UWFPS 2017 daily cold-air pool forecast valid Monday 01/29/2017 10:00 MST (8:00 am local time)

Summary: The most significant cold-air pool episode of the 2016-2017 winter is underway. Unfortunately for aircraft operations, low clouds set up early Monday morning along the Wasatch Front. Pollution concentrations will increase daily through Thursday. Mix-out time is uncertain as the system next Friday-Saturday may be too weak to remove Valley inversions which could potentially last beyond Sunday 5 Feburary.

Current weather synopsis and short-term forecast for Monday 29 January:

The temperature inversion continued to lower and strengthen Sunday night (for the third night in a row). In addition, a stratus layer formed at the bottom of the strong capping inversion.

1200 UTC Monday 30 Jan KSLC sounding. Note the very strong temperature inversion that starts near 825 hPa.

Unfortunately for aircraft operations, the cold-air pool has in the Salt Lake and Utah Valleys  has turned into a stratus-capped "cloudy" cold-air pool. As of 9 am, foggy conditions with visibilities around 1-5 miles with ceilings of  were observed in the Salt Lake and Utah Valleys. Localized dense fog that was reported overnight in the Cache and portions of the Utah Valley appeared to be lifting at 9 am.

 

Ceilometer image overnight in the Salt Lake Valley showing the fog formation after midnight Sunday night.

Now that low clouds have 'set up shop', a persistence forecast will be used at this time so additional low clouds are likely over the next few days. However, how much the cloud deck elevation evolves or whether it burns off in the afternoons, remains to be seen.

Visibility/ceiling/clouds: Coudy in the morning transitioning to mostly cloudy in the afternoon. Ceilings rising to around 2000 feet and visibility in the 2-4 mile range.

Winds: light and variable.

Tuesday 31 January:

Persistence forecast (similar evolution to Sunday night into Monday) used at this time.

Visibility/ceiling/clouds: Coudy in the morning transitioning to mostly cloudy in the afternoon. Ceilings near 700 feet with 1 mile visibility rising to around 2000 feet and visibility in the 2-4 mile range. Patchy dense fog with 0.25 mile visibility before 10 am in Cache and Utah Valley.

Winds: light and variable.

Wednesday 1 February- Friday  3 February:

The cold-air pool will continue with pollution levels continuing to climb through at least Thursday, and likely into Friday.

If the current stratus deck rises and deepens, than pollution levels will not climb as fast as if conditions are less cloudy or the clouds stay at a lower height. We will have to watch. Mid- and high-level clouds will also impact the cold-air pool during this time, resulting in less cooling at night, and a decreased chance of dense surface fog. Winds will also begin to increase and be channelled into the Salt Lake Valley from the south through the Jordan Narrows gap Wednesday into Thursday as the storm system approaches California, potentially resulting in impressive gradients in pollution between the Utah Valley and southern Salt Lake Valley where mixing of clean air will potentially reduce pollutants and the northern Salt Lake Valley and Great Salt Lake where pollution will have accumulated for several days as nocturnal drainage winds take pollutants out over the Lake. The top of the inversion will lower to ~600 m above ground level, with most pollution below 400 m AGL in the Salt Lake and Utah Valleys. These southerly winds are expected to start eroding the top of the cold-air pool on Thursday, making for a shallower polluted layer that could be harder to sample with the aircraft.

Visibility/ceiling/clouds: 60% chance of stratus and fog. Ceilings less than 1000 feet and visibility less than 1/2 mile possible if low clouds and fog occur. 

Partly cloudy. Cold, highs in the 20s, lows in the 5-10 F range.

Winds: light and variable within cold-air pool near surface. Elevated 10-20 knot southerly flows between 200 and 800 m above ground level in the Utah and Salt Lake Valley.

 

Saturday 4 February – Monday 6 February:

Complex potential decay period of the cold-air pool, with precipitation likely Friday into Saturday. The unknown is whether the mixing and cold air with the system are sufficient to overturn the toxic soup that will be in the Utah basins. Details are uncertain at this time as models do not agree yet on either the strength of the remaining temperature inversion or if the storm system will be cold enough to easily scour out pollution from the low elevations of the Great Salt Lake or Cache Valley. Stay tuned as we get closer and model skill improves.