UWFPS 2017 daily cold-air pool forecast valid Tuesday 01/31/2017 8:00 MST (8:00 am local time)

Summary: The most significant cold-air pool episode of the 2016-2017 winter continues, with a multi-day period with PM2.5 above the National Ambient Air Quality Standard in Utah, Salt Lake, and Cache Valleys expected through Friday.  A weakening and dilution of pollution within the cold-air pool is likely Saturday as a storm system crosses the area, however complete mix-out remains uncertain as only minor changes between model solutions will be the difference between complete or partial mix-out. Weaker valley inversions could potentially continue until another possible storm Monday or Tues 6 or 7 February.

Current weather synopsis and short-term forecast for Tuesday 30 January:

A shallow and pronounced temperature inversion contines on day 5 (Tuesday) of the cold-air pool. A stratus layer re-formed Monday night at the bottom of the strong capping inversion very similar to Sunday night.

1200 UTC 31 January KSLC Sounding

The low clouds appear to be very similar to those observed yesterday. Thus, persistence forcast with the cloud deck lifting by mid-morning is the most likely scenario. However, very minor changes in temperature and moisture structure can lead to the fog not lifting, so it is a 'waiting game' in terms of aircraft operations.

Visibility/ceiling/clouds: Cloudy/foggy in the morning with visibility in 1-3 mile range and ceilings in the 500-1000 ft agl range. transitioning to partly cloudy and hazy in the afternoon. Ceilings rising to around 2000 feet and visibility in the 2-4 mile range.

Winds: Southerly 5-10 kts below inversion height in the morning in the Salt Lake and Utah Valley. Turning northwesterly 5 kts in the afternoon. light and variable in Cache.

Wednesday 1 February:

Persistence forecast (similar evolution to Sunday night into Monday) used at this time. However, this forecast will be updated this afternoon/evening. It is possible that the nocturnal cloud deck dissipates with the arrival of mid-level moisture either Tuesday night or Wednesday night.

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 will also begin to increase and be channelled at times into the Salt Lake Valley from the south through the Jordan Narrows gap  afternoon on Wednesday.

Winds: Southerly 10-15 kts below inversion height in the Salt Lake and Utah Valley.  light and variable in Cache.

Thursday 2 February- Tuesday 7  February:

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

Key events to watcch during this peruod:

• Rain and snow and potential mix-out or partial mix-out Friday night into Saturday morning
• Second storm system Monday night-Tues morning that will potentially complete the mix-out if first one does not do the job.

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 continue to 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 at times 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 night into Friday night, making for a very shallow polluted layer that could be harder to sample with the aircraft, as well as gusty southerly channelled winds in the 20-30 kt range.

Precipitation is 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. A brief return to high pressure on Sunday will be followed by potentially another storm Monday into Tuesday. 

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.

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

Summary: The most significant cold-air pool episode of the 2016-2017 winter is underway. 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.

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

The temperature inversion continued to lower and strengthen Saturday night (for the second night in a row), and this trend is expected to continue into Monday.

1200 UTC 29 January KSLC ounding. Note the strengthening and descending temperature inversion compared to past days.

Fortuitously for aircraft operations, the cold-air pool has started out without much dense fog.  As of 8 am, mostly clear conditions with visibilities around 5 miles in haze were observed in the Salt Lake and Utah Valleys. Localized dense fog was being reported in the Cache Valley.

 

Satellite fog product at 8 am. Note the lack of yellow colors (fog) over all areas.

Given that boundary-layer moisture is only modestly increasing due to minimal snow melt and cold temperatures, persistence forecast will be used at this time. However, an expansion of low clouds and fog is probable over the next day or so.

Visibility/ceiling/clouds: Mostly sunny 10 am- 5 pm all areas, with the possible exception of Cache. Cold with highs in the 20s, lows in the 5-10 F range. 30% chance of low clouds/stratus development in the Salt Lake and Utah Valley and 50% chance of fog in the Cache Valley after 6 pm. Ceilings most likely to remain above 2500 ft AGL with visibility in the 3-5 mile range in Salt Lake Valley. Ceilings less than 500 feet and visibility less than 1/2 mile possible in Cache.

Winds: light and variable.

Monday 30 January:

The cold-air pool will continue to strengthen Monday, and pollution levels should continue ramping up each day through Thursday with the combination of a descending strong, low-level capping inversion and increased emission rates during the work week.  Partly cloudy skies expected with morning fog burning off by 10 am in Cache Valley. Cold, highs in the 20s, lows in the 5-10 F range.

