UWFPS 2017 daily cold-air pool forecast valid 01/12/2016  1830 MST

Current weather synopsis and nowcast (0-12 h valid 1830 MST 12 January 2017 to 0630 MST 13 January).

The unusually wet, unsettled weather continued today in northern Utah with light accumulations of snow (dusting-1" in Utah and Salt Lake Valleys). The moist air mass combined with the saturated soils resulted in fog, some dense, for the morning hours. Essentially all of the snow has melted in the Salt Lake and Utah Valleys. 8-11" snow remains on the ground in the Cache Valley.

Light flurries and low stratus will result in IFR conditions at the Salt Lake Airport through 2300 MST. Clearing is expected after midnight.  The Utah Lake remains frozen. The current saline Great Salt Lake temperature is just below freezing around -1.0 C.

Visibility/ceiling: Dense fog possible in low lying areas, visibility 1/8-1/4 miles, IFR or lower through midnight.

Winds: light and variable.

Cloud cover: Overcast in stratus and flurries through midnight, then partial clearing.

                             

Short-term forecast (12-36 h forecast Friday 13 January):

A break in the seemingly endless Pacific storm systems begins Friday. Partly to mostly cloudy skies and cool temperatures (30-35 F) are expected along the Wasatch Front and in the Cache Valley as the recent storm system departs.

Visibility/ceiling: no restrictions

Winds: light and variable less than 6 mph

Cloud cover: decreasing increasing mid-level clouds clearing by Friday afternoon.

Mid-term forecast (Saturday-Monday 14-16 January):

A low-amplitude ridge of high pressure will approach Utah from the west on Saturday.  Warm air advection during at mountaintop will warm 700 mb temperatures from -10 C on Thursday to around -5.0 C on Sunday and Monday. This will resulting in a low-intensity capping temperature inversion or cold-air pool over the Salt Lake Valley beginning Saturday night and continuing through Monday.

Mostly clear skies and brisk northerly winds are expected during Sunday afternoon. Sounding profiles in some of the models now unfortunately produce fog and stratus formation in the Salt Lake Valley due to the unexpectedly wet day on Thursday. please stay tuned.  Persistence is a powerful forecast in these episodes, so Saturday will provide a decent sense of the potential fog issues for Sunday.

Visibility/ceiling: Low clouds and fog possible.

Winds: generally light and variable.

Cloud cover:Mostly clear about boundary-layer.

NAM forecast sounding for Sunday 1400 MST. Note the shallow fog/low cloud saturated layer forecast near the surface.

Long-term forecast (Tues-Fri Jan 17-20):

The weak to moderature intensity cold-air pool that will begin on Sunday will potentially last until mid-week before the cold-air pools ends as another potential period of copious moisture and heavy rain/mountain snow begins next Thur/Fri 19-20th Jan. The cold-air pool is currently forecast to peak in intensity on Tuesday (see forecast sounding below). However, this episode will only be a "modest inversion", given the lack of snow cover (except in Cache Valley), periods of high clouds that limit surface radiational cooling, no really cold air in the boundary-layer to start the episode, and temperatures at mountaintop level that are not that warm (not a strong high pressure center).

GFS Forecast sounding on Tuesday 1100 MST during peak cold-air pool intensity.

Key cold-air pool meteorology definitions:

cold air emplacement --> Cold-air left behind from a synoptic weather system; significant cold-air emplacement in the boundary-layer will lead to a more intense cold-air pool when warming aloft commences.

warm air advection at mountaintop --> A key driver of cold-air pool intensity. Very warm temperatures at 700 mb relative to surface temperatures result in a more intense capping inversion.

lake-air temperature differential --> When the lake surface is warmer than the overlying air (most typical scenario), the lake surface acts as a moisture and heat source, increasing chances for dense fog formation.