold low-levels, an inversion may begin on Thursday. A weak short-wave trough will punctuate the ridge on Saturday but may be to weak to remove the inversion. More pronounced high pressure may build late next weekend, but it is too early to tell at present, we will have to wait and see.
-Erik
16 Jan-2011 3:00PM
T-0->T+24 (Sun AM - Mon AM)
Temperatures at 700mb stay at around 0C throughout the day. There will be considerable cloudiness as well as the chance for some rainfall. The dirty ridge affecting our area will start to break down by Sunday morning as cooler air advects in from the Northwest bringing us to -4C.
T+24->T+48 (Mon AM - Tue AM)
The chance for rain continues through Monday but should taper off by the afternoon. Cold air advection continues as temperatures aloft cool to -6C, which, combined with the rain in the morning gives us a chance to mix out the cold pool.
T+48->T+72 (Tue AM - Wed AM)
The ridge restrengthens on Tuesday, however cold air advection continues bringing us down to -8C at 700mb. There is still a good chance for a nocturnal inversion on Tuesday night because the should be a reasonable amount of clearing throughout the day Tuesday. However, by Wednesday morning another shortwave trough starts its push through.
Extended:
If the cold pool does not get mixed out on Monday, the trough on Wednesday certainly looks like it will do the trick. After Wednesday, the pacific jet looks to line up directly over Utah, bringing us unsettled weather for the rest of the week.
-zach
15 Jan-2011 9:30 AM
T-0->T+24 (Sat. AM - Sun AM)
Periods of clouds today. There may be some clearing in the mid morning through around noon before the next wave of clouds begins to work in. There is a strong surface based inversion still in place this morning which may contain low clouds in some locations. Lidar plots indicate something close to a low stratus layer at the ISS site only about 200 m AGL. There will be a chance for some form of boundary layer clouds throughout the day. The inversion top is around 820 hPa with another strong stable layer found at crest level. PM2.5 remains high.
The surface temperatures are just below freezing this morning, but will likely climb to a few degrees above freezing within a shallow well mixed CBL. This may lead to additional melting of the snow pack and further moistening of the boundary layer.
Some very light precipitation today is possible but is not expected to play a significant role.
Broken to overcast mid and high level clouds are likely tonight, which will prevent strong radiational cooling at the surface.
T+24->T+48 (Sun AM- Mon AM)
A moister shortwave will impact the region on Sunday bringing clouds and a chance for light rain/snow/wintry mix. The surface temperature within the inversion as well as the max temp in the column will matter for determining P-type. Best guess as of now is some light snow or FZRA early transitioning to RA.
Warm air advection near crest level throughout the day will help to maintain the CAP, altering the structure somewhat by coupling the mid level stable layer with the surface based stability.
Wind speeds at crest level will increase steadily throughout the day from the NW. Speeds up to around 40 knots are possible. However due to the stability in place models are quite conservative with the amount of downward momentum mixing, keeping the boundary layer winds much weaker. WAA at 700 hpa is expected which may actually strengthen the CAP. Interestingly, some indication of a low level southerly jet forming above surface based inversion is found. This could lead to some spatial variability throughout the valley in surface temperatures especially proximal to the traverse range.
T+48->T+72 (Mon AM - Tues AM)
Crest level winds will continue to increase on Monday pushing to near 50 mph. Momentum mixing looks a bit better and we may see some spatial wind driven erosion of the CAP in NW flow. Light to moderate rain will be likely with mostly cloudy skies.
-neil
14 Jan-2011 12:00 PM
T+0-T+24 (Friday Noon - Saturday Noon)
The short-wave trough that brought several inches of snow as well as observatons of freezing rain in the Salt Lake Valley has now moved east of the area. Fog and low clouds are widespread in the Salt Lake Valley and it will likely stay that way for the next several days. A dirty ridge of high pressure will reassert itself over the Western US during the next 24 hours. However, copious amount of mid and high clouds will continue to stream across the area as temperatures aloft warm from -8 to -2 degrees C by midday Saturday. The low-level airmass will slowly warm under the thick clouds and fog.
T+24-T+48 (Thursday Noon - Friday Noon)
A fast-moving shortwave trough crosses Utah overnight Thursday into early Friday morning (700 mb temps cool to near -6 C by Friday morning and NW winds increase to 25 kts). Models continue to indicate around 0.25" of precipitation, probably in the form of snow falling right near 0 degrees C at the surface (1-2") given a nearly isothermal temperature profile during this time. Model profiles indicate that the inversion will not completely mix-out as the environmental lapse rate is forecast to be insufficient for deep mixing along a moist-adiabat for the duration of this precipitation event.
T+48-T+72 (Friday Noon - Saturday Noon)
The dirty ridge scenario under broad northwest flow looks to continue with little change in temperatures aloft and copious amounts of high/mid-level cloudcover crossing the region. A weak temperature inversion with near-isothermal profiles is expected.
Extended: Several shortwave troughs will skirt Northern Utah Sunday into Tuesday as the flow becomes increasingly westerly and the ridge flattens. Models are trending toward wetter and slightly cooler with these systems. We will have to wait for future model runs to determine if IOP6 will end with one of these troughs.
--Erik
12 Jan-2011 12:30 PM
T+0-T+24 (Wednesday Noon - Thursday Noon)
A dirty high pressure ridge will amplify over the Western U.S. for the next 24 hours. Mountaintop temperatures (700 mb) will warm from -8 to near -2 degrees C by Thursday midday. The overall inversion strength is not expected to increase significantly as low-level waming of the valley cold air pool is forecast to occur at the same time it is warming aloft. This moderation of low-level temperatures is likely due to radiative effects of the thickening cloudcover. Mostly cloud skies ahead of a weather disturbance will continue during this period.
T+24-T+48 (Thursday Noon - Friday Noon)
A fast-moving shortwave trough crosses Utah overnight Thursday into early Friday morning (700 mb temps cool to near -6 C by Friday morning and NW winds increase to 25 kts). Models continue to indicate around 0.25" of precipitation, probably in the form of snow falling right near 0 degrees C at the surface (1-2") given a nearly isothermal temperature profile during this time. Model profiles indicate that the inversion will not completely mix-out as the environmental lapse rate is forecast to be insufficient for deep mixing along a moist-adiabat for the duration of this precipitation event.
T+48-T+72 (Friday Noon - Saturday Noon)
The dirty ridge scenario under broad northwest flow looks to continue with little change in temperatures aloft and copious amounts of high/mid-level cloudcover crossing the region. A weak temperature inversion with near-isothermal profiles is expected.
Extended: Several shortwave troughs will skirt Northern Utah Sunday into Tuesday as the flow becomes increasingly westerly and the ridge flattens. Models are trending toward wetter and slightly cooler with these systems. We will have to wait for future model runs to determine if IOP6 will end with one of these troughs.
