Before PCAPS began, the team of U of U scientists met with some of the experts from the National Weather Service (NWS) here in SLC. We were curious as to what they considered to be the most pressing forecast challenge associated with Persistent Cold Air Pools (PCAPS). The onset of dense fog, particularly at the airport, was the unanimous reply.
Fog in some ways seems like an easy forecast issue. For fog to form the ambient air temperature must cool to its dew point, at which point water vapor is forced into condensate in the form of fog (or cloud) droplets. Alternatively, sufficient moisture can be added to air through evaporation from the ground (water vapor flux) or through evaporation from rain to bring the air to saturation. Easy right? In principal yes, in reality no.
Throughout much of IOP 1 the atmospheric boundary layer seemed continually on the verge of forming fog, and in fact on a few occasions the airport reported mist (which is basically thin fog), but no truly dense fog. In some ways the forecast question became "why isn't dense fog forming?", after all the air was nearly saturated, the ground was wet and cold with snow and melted snow, and overnight temperatures were within measurable accuracy of the dew point.
Things changed on Sunday Dec 5th.
Light rain fell on Saturday evening across the Salt Lake Valley and much of the northern Great Basin. As skies cleared behind the departing weak system and temperatures dropped, dense fogged formed over much of NW Utah and NE Nevada.
However, dense fog did not form within the Salt Lake Valley. There were some areas of mist overnight and in the morning, but nothing like what was seen over the west desert. Why? I have no clue. Something to do with the urban environment? maybe there were more clouds overnight due to the proximal Wasatch Range? Very tough to say.
Despite the lack of in situ fog production, SLC would not remain fog free for long! With strong differential daytime heating between fog covered areas and the sunnier Salt Lake Valley organized northerly flow developed which would soon advect a 'fog front' from our west and north into the valley. The airport was the first to see its arrival, which was marked by visibilty plummeting from 2 miles (haze and mist) to less than 1/8 of a mile which caused numerous delayed flights. By early afternoon downtown Salt Lake disappeared into the murky ground based cloud. A ragged boundary of fog continued to progress south through the valley throughout the afternoon, with all of the valley eventually 'going under' by dark. Visibility was minimal, and may even have played a major role in the crash of a small aircraft attempting to land near Ogden.
What tipped the scales to allow for dense fog versus thin mist? Was the precipitation contribution of moisture to the cold pool the straw that broke the camels back? What role did the melting snow play in both forming and maintaining the fog? And why did the Salt Lake valley behave so differently?
It is tough to draw any conclusion as of yet, but fortunately PCAPS has two phases: 1.) observing and 2.) analysis. The analysis of the data we collect now will be ongoing for years to come, and perhaps we'll be able to answer some of these questions.