Monday, November 29, 2010

And so it begins. IOP 1 sort of...

Following the passage of a cold trough that brought significant snowfall to all of the Salt Lake Valley conditions are seemingly ripe for a cold pool to form as a ridge builds into the region from the west. Strong subsidence warming is expected near crest level, but at issue is the degree to which a nocturnal surface based inversion forms under the partly cloudy skies on Monday night. Models have suggested that the boundary layer will remain fairly well mixed through the night... however forecaster intuition would suggest that the widespread snow pack and only partly cloudy skies will favor rapid cooling at the surface. While the 00UTC sounding from the Salt Lake airport does indicate a relatively well mixed lower troposphere,

surface observations show rapid cold pool formation in the neighboring valleys and more modest cooling within the Salt Lake Valley proper. This pattern is quite familiar as the less urban and more natively vegitated Rush and Tooele valleys tend to develop stronger nocturnal cold pools. Within the Salt Lake Valley many stations have shown their typical diurnal windshift to the SE which is a well known indicator thermally driven drainage flows, and presumably a precursor to the formation of a nocturnal surface based inversion.

Infrared satellite images after sunset also indicate that skies are mostly clear and the surface is in fact largely snow covered and quite cold especially compared to the nearby Great Salt Lake. Good conditions for a cold pool if you ask me.

Beyond tomorrow the future of this event becomes less certain with thick altostratus possible, a very weak shortwave trough, and then continued 'dirty' ridging expected. It will be of great interest to our group to see how the cold pool responds to these variations.

With the official start of PCAPS just two days away, if this event comes together as we currently anticipate we will start operations in the midst of a Cold Air Pool. The PCAPS team hopes to use this event as our first IOP, though we will be scaling back from full operations and using this event as a shakedown for our operations, forecasting, data analysis, communications and decision making processes. The current plan, pending revision, is to use twice daily (18 and 06 UTC) NCAR ISS balloon launches to supplement the NWS operational soundings and to monitor the continuously operating observation platforms.

Now I guess we'll just have to wait and see how much cooling occurs tonight.

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