--Erik
11 Jan-2011 1:15PM
T+0-T+24 (Tuesday Noon - Wednesday Noon)
A radiation inversion formed Monday night under clear skies while temperatures warmed aloft to -10 C by the 18 Z Tuesday ISS rawinsonde launch. A dirty high pressure ridge will remain over the region for the next 24 hours. However, thick high clouds associated with a splitting upper-level disturbance over California will result in filtered sunshine today and most likely overnight tonight. High temperatures Tuesday will struggle to reach the mid-20s F. Overnight lows Tuesday night will remain elevated compared to Monday night due to continuing cloudcover. However, the GFS WRF is hinting at partial clearing Tuesday night which would result in more cooling that the NAM is indicating. Temperatures aloft will warm slightly from -10 to -6 degrees C during this forecast period and winds aloft will remain relatively weak at 10-15 kts.
T+24-T+48 (Wednesday Noon - Thursday Noon)
The shortwave ridge amplifies slightly (700 mb temps near -4 C by Thursday morning) but not enough to shunt pacific moisture riding over the top of the ridge away from Northern Utah. The 700 mb flow remains relatively weak (15 kts or less) Consequently, mostly cloudy skies will continue over this period, with model soundings showing a slow moderating of low-level inversion strength presumably due to radiative heating of the ground by thick clouds aloft.
T+48-T+72 (Thursday Noon - Friday Noon)
A quick-hitting short-wave trough riding along the periphery of the ridge may bring a period of snow to the region Thursday afternoon and night before clearing Friday morning. At this time model are indicating around 0.25" precipitation (2" snow) with 700 mb temperatures around -4 to -6 degrees C. Winds aloft become more zonal and increase to 20-30 kts during this period. Forecast sounding indicate a deep isothermal layer during this event which would result in snow at all elevations and the continuation of a weak low-level cold air pool.
Extended: A series of fast moving weather disturbances look to clip Northern Utah during the extended forecast but the details will surely change in future model runs. The basic story is that systems brushing Northern Utah along the periphery of the high will keep the pattern complicated and we are unclear as to when a total mix-out of the current CAP will occur.
--Erik
10 Jan-2011 2:15PM
T+0-T+24 (Monday Noon - Tuesday Noon)
The deep, cold upper-level trough that brought cold temperatures, relief from the poor air quality of recent days, and light snow to the Salt Lake Valley will rapidly progress east during the next 24 hours as a low-amplitude (dirty) ridge of high pressure builds in from the west. The result will be rapidly warming temperatures at mountaintop (from -20 on Monday to to -9 C by noon Tuesday). Cold air will remain trapped in the valleys, resulting in a rapidly developing temperature inversion. Patchy fog is a concern with clearing skies and cold overnight temperatures in the upper single digits Farenheit in the Salt Lake City metropolitan area.
T+24-T+48 (Tuesday Noon - Wednesday Noon)
Thick mid-level clouds and even a few flurries are possible as a weak shortwave embedded within the ridge moves onshore and weakens as it translates east. 700 mb temperatures will change little warming from -9 to -7 C during this period. Little sunshine is expected during the day on Tuesday and cold air is expected to remain entrenched in the valleys with high temperatures remaining in the mid-20s. Tuesday night should be warmer than Monday night with thick mid-level clouds remaining.
T+48-T+72 (Wednesday Noon - Thursday Noon)
Dirty ridge axis shifts from Western Nevada to Western Utah during this time period. Periods of high clouds are likely, and temperatures will warm at 700 mb from -7 to -2 degrees C. Cool air will remain in valleys and overall inversion should be peaking in intensity since the start on Tuesday morning.
Extended: A dirty ridge will redevelop to the west of Utah. A quick-hitting shortwave riding over the ridge will bring a chance of snow on Wednesday night, but temperatures aloft will likely remain too warm for a cold air pool mix-out. Further amplification of the ridge is possible thereafter although systems brushing Northern Utah along the periphery of the high will keep the pattern complicated.
8 Jan-2011 2:15PM
T+0-T+24 (Saturday Noon - Sunday Noon)
All eyes are on the next 18 hours as a Pacific weather system and associated short-wave trough bring sharply colder temperatures aloft which will hopefully end the inversion. 21 UTC (1:15 pm) sounding indicates around 2 degrees C of cooling above 800 mb since the 18 UTC (10:15 am) radiosonde at ISS. Current 700 mb temps at 21 UTC are around -5 C and will cool another 11 C to around -16 C in the next 12-18 hours. Along with the cooler air aloft associated with the southeastward-moving baroclinic zone will be a period of positive vorticity advection and large-scale lift. While moisture is limited, an inch or two of snow is possible with snowfall starting after midnight and into the morning on Sunday. We expect the cooling aloft in addition to lifting of a near-saturated atmosphere along a moist adiabat to do the trick of mixing out the polluted soup we have been living in the last week.
Extended: A messy weather pattern looks to continue for the Monday-Thursday period. A full-scale IOP looks to be unlikely but more information on this forecast period will be provided in Sunday's weather discussion.
--Erik
7 Jan-2011 1:30PM
Visible satellite and surface based observations all show that the dense
cloud/fog layer of this morning is substantially thinning as we move
into the afternoon hours. This is the case across all of NW utah and is
probably driven by the diurnal cycle. There is a hint of preferential
clearing along the eastern half of the valleys which may be related to
slight NE flow coming over and across the Wasatch. (Fortunately we are
in good shape to observe these impacts with a number of east west data
transects conducted today).
Tonight we will start out clear and thus expect considerable cooling in
the hours immediately following sunset. This may favor the redevelopment
of truly dense fog in most locations.
On Saturday a slow mix out process will begin as a progressive and
flattening shortwave trough moves into the region. Gradual but continual
cooling near and below crest level are forecast throughout the day
Saturday. This cooling will alter the vertical structure of the
inversion, forcing the profile towards weak and weaker stability (in
theory). I anticipate that the 'inversion top' will rise tomorrow with
an enhanced mixing volume by afternoon. Clouds will increase overhead
and some light snow is possible. Winds will be increase from the NW at
crest level with weaker southerly flow below. (One possibility is the
maintenance of a low level and sharp inversion due to this sheared flow.
The southerly flow could cause some decent warm air advection in the
lowest few hundred meters beneath the cooling above.) The lower levels
southerly wind also suggests that it is possible that we will see some
of the now familiar spatial variability near the Traverse Mountains.
However this is not a strong southerly mix out scenario.
On Saturday night the true surface cold front slowly moves into the
region with surface northerly flow and some light to moderate snowfall,
with the back edge of the baroclinic zone arriving Sunday morning. I
anticipate that the last of the cold air pool will be removed by this
final push of the cold front.
So to summarize: One more night with fog/clouds possible. Cooling aloft
on Sunday removing much of the stability except possibly the lowest
levels, then the final death blow by Sunday morning.
--Neil
6 Jan-2011 2:00PM
T+0-T+24 (Thursday Noon - Friday Noon)
Temperatures aloft stay at (-2C). The cutoff low has moved into Southern Utah/Northern Arizona seems like it will only bring a few high clouds at a maximum. Thursday night has a good chance to develop a nocturnal inversion as well as dense fog.