Visibility/ceiling/clouds: Mostly sunny 10 am- 5 pm all areas. 30% chance of low clouds/stratus development in the Salt Lake and Utah Valley and 50% chance of fog in the Cache Valley before 9 am and after 6 pm. Ceilings expected to remain above 2500 ft AGL with visibility in the 3-5 mile range in Salt Lake Valley. Ceilings less than 500 feet and visibility less than 1/2 mile possible in Cache.
Winds: light and variable.

Tuesday- Thursday 31 January - 2 February:

The cold-air pool will continue with pollution levels continuing to climb.

If a stratus deck forms during this period, then pollution levels will not climb as fast as if conditions are less cloudy. 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: 30% 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 5-15 knot southerly flows between 200 and 800 m above ground level in the Utah and Salt Lake Valley.

 

Friday 3 February – Sunday 5 February:

Complex potential decay period of the cold-air pool. 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.

UWFPS 2017 daily cold-air pool forecast valid Saturday 01/28/2017 8:00 MST (8:00 am local time)

Summary: A major cold-air pool episode is underway. Minimal stratus and low clouds have formed at the onset of the episode, and will likely not affect flight operations tonight. Pollution concentrations are expected to increase daily through Wednesday. Increasing mid-level clouds, southerly winds at the top of the shallow cold-air pool, and top-down erosion of cold-air pool is expected Thursday, with a storm system and mix-out possible Friday.

Current weather synopsis and short-term forecast for Saturday 28 January:

The temperature inversion has lowered and strengthened overnight, and this trend is expected to continue into Monday.

 

SLC sounding at 5 am. Note the descending capping layer at 800 hPa.

The feared widespread low clouds and fog that plague other basins within the western US have not materialized thus far for the Utah basins, with drier air filtering into the area helping to prevent the development of widespread stratus and fog. However, as of 8 am, low clouds were expanding over the Western Desert and northern Great Salt Lake area (yellow colors):

 

Satellite low cloud/fog product 7 am 1/28/2017. Low clouds are in yellow.

Given the low model forecast skill with fog, persistence is generally the best forecast in the 24-hr range. Thus, a 60% chance of mostly clear conditions is expected through the flight tonight although the expansion of the clouds noted over west desert will need to be watched.

Visibility/ceiling/clouds: Mostly sunny and cold highs in the 20s, lows in the 5-10 F range. Mostly sunny and hazy conditions are expected through the period at this time. There is a 30% chance of low stratus deck or dense fog forming in the evening after sunset and will need to be monitored.

Winds: light and variable.

Sunday 29 January:

The cold-air pool will continue to strengthen Sunday. Partly cloudy. Cold, highs in the 20s, lows in the 5-10 F range. 30-50% of dense fog at Salt Lake Airport and Logan Airports in the morning, 30% chance of fog and stratus cloud deck forming in all basins and will need to be monitored.

Visibility/ceiling/clouds: 30% chance of stratus and fog. Ceilings less than 1000 feet and visibility less than 1/2 mile possible if low clouds and fog occur. Winds: light and variable.

Monday 30 January – Wednesday 1 February :

Pollution levels should ramp up notably each day Monday-Wednesday with the combination of a descending strong, low-level capping inversion and increased emission rates as the work week starts. The cold-air pool will continue to strengthen Monday and remain strong Tuesday. Partly cloudy. Cold, highs around 30, lows in the 10-15 F range.

Visibility/ceiling/clouds: 40% chance of stratus and fog during this period, although confidence is low. Ceilings less than 1000 feet and visibility less than 1/2 mile possible if low clouds and fog occur. Winds: light and variable.

Thursday 2 February – Friday 3 February

Complex decay period of the cold-air pool. Details are uncertain at this time. Southerly winds are expected to erode the top of the cold-air pool on Thursday, making for a shallower polluted layer that could be harder to sample with the aircraft. The southerly winds may also ‘clean-out’ the Utah and southern Salt Lake Valley before other areas on Thursday. A storm system with rain/snow mix at this time is also possible Friday. The combination of the increasing southerly winds and storm system result in complete mix-out of the cold-air pool on Friday in current numerical model prediction. Stay tuned as we get closer and model skill improves.