Building a Valley Cold Pool the Hard Way

If you want to build a persistent valley cold pool in northern Utah, often referred to colloquially as "an inversion," the best way to do it is to have a strong cold surge followed by the development of an upper-level ridge. The cold surge provides the cold air at low levels while the ridge then increases the temperatures aloft.  During the winter, there's insufficient heating from the sun to warm the cold air at low levels, so one ends up with a persistent valley cold pool that is topped by a stable layer or inversion that prevents the dispersion of air pollution. This process is even more effective if there is snow on the ground.  

The past two weeks, however, provided something a little different.  On December 6 we had an atmospheric river move through the area (indicated by green below...note that the meteograms for temperature and wind do not quite line up vertically).  This was followed by a period with occasional mid and high clouds, somewhat enhanced southerly flow, and rising temperatures.  At the Salt Lake City airport, the afternoon wind often switches to northerly or northwest, but those periods were pretty limited due to the enhanced southerlies.  The southerlies were not strong, but combined with a lack of snow cover and occasional clouds, they allowed us to remain relatively mixed and mild in the valley.  

Source: MesoWest

However, things changed after about December 11.  The flow weakened.  We also saw clearer skies.  The result was the slow development of a valley cold pool with temperatures falling each day.  On the 11th, the maximum temperature at the Salt Lake Airport was 62°F.  Yesterday it was 49°F.  

In contrast, if we look at the temperatures on Hidden Peak at the top of Snowbird (11,000 ft), they show a general warming trend for the period.  

Source: MesoWest

Essentially, we are building a persistent valley cold pool the hard way, without a prior cold-air surge. The ridge is building in, increasing temperatures aloft, but instead of having cold air at low levels to start, we were relatively mild.  Since December 11, however, with the flow weakening and less cloud cover, we are seeing temperatures declining with a valley cold pool slowly strengthening.  Yesterday afternoon we had a bonafide inversion with a surface temperature at the airport of 45°F and a temperature at 7200 feet of 50°F,

Source: SPC

And with the strengthening cold pool and inversion, we are starting to see the buildup of pollution in the valley, as evident from the photo I took yesterday at the top of Mt. Van Cott above the University of Utah campus showing smog in the Salt Lake Valley.

Really we're fortunate not to have had that prior cold surge or snow on the valley floor.  We're also fortunate to have a cold front coming in on Wednesday that will mix out the pollution.  

Note: We are moving into a more active pattern this week, but will be taking a break from blogging for a few days.  Good luck!