T+24-T+48 (Friday Noon - Saturday Noon)
Cold air advection lowers temps aloft to (-4C) Friday afternoon and to (-7C) by Saturday morning. Clouds ahead of the trough may reach the Salt Lake Valley by Friday night, but should be mainly clear during the day Friday. Fog once again has a good chance of developing.
T+48-T+72 (Saturday Noon - Sunday Noon)
Cold air advection continues as the trough continues to dig into our region, down to (-14C) by late Saturday and (-16C) by Sunday morning. The trough also brings a chance for snow on Saturday, although the accumulations will not be very large in the valley. This storm looks to have us mixed out by Saturday night.
Extended:
Sunday continues to give a chance for snowfall, although once again there is little chance for any great accumulations. A shortwave ridge looks like it will set in for a couple days before strong, moist, and cold zonal flow will bring a much larger storm to our area.
-Zach
5 Jan-2011 2:00PM
T+0-T+24 (Wednesday Noon - Thursday Noon)
Fog could develop throughout the day today, with high clouds moving out of the region. Temperatures aloft warm from (-8C) to (-4C), as the shortwave trough leaves the area. By the nighttime, the cutoff low sitting off the California coast should start to move inland and give us another chance for high clouds. However it still looks like we could develop a nocturnal inversion overnight.
T+24-T+48 (Thursday Noon - Friday Noon)
Warm air advection continues as temperatures aloft warm to (-1C). The cutoff low continues to give a chance for some high clouds. There is also a chance for fog again, but otherwise it should be mostly clear (ignoring the smoggy air).
T+48-T+72 (Friday Noon - Saturday Noon)
The cutoff low continues to pass through Arizona, while another trough looks to begin is passage through the west. The cutoff could bring high clouds during the morning and afternoon, but by late afternoon/night, the trough should bring lower clouds and a slight chance for precipitation. This trough also cools us from (-1C) on Friday to (-8C) on Saturday night/Sunday morning. This event will give a very good chance for mixing out our inversion.
Extended:
The main body of the trough will pass through during Sunday morning, eventually cooling us to (-16C) at 700mb. While the trough doesn’t look to provide us with substantial precipitation, the cold air advection should do well to mix us out. After the trough, another large ridge looks to push us right back into a cold pool.
-Zach
3 Jan-2010 12:30 PMT+0-T+24 (Monday AM - Tuesday AM)
Decreasing mid-level clouds and snow flurries, with high clouds moving in throughout the day; chance for boundary layer clouds to form. Temperatures aloft warm from (-12C) to (-9C) through tonight. A cutoff low sitting off the California coast will is currently pushing in clouds to our south, they are not expected to reach the salt lake valley. A nocturnal inversion may develop overnight.
T+24-T+48 (Tuesday AM - Wednesday AM)
A small shortwave trough brings some cloud cover but in the morning, by late Wednesday afternoon/night, a shortwave ridge pushes in over the cutoff low that is currently sitting off the California coast, warming us to (-2C) at 700mb. Considerable cloudiness may continue throughout the day on Wednesday.
T+24-T+48 (Wednesday AM - Thursday AM)
The cutoff low starts to push inland trough Arizona and southern Utah, while the leading edge of a large trough starts to dip through the Pacific Northwest. Both systems have a small chance to bring high clouds to the area. Temperatures aloft warm to (-1C). There looks to be a good chance for a nocturnal inversion Thursday night.
Extended: The models are now in good agreement that a large trough will push through on Saturday, mixing out the region and giving us some relief to the air pollution we are currently experiencing. Temperatures cool once again to (-16C), while at the same time there is a good chance for precipitation. After the storm, the models are unclear about whether a high amplitude ridge sitting off the west will move inland, or continue to drive arctic air into the intermountain west.
-Zach2 Jan-2010 11:00 AM
T+0-T+24 (Sunday AM - Monday AM)
Temperatures aloft stay at around (-10C) for the period. The dew point is also expected to increase significantly overnight for reasons that are not clear. A cutoff low to our southwest, as well as a shortwave trough to our north may bring in some high clouds over the salt lake valley by late afternoon.
Temperatures aloft show little change staying between (-10C) and (-8C). There is a good chance for low clouds and freezing fog given model forecasts. Unsure about the mechanism for the creation of low clouds, however, the models are in good agreement. Winds stay light, and there is still a chance of further radiational cooling Monday night depending on how thick the cloudcover is.
700 mb temperatures warm to (-6C). Clouds may continue, and there is a small chance for precipitation, possibly in the form of freezing rain/light snow. Another cutoff low sits off the California coast, and is what gives us a chance for very light precipitation as it slowly drifts across Southern Utah during this forecast period.
Extended: Temperatures aloft eventually warm to (-3C). A very large ridge looks to build over the west during this period if the 12Z GFS verifies, with no potential for a mixout in the near future. Clearing will happen Wednesday, giving us clear skies and a chance to strengthen the inversions overnight. Air pollution will continue to be an issue.
-Zach/Erik
01 Jan-2011 02:30 PM
T+0-T+24 (Saturday AM - Sunday AM)
Skies should remain clear through Saturday evening as weak high pressure starts to build in from the west. Temperatures aloft will warm from (-21C) to (-10C). The subsidence inversion also lowers, implying that we will have a quite deep inversion by Sunday morning. A chance of freezing fog exists for Sunday morning.
T+24-T+48 (Sunday AM - Monday AM)
High clouds move in by Sunday evening associated with an upper-level low centered over California. Clouds may be thin and patchy enough that surface cooling remains robust Sunday night although less than on Saturday night. Temps at 700 mb stay at around(-10C). By Monday morning, there is a greater chance for high clouds as well as surface fog. Temperatures should remain below freezing. During this period models indicate a shift in inverion type toward a cloud-topped inversion.
T+48-T+72 (Monday AM - Tuesday AM)
Salt Lake City remains in a weak but large-scale zone of high pressure over the Western US with an embedded cutoff low over California continuing to spread clouds north into region. A greater chance of cloudiness at lower levels as models indicate a cloud-topped inversion holding strong in the Salt Lake Valley . Temperatures warm aloft to around (-7C). Winds remain light. The subsidence inversion looks to stay low, however, because of the cloudiness, the surface inversion may not strengthen overnight.
Extended: Cloudiness looks to remain through Thursday. Temps look to raise up to (-4C) by Thursday as well. A ridge looks to last for a day or two before another trough gives a chance to mix out the valley. Air pollution could become a major issue by mid next week.
--Zach/Erik
31 Dec-2010 12:10 PM
T+0->T+24 (Friday AM - Saturday AM)
Clearing should begin Friday afternoon, with a slight potential for snow flurries. Temps aloft warm slightly during the period from (-21)C to (-18)C. Moist NW flow early in evening gives a slight chance for lake effect snow over the Tooele Valley and west side of SL Valley. Expect the flow to decrease from 20kts to 5kts toward dawn Saturday. How much mixing we have overnight is the largest forecast concern. Lingering cloudcover and deep mixing with cold air aloft could keep temps in the low teens tonight. However, given deep snowpack and clearing sometime after midnight forecaster intutition is that temps should cool below 10F with a weak surface inversion. There is a potential for fog Saturday morning given higher dewpoints over the lake and widespread snowcover.
Temperatures aloft warm from (-18C) to (-10C). Some high clouds may move in Saturday night due to a cutoff low over California, but there should still be ample radiational cooling. The subsidence inversion continues to lower; a surface inversion should be present Sunday morning in addition to the deeper inversion due to the warming aloft. Winds stay at 5kts at 700mb. Chance of fog continues for Sunday.
Temperatures stay steady at (-10C) aloft. High clouds increase as the cutoff low moves over southern Utah. The NAM continues to lower the subsidence inversion, as well as strengthening the surface inversion. Chance of fog continues for the Salt Lake Valley.
Extended: The cutoff low will pass over southern Utah, bringing some more high clouds and a chance to weaken but not break through the inversion. Another cutoff low sits over California but does not seem to impact northern Utah. With large scale ridging falling into place and temperatures at 700 mb warming to -2 C we can expect to inversion to persist and/or strengthen for quite some time.
23 Dec-2010 2:10 PM
T+0->T+24 (Thurs AM - Friday AM)
Finally a break from the recent wet, unsettled weather! The departing short-wave trough centered to our south will slowly drift east, with drier and slightly warmer air approaching at mountaintop level by Friday morning associated with a building short-wave ridge (warming from -5 C at 700 mb to around -3 C). Areas of dense fog are expected in the valley tonight with clearing skies aloft allowing for the coldest night in some time. Temperatures will bottom out in the low to mid 20's across the Salt Lake Valley.
T+24->T+48 (Friday AM- Saturday AM)
A high-amplitude short-wave ridge axis will be centered over Utah during this period. Fog will likely be slow to burn off Friday morning in the stable environment. Temperatures aloft will warm significantly from -3 C to + 2 C during this period, and cloudcover is expected to be minimal. Winds at crest level will be less than 10 kts during Friday but will start to increase to above 15 kts by Christmas morning. Strong radiative cooling is expected Friday night which will further strengthen the cold air pool. Redevelopment of low-level fog, particularly near the Great Salt Lake will be a concern Friday night and Christmas morning.
T+48-T+72 (Sat AM - Sun AM)
The short-wave ridge axis will shift east of Utah, giving way to increasing southwesterly flow ahead of an approaching Pacific weather system. By Sunday morning, 700 mb level winds are forecast to be near 30 kts. Temperatures at 700 mb will remain in the +2 to + 4 C range, allowing for a pronounced inversion to persist. High clouds may limit incoming solar radiation on Christmas day.
Extended: During the day on Sunday strong southwesterly flow of near 40 kts may be sufficient to remove the cold air pool (we will have to wait and see) and this event may be a nice example of 'pre-frontal mix out'. As a compact short-wave trough crosses the region Sunday night temperatures will cool further to around -7 C and showers are a possibility. If the prefrontal flow does not mix out the cold air pool the cooling aloft combined with precipitation Sunday night may do the trick. Monday and Tuesday Utah remains in zonal flow with moderate winds and cool temperatures aloft (-5 C range). We are still waiting for our weeklong giant ridge.
-Erik
22 Dec-2010 12:30 PM
T+0->T+24 (Wed AM - Thurs Am)
The deep plume of subtropical moisture will finally exit the region later today as a collapsing upper level trough moves east across the western US. The trough axis proper will cross the region late tonight/tomorrow morning, and as it does it will close off the tap of deep moisture. Before then, significant moist and warm southerly flow will continue across Utah bringing a chance for light to moderate rain to the Salt Lake Valley.
T+24->T+48 (Thus AM- Fri AM)
The trough will slowly shift east of the region while a ridge build just upstream during the day on Thursday. Some lingering showers will be possible early in the day, followed by progressively clearing skies in the afternoon. Temperature near crest level will warm throughout the day and into Thursday night as a subsidence inversion and warm air advection bump temperatures from near -6 C up to -4 C. Mostly clear skies overnight should allow for considerable nocturnal cooling and the development of a surface based inversion by Friday morning.
T+48-T+72 (Fri AM - Sat AM)
The ridge will amplify across the region on Friday bringing more substantial warming aloft, pushing temps at 700 hpa to around +2 be early Saturday morning. Patchy debris cirrus sheared from a pacific storm are expected to move overhead, arcing up and over the ridge axis. With solstice sun, the solar forcing for the diurnal destruction of the cold pool is expected to be very weak, and persistence is anticipated. Continued partly to mostly clear skies should allow the cold air pool to strengthen into Saturday morning.
Extended: On Saturday the ridge will remain in place with increasing clouds as an elongating and negatively tilting pacific trough begins to move onshore in california. By Sunday morning this compact and vigorous system will swing through northern utah, generating strong southerly winds and an eventual frontal passage. The strong winds may be sufficient to remove the cold air pool and this event may be a nice example of 'pre-frontal mix out'.
-Neil
21 Dec-2010 12:30 PM
T+0->T+24 (Tuesday AM- Wed AM)
A deep plume of subtropical moisture remains in place, centered a bit south of the region. A trough just off of the California coast is digging slightly and collapsing in wavelength. This will favor an increasingly meridional flow from south to north across Utah later today and tonight, shunting some of the deeper moisture back north as well as ramping up temperatures at 700 hpa from -7C this morning to -4 C this afternoon. This should favor a changeover to rain in the valleys, though with some snow on the ground some locations may not make the switch over.
T+24->T+48 (Wed AM-Thurs AM)
The trough will continue to weak and collapse in wavelength as it becomes mobile and advance from west to east across the great basin. The Vort Max is currently projected to slide to our south, which should keep the most organized precipitation at bay. However, with considerable moisture still in place some light rain/snow will still be likely across northern Utah. Temperature aloft will be between -5 and -6 C, which would generally support rain, however, with some cold air near the surface overnight, a mixed bag will remain possible.
T+48->T+72(Thurs AM-Fri AM)
The short wave trough will slowly exit to the east during the first half of the day as a ridge begins to build to our west. Skies should progressively clear through the day, becoming mostly clear by evening. The ridge will continue to amplify during the overnight hours, and with clear skies in place and some spotty snow on the ground cold pool conditions are expected to develop.
Extended: Friday: Ridge amplifies significantly, penetrating well to the north. However, some clouds are likely to shear off of a Pacific NW trough and arc up and over the ridge axis providing some high clouds during the day and overnight. This may impact both diurnal heating and cooling.
Saturday: Ridge remains in place, with some clouds continueing to stream by overhead.
Sunday: A weak short wave orbiting the larger trough over the eastern pacific will ripple across the great basin. Strong winds near crest level are possible and there will be some chance of a mix out or partial mix out of the extant cold pool.
-Neil
20 Dec-2010 2:58 PM
T+0->T+24
Strong west-southwest flow aloft
Cloudy with scattered light to moderate snow, 1-2 inches possible
Temps at 700mb vary between (-8)-(-10)
T+24->T+48
Compacting trough
Scattered rain showers at temperature aloft warm to ~(-5)
Majority of precip looks to stay to the south, with scattered light snow/rain
T+48->T+72
Trough becomes mobile and compacts as axis passes trough the area
Cloudy with rain/snow showers possible
Temperatures aloft stay between (-4)-(-5)
Extended outlook
A ridge rapidly intensifies Friday through Saturday
Clearing over Christmas eve day hints at a strong possibility of a cold pool
Temperatures look to stay at around (-4) for the forecast period
-zach
17 Dec-2010 2:58 PM
T+0->T+24 (Fri AM- Sat AM)
Thick clouds will overspread the region and lower in the afternoon. Some light snow will be possible by evening and increasingly likely through the night. Minor accumulations will be possible by morning (< 1 inch). Weak cold pool conditions currently in place will likely persist through this period.
T+24->T+48 (Sat AM - Sun AM)
Thick clouds continue with a chance of light snow in the morning then a chance of rain/snow in the afternoon. Any accumulations will continue to be light.
By Saturday night precipitation will increase in intensity somewhat. In heavier periods it may still mix with snow, especially for the bench locations. The fate of the cold air pool will depend on the intensity of precipitation during this time, though in general it is expected to dissipate.
T+48->T+72 (Sun AM - Mon AM)
Moderate to occasionally heavy rain will be likely.
Extended:
Temperatures drop on Monday with some light snow possible in town, though only minor accumulations look likely as of now. Things warm up again on Tuesday with another surge of deep moisture. Heavy rain is possible. Snow will again be possible on Wednesday.
-Neil
16 Dec-2010 11:17 AM
T+0->T+24 (Thurs AM - Friday AM)
A weak shortwave ridge will build across the region on Thursday, with split flow driving a storm south of the forecast region. Skies will be mostly clear. Temperatures will remain cold as -12 C air is still in place at 700 hpa. The morning sounding indicates a weak cold pool in place, and with some snow, cold air, and near solstice sun the present cold pool should easily persist through the day. As we move into the overnight hours, weak warm air advection will begin with temperatures climbing from -12 to -10 (at 700 hPa) by tomorrow morning. Continued clear skies should allow for considerable cold air production tonight which will strengthen the current inversion.
T+24->T+48 (Friday AM - Saturday AM)
A cold pool will be in place on Friday morning, however stormier weather will move in during the day. A large plume of subtropical moisture will overspread the region in WSW flow. Thick clouds will accompany strong WAA. Temperatures will climb from -10->-6 by late Friday night. Precipitation will be increasingly likely throughout this period with mountain snow and valley snow then rain possible. The fate of the cold pool is unclear with this first event as WAA aloft will help to keep strong stability in the lower levels. The amount and intensity of precipitation will play an important role in maintenance vs destruction.
T+48->T+72( Saturday AM - Sunday AM)
A continued stream of moisture will be advected into the region during the day Saturday. Clouds and rain will remain likely, with temperatures climbing as high as -2 C aloft. Winds will also be stronger with fast SW flow aloft and strong southerly flow near the surface.
Extended:
Unsettled and moist conditions will continue into next week. Things might cool off on Monday as we fall back to the cold side of the baroclinic region draped across the western US. However a warm up again looks likely as an amplifying trough off the west coast promotes a more meriodional southerly flow. Strong and persistent cold air pools are not anticipated through Tuesday.
12 Dec-2010 2:00
T+0->T+24 (Sunday AM - Monday AM)
High clouds are taking their time to clear over the Salt Lake Valley as of early afternoon Sunday which may prevent the Salt Lake Valley from mixing as well as forecast this afternoon. A short-wave ridge of high pressure will be centered over Utah tonight through Monday morning with clear skies, strong radiational
cooling and light winds allowing for the first chilly night in a while. 700 mb temperatures will warm to about 3 C late tonight before cooling a few degrees Monday as the short-wave ridge begins to flatten.
T+24->T+48 (Monday AM-Tuesday AM)
The short-wave ridge responsible for IOP3 flattens to near zonal flow during this period. 700 mb temperatures fall from +2 to -2 during this 24-hr period and extensive high clouds will approach the region during Monday evening.
Current thinking is that high clouds will lower and thicken throughout the night Monday which will limit radiational cooling. Winds aloft will remain fairly constant in the 15-30 kt range at mountaintop level, which should be insufficient to mix out the inversion during this time.
T+48->T+72 (Tuesday AM-Wednesday AM)
A vigorous short-wave trough will rapidly dig south and east, with an associated
baroclinic zone crossing Utah on Tuesday evening and night. South-southwesterly flow will increase below mountaintop level to between 20 and 40 kts which may partially or completely mix out the cold air pool in the Salt Lake Valley Tuesday afternoon. Model guidance keeps the cold air pool largely intact until the strong dynamic lift associated with cold frontal passage around 9 pm Tuesday night. 700 mb temperatures crash to -12 C, the coldest in several weeks. A period of snow on either side of the cold front could set the stage for more cold pools if we ever get another high pressure ridge to set up in the Western U.S.!
Extended:
A large-amplitude trough is forecast to dig over the eastern Pacific Ocean beginning Wednesday and lasting through next weekend. Moist, zonal flow off the Pacific is forecast with extensive cloudiness, showers, and and strong flow at mountaintop preventing the development of cold air pools during this time period.
11-Dec-2010 11:30
T+0->T+24 (Sat AM - Sun AM)
This morning a very shallow nocturnal inversion was found across the salt lake valley. An additional isothermal layer was found midway up through the basin (half way up the mountains). The surface inversion is expected to erode during daytime heating.
An amplifying ridge to the west of the region will rotate yesterdays baroclinic zone into a more meridional orientation across Utah. Then as the ridge further amplifies this region of strong temperature gradients will advance across the state as a warm front (sort of). By late in the day Saturday, thick clouds will again overspread the Salt Lake Valley and very strong winds (up to 50 kts) will develop near mountain top level within the stable layer associated with the baroclinic zone. The extent of momentum mixing from aloft to the surface is not clear. Nonetheless, a surface based inversion is not particularly likely due to minimal radiative cooling and possibly strong winds. Light precipitation is outwardly possible.
700 hPa temps climb from -8 to near 0 C by Sunday morning.
T+24 -> T+48: (Sun AM - Mon AM)
Clouds will begin to lift to the north and east during the day on Sunday as the ridge amplifies and shifts to the east. This should set up for clearing skies. A subsidence inversion will be present near or now below crest level, which will on the large scale trap air within the valley. Surface heating and the lack of a robust surface inversion from Saturday night will likely created a fairly well mixed profile in the low levels.
Skies will remain mostly clear on Sunday night, which should provide a period of strong radiative cooling and the development of a nocturnal inversion.
T+48->T+72 ( Mon AM - Tues AM)
The ridge will begin to flatten a bit on Monday as a weak short wave trough arcs north over the ridge axis. The ridge axis itself will remain positioned over Utah. Some clouds may move into the region and modest cooling aloft is expected.
Extended: On Tuesday a more substantial trough with much colder air will begin to approach from the NW. A surface pressure gradient is expected to develop favoring more southerly flow. Winds will increase especially in the afternoon, and a frontal passage is expected sometine Tuesday night into Wednesday morning. Either the strong winds in advance of the front, or the front proper is expected to destroy the inversion.
10-Dec-2010 13:38
T+0->T+24 (Friday AM - Saturday AM)
What a difference a day makes! Yesterday's numerical guidance did not handle the current baroclinic zone intensity and placement well which resulted in a
total mix-out of the inversion (end of IOP 2) Friday morning followed by a surface frontal passage from the northwest and a period of moderate
rain across the Salt Lake Valley. Moist northwest flow will persist into Friday evening with scattered snow showers continuing. Orographic enhancement will mean a good snowfall for the mountains with this event. Snow levels will fall to the valley floors (700 mb temperature of -8C) by late evening, but only minoraccumulations (1-2") are expected below 5000 ft as large-scale dynamics will be lacking.
T+24->T+48 (Saturday AM-Sunday AM)
Building high pressure will result in a zone of warm air advection over northern utah and light precipitation on Saturday. At this time most of the precipitation is expected to be light and confined to the mountains north and west of Salt Lake City. Temperatures at mountaintop level will slowly warm to near -2 C by Sunday morning. Skies will likely remain overcast throghout Saturday and Saturday night. Winds look to remain fairly strong aloft throughout Saturday, so the Salt Lake Valley below crest level may remain mixed through Sunday morning.
T+48->T+72 (Sunday AM . Monday AM)
The axis of a progressive short-wave ridge of high pressure will approach Northern Utah by 12Z Monday morning.
During this period large-scale winds will rapidly decrease and a notable subsidence inversion associated with the short-wave ridge will be apparent.
Sunday night the combination of clear skies and light winds should allow for astrong radiation inversion.
Extended:
The short-wave ridge axis will be over Northern Utah during the day on Monday. By Monday night into Tuesday, the ridge rapidly gives way to moist westerly flow with mid-level clouds. At this time it looks like Monday night will be clear with another round of good radiational cooling, and
high clouds may move in and limit insolation during the day on Tuesday. Winds aloft will strengthen during this period but are not expected to be strong enough to mix out the inversion. Late Tueday night a strongly forced weather system will likely bring an abrupt end to IOP 3 (if forecasts do not dramatically change) with -10 C air at 700 mb and accumulating snowfall in the Valley.
09-Dec-2010 14:57
T+0->T+24 (Thurs AM - Fri AM)
A very weak PV filament (or vort max) embedded in generally zonal flow will
move across Utah during the day Thursday. Partly to mostly cloudy conditions
and some light rain are expected with this feature. Rainfall amounts will be
negligable for the most part. Temperatures aloft will remain near -4 C and
current cold air pool conditions are expected to persist. During this time a
compact and fast moving shortwave trough will move into the pacific northwest.
T+24->T+48 (Fri AM-Sat AM)
The compact PAC NW shortwave will glide a bit to the north of the forecast
area as a ridge begins to amplify just off the west coast. A modest baroclinic
zone will drop south into Northern Utah bringing cloud and a chance for valley
rain and mountain snow. Models are much less aggressive with the cold air
associated with this feature in todays runs, and the potential for mix out is
now marginal with 700 hPa temps of -6 C (as opposed to -10 C). Precipitation
looks to be maximized north of Salt Lake and it is not clear how focused on
the mountain the event will be. The high-resolution models show mostly
orographic snowfall with little rain in the valley. This would make mix out
even less likely. Limited sun should minimize the impact of surface diabatics.
T+48->T+72 (Sat AM – Sun AM)
On Saturday the ridge amplifies significantly and shifts a bit east. The
amplification will cause the baroclinic zone that provided cooling on Friday
to shift back north and east bringing strong warming over Utah. However
considerable moisture will still be advected across the ridge axis and arrive
in Utah on strong NW flow. Thick clouds will be possible for the first portion
of the day along with a chance for some light rain. Temperatures will increase
from near -6C to around 0 C at 700 hPa. Both WAA and subsidence are expected
to be playing a role in the warming. Winds at 700 hPa will be as high as 40
Kts (20 m/s). At the surface a regional pressure gradient will promote stronger
southerly winds channeled through the valleys topography. It is unclear how
the cold pool (assuming it is still in place) will respond to this scenario.
Extended:
Very strong ridging is expected for Sunday and Monday with temps at 700 hPa
climbing to +4 C. Cold pool conditions are expected. The ridge then flattens
as the area comes under the increasing influence of a broad trough to the NW
-Neil
08-Dec-2010 12:49
T+0->T+24:
Ridge axis shifts to the east and the ridge rapidly flattens opening the
region to strong westerly flow. Accompanying the westerly flow is a weak
baroclinic zone, thick cirrostratus and altostratus. Some light precipitation
currently over Northern Nevada ans SW Idaho will move into the region.
Temperatures at 700 hpa will fall from the current +2 C back to about -4 -> -5
C. Current cold pool conditions are expected to continue as surface
temperature will still remain at a deficit. However the cooling aloft will
constitute a weakening of the Cold Pool. Surface cooling is only expected to
be weak due to the cloud cover. One complication is the possible intrusion of
the colder dirty and foggy lake air into the north end of the salt lake
valley. The extend of this intrusion is highly uncertain.
T+24->T+48:
Zonal flow will dominate with 700 hPa temps bouncing around between -4 and
-6 C. Some light precipitation and thicker clouds are possible in the morning,
with a slight trend toward less cloud cover in the afternoon. The cold pool
may persist through this time, but it is not expected to be particularly
robust.
T+48-.T+72:
A fast moving and compact shortwave trough will approach and eventually
cross the forecast area during this time window. Clouds will again increase
with a chance for light rain and/or snow by midday. A weak trough axis will
cross the region in the evening, bringing the best chance for organized
precipitation and dropping temperatures back to -8 C. Winds are expected to be
fairly strong from the NW. The combination of significant cooling, strong
mixing due to winds, and precipitation are expected to remove any lingering
cold pool from the valley by Saturday morning.
Extended: On Saturday a strong ridge will build west of the forecast area,
keeping northern Utah under NW flow. Moisture spilling over the ridge will
keep cloud cover in place while significant warming at 700 hPa occurs. On
Sunday the ridge axis will shift east into the great basin bringing clearing
skies and very warm temperatures aloft (+4 C). Cold pool conditions are
expected to develop during this time however the expected duration of the
event is not clear due to model uncertainty.
-Neil
07-Dec-2010 17:07
T+0->T+24 (5 am Tuesday - 5 am Wednesday):
As the storm system that ended IOP 1 pulls away from the region a progressive short-wave
trough will amplify and cross the region over the next 24 hours. 700 mb
temperatures of around -4 C at 12 UTC Dec 7th will warm to near +2 by 12 UTC
December 8th, with a lowering subsidence inversion combining with strong
radiational cooling overnight on December 7th will likely make for an impressive
inversion on Wednesday morning.
T+24->T+48 (5 am Wednesday - 5 am Thursday):
Temperatures will slowly cool from +2 C to -4 C aloft between forecast hours
24-48 as the shortwave ridge rapidly gives way to moist, westerly flow with
a weak embedded disturbance passing near hour 48. However, similar to the IOP1,
the coolest air and moisture remains to the north of the region and it is
unlikely that the inversion will be removed in this scenario, although some
weakening is possible during this period.
T+48->T+72 (5 am Thursday - 5 am Friday):
Westerly zonal flow, extensive mid-level cloudiness and 700 mb temperatures near -4 C will characterize this
period. If a cold pool is in place during this time, it is unlikely to go
away, although a lack of radiational cooling will preclude any
intensification.
Extended outlook:
A fairly robust short-wave trough with 50 kt winds and -8 C temperatures at
700 mb is expected to arrive Friday into Friday night. This system will be
diving southeast along the periphery of a building ridge of high pressure, and
predictability on the placement and intensity of these kind of "clipper"
disturbances is low. At this point it looks likely to mix out the valley and
bring over a quarter inch of snow/rain mix to the Salt Lake Valley. Long-range
models indicate a large region of high pressure may build over the western
U.S. next weekend into early next week, with further persistent cold air pools
likely in this scenario.
05-Dec-2010 09:51
T+0->T+24:
Short wave ridge axis shifts east of the forecast area today as a compact and flattening shortwave trough moves onshore through California and into the Great Basin tonight. Clouds from last night are starting to exit the region with clearer skies upstream over Nevada. This should allow for a sunnier day today with scattered mid level clouds, before clouds again increase and thicken tonight. The 00 UTC sounding shows that the boundary layer up to ~840 hPa is nearly saturated, which may allow for low altitude strato-cumulus to form. Lapse rate is close to isothermal up to 790 hPa, with drier better mixed air above. 700 hpa temps start out near -5 C but do to warm air advection will rise to around -2 by this evening.
T+24->T+48:
The flattening shortwave trough arrives in the region midday monday with the coldest air and trough axis progged to arrive in the late afternoon (around 00Z). Some light precipitation is expected by mid day in the increasing SW flow prior to the trough axis working through, continued scattered precipitation, especially in the mountains is expected through Monday night. Air at 700 hpa is expected to cool to around -6 -> -7 C. Model soundings/time heights show a deep moist neutral lapse rate which would effectively mix out the cold pool. The mixed layer remains in place through about 12Z on Tuesday morning, before strong subsidence aloft once again forces a stable layer down from aloft to and eventually below crest level (so it is possible to see a surface mixed layer capped aloft on Tuesday). However, if precipitation is less light, -7 C would still come up just shy of removing the current cold pool, so mix out is still not a certainty but is much more likely than in all the other marginal situations we've seen thus far.
T+48->T+72:
Models indicate that a subsidence inversion will descent from 600 hpa down to 800 hpa during this time. Skies will be mostly clear Tuesday and through the first half of tuesday night before some more midlevel clouds arrive. Daytime clearing coupled with weaker ambient stability in the lower layers will help to favor some overturning below crest level for the first part of the day. However as the subsidence inversion descends it will prevent this mixing from being deep enough to ventilate the valley, effectively reforming a cold pool even if we remove the current one. By Tuesday evening clear skies will likely help to generate a surface based inversion which may couple with the subsidence inversion to once again form a robust cold pool.
Extended outlook:
A more zonal flow is expected to develop for Wednesday. Strong stability in the lower mid troposphere will persist along with mid level clouds. Forecasting thinking is that if a robust cold pool develops on Tuesday, that it will be able to persist through this period.
04-Dec-2010 14:13
T+0->T+24 (5AM Saturday-5AM Sunday)
A weak baroclinic zone (warm frontal) with several minor embedded disturbances will bring clouds and light precipitation to northern Utah Saturday morning. This baroclinic zone and moisture will lift north during the day with moist SW flow aloft keeping a thick midlevel cloud deck in place through Sunday night. The GFS has the moisture and baroclinicity moving north early afternoon on Saturday while the NAM retains clouds and showers across the region into Saturday night. If the NAM is correct, some near moist-neutral conditions could result in mix-out (at least partial) on Saturday night. However, confidence in this is low and given the resilience of the inversion for the past few systems forecaster intuition is that the inversion will remain in place Saturday night. While confidence is hight that Saturday night will remain mostly cloudy, some breaks in the clouds and plentiful low-level moisture in place could result in fog redevelopment, possibly more dense in the Salt Lake Valley than that observed last night if the breaks in the mid-level cloud deck are sufficient.
T+24->T+48 (5AM Sun-5AM Monday)
Southwest flow ahead of a short wave trough approaching California will spread high clouds into the forecast area. Temperatures aloft will remain around -2 C
at 700 mb for the 24-48 hr forecast period. There is some uncertainty on the amount and thickness of the cloudcover during the day Sunday and overnight
Sunday night. The NAM has less high clouds Sunday night than the GFS. How much clear skies and cooling are observed Sunday night and the impacts on fog redevelopment will also be important in the evolution of the cold air pool.
T+48->T+72 (5AM Monday- 5 AM Tuesday)
All eyes are on the compact short-wave trough that will be centered over Salt Lake City around hr 60 (Monday afternoon). The system is not particularly cold
or strong, although QPF amounts indicate we will get more than a sprinkle (0.10-0.25" rainfall). Temperatures will cool to around -6 C aloft and during this period we stand the best chance of moist-neutral conditions and at least a partial mix-out. We will have to wait and see.
Extended: Looking ahead to Tuesday at short wave rigde builds with 700 mb temperatures warming rapidly to +2 C. A clear, cold night is likely Wednesday morning. The short-wave ridge quickly moves east and is replaced by zonal flow with plenty of high clouds. Salt Lake City remains on the southern edge of this flow, however, indicating that an inversion is likely to persist.
03-Dec-2010 15:44
T+0->T+24 (5AM Friday-5AM Saturday)
A weak baroclinic zone with associated clouds and light precipitation will be in place across northern Utah during the day. Despite modestly colder air
aloft cold pool conditions will remain in place with a potent near surface inversion layer. As we move into the evening and overnight hours the baroclinic
zone will lift slightly to the north, then north east. However moist SW flow will keep skies mostly cloudy with light rain possible.
T+24->T+48 (5AM Sat-5AM Sunday)
In general moist SW flow aloft will continue with cold pool conditions in place at the surface. Clouds and light precipitation will remain likely. Embedded
within the SW flow a modest vorticity maxima (sheared off from a deeper trough in the Eastern Pac) will arc through the northern portions of the great basin
. A pocket of modestly colder and moister air may accompany this feature and the NAM seems to think that a mix out may occur Saturday evening with a
saturated near moist nuetral profiling developing. Uncertainty remains high for this 'event' as the GFS differs in the location of the voriticity maxima and
keeps northern Utah drier and more stable through that period. It is also unlikely that the Nam is really resolving the strength of the inversion currently
in place (how could it with so much stability packed into some very shallow layers)
T+48->T+72 (5AM Sunday- 5 PM Sunday)
On Sunday and sunday night utah will remain moist SW flow as a rapidly weakening pacfic trough slowly approachs the region. Skies will be cluttered with
variable mid and high level clouds. Temperature advection remains weak. No major changes in cold pool structure seems likely at this time.
Extended: Looking ahead to monday the dying pacific trough pushes into the region bringing deeper moisture and slightly colder air. Precipitation,
especially in the mountains looks to be more significant than that which we've seen over the past few days and a mix out or modification of the cold pool is more likely.
02-Dec-2010 13:57
t+0->t+24 (5AM Thurs - 5 AM Friday)
A strong cold pool is in place with a sharp temperature inversion between the
surface and 800 hPa. The air within this shallow inversion is moist, with some
fog observed early, and quite polluted. The air aloft is better mixed, drier,
and much cleaner. Winds are southerly withing the cold pool and westerly aloft,
veering across the the top boundary of the inversion. Warm air advection and
possibly some subsidence warming is occuring near 800 hpa.
An meridionally elongated trough is currently moving east from the Pacific into
the western US impinging on the broad ridge currently in place across the
interior SW. As it does so the northern portions of the PV anomaly will shear
away from the more southern reaches. The northern portion of this system will
drag a modest baroclinic zone with associated clouds and light precipitation
into Nevada and Idaho. Across northern Utah cloud cover will increase by
morning, as will winds aloft (WSW near 40 kts at 700 hPa). Neutral or weak warm
air advection is expected. The degree of overnight cloud cover is uncertain as
are the effects of the strong winds. Some pressure channeling of the winds is
possible with surface high pressure developing SE of the region and surface low
pressure over Idaho and Montana.
T+24->T+48: (5AM Friday - 5AM Saturday)
For the firt twelve hours of this forecast period the baroclinic zone sags south
into northern utah bringing cooling and moistening throughout the column as well
as strong WSW winds aloft and southerly flow near the surface. A weak surface
confluence boundary is may just reach the Salt Lake Valley during this time.
Light mixed precipitation is possible.
During the second half of the forecast period the baroclinic zone and associated
features will migrate back to the north. Warm air advection will become dominant
during this time. The fate of the surface cold pool throughout this period is
highly uncertain as the degree of wind, cooling, precipitation, and the strenght
of the cold pool itself are all unclear.
T+48->T+72:
Moist SW flow will be in place. Periods of clouds and light rain are possible,
though most of the precip seems to be focused on the orography.Warm air
advection will oppose some dynamic cooling, holding crest level temps near -4 C.
01-Dec-2010 12:02
t+0->t+24:
Ridge amplifies across the western US, weak warm air advection at 700 hpa with temperatures climbing to -1 C. Mid level clouds will continue through the first portion of this forecast period, though partial clearing will be possible overnight as the amplifying ridge nudges clouds to the north. The extend of this clearing may have significant impacts on the formation of a surface cold pool during the night time hours.
t+24->t+48:
Ridge axis shifts to the east placing northern Utah under WSW flow aloft. A flattening trough and associated baroclinic region approaches for Thursday night, bringing clouds, colder air aloft and at the surface, and a chance for either rain or snow. Windspeeds at 700 hpa are expected to accelerate to near 35 Kts from the WSW as the leading edge of the baroclinic zone approaches. Collectively these processes are expected to bring the demise of the cold pool sometime after 00 UTC on Dec 3rd. However, the GFS and NAM differ on the southward extend of this baroclinic zone, with the GFS holding it further north which may make mix out somewhat less likely.
t+48->t+72:
Colder air and clouds for the first half of this period are expected followed by considerable warm air advection in SW flow. Cold pool formation does not seem particularly favorable during this period.
Extended outlook:
Warm SW flow continues for Saturday in advance of a trough ejecting from the southeastern Pacific into the mountain west. Clouds will increase into Sunday with the trough expected to bring valley rain and mountain snow late in the day on Sunday.
30-Nov-2010 14:30
T0-T24:
Strong subsidence inversion is expected to lower from crest level downwards to about 850 mb. The surface based nocturnal inversion looks to be eroding due to surface heating in some but not all locations. High clouds will lower and thicken this afternoon and evening, though some breaks are still possible. This thick altostratus may initiate some light precipitation, though nothing more than a trace is currently expected. Clouds are expected to limit cold air production overnight and a strong surface based nocturnal inversion is thus not expected.
T24-T48:
A very weak shortwave trough will pass just north of the region keeping scattered clouds over the valley with thicker clouds overnight, slight chance of precip still exists. Strong static stability will remain in place in the lower troposphere throughout the day and a limited diurnal range is expected due to the cloud cover.
T48-T72:
Ridge amplifies shunting clouds to the north. Weak warm air advection. Cold pool may reforms/restrengthens if the phasing of the clearing coincides with even a portion of the nightime hours.
Long Term: Another trough looks to bring light to moderate precipitation (currently expecting mostly rain, though surface temps may be an issue). Models suggest that moistening of the lower troposphere will force the atmosphere closer to moist adiabiatic and may allow mix out the cold pool sometime on Dec 3rd, after that the ridge looks to reform and last for a few days.
29-Nov-2010 12:23
T-0->T+24:Cold trough exits to the east with ridging building in from the the west. Clear skies are expected through the overnight hours with relatively calm winds. Strong warming in the mid levels do to weak warm advection and strong subsidence. 700 hpa temps go from -16C to -4 -> -6 C during this period. Mid level warming and surface cooling should develop a robust cold pool scenario.
T+24->T+48:
Ridge axis shifts east across region and ridge is weakend late in the period due to an impinging short wave trough. High clouds are expected from midday on thickening and lowering into altostratus during the overnight period. Light precipitation is possible. Strong stability remains in place near crest level (700 hpa). Cooling with the weak short wave is insignificant and not expected to remove the cold pool. Surface conditions are an unknown forecast problem with cloud cover strongly impacting the surface radiation balance. Light precipitation could play a role in moistening the lower levels which may help in fog generation.
T+48->T+72: Amplifying ridging over the mountain west with significant moisture spilling across the ridge axis, which may lead periods of mid and upper level clouds. Northern Utah will be on the fringe of these cloudy areas so considerable forecast uncertainty remains. Modest warming due to WAA pushes 700 hpa temps to near 0C. Stability at and below crest level is expected to remain in place allowing the cold pool to persist.
Long Range: Sometime on Dec 4-5 a series of troughs may impact the region, though current model runs suggest splitting and flattening disturbances. The demise of the cold pool is uncertain, though currently projected to occur on Dec 4th. If these systems are weaker than expected the possibility of a very long lived cold pool exists.
Forecasters: Neil Lareau, Zach Hansen, and Erik Crosman.
