UWFPS 2017 daily cold-air pool forecast valid 12/27/2016  0900 MST

Current weather synopsis and nowcast (0-12 h valid 0900 MST 27 December to 2100 MST 27 December):

The cold weather system on Christmas day brought cold air emplacement and significant fresh snow cover to the region, setting the stage for a significant cold-air pool episode for the next several days. Current snow depths range from 7-14" across the region.  Utah lake remains unfrozen but if cold weather continues over the next week it will likely freeze. The current Great Salt Lake temperature is just above freezing around 1 C.

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

Winds: light and variable.

Cloud cover: Partly cloudy outside of foggy areas in low-lying areas.

                                    MODIS Image from 26 Dec. Blue areas are snow covered.

                                              Skew-T with strong temperature inversion 0500 MST 27 Dec

Short-term forecast (12-36 h forecast):

Cold-pool conditions expected next 36 h.

A strong inversion is noted this morning from the surface to 600 mb. A weak weather system will graze Utah over the next 24 hr, cooling temperatures aloft from -4 C to -10 C and weakening the cold-pool but likely not removing it.

 The depth of the polluted layer is expected to be ~200 m in Cache Valley, and 300-500 m in Utah and Salt Lake Valleys.

Visibility/ceiling: Dense fog in low lying areas, visibility 1/8-1/4 miles, IFR or lower through morning. Visibility rising to > 1 mile with ceiling heights > 2000 m in afternoon

Winds: southwest winds 5-10 mph through noon, reversing to light northwest winds in the afternoon at KSLC

Cloud cover: increasing mid-level clouds associated with weak weather system.

Mid-term forecast (2-4 days):

A short-wave ridge will traverse northern Utah between Wednesday and Friday.  Warm air advection at mountaintop will warm 700 mb temperatures from -10 C to ~+1 C, resulting in a high-intensity capping temperature inversion over the Salt Lake Valley from through Friday. Peak cold-air pool intensity expected on Thursday 29 Dec. The strong capping inversion height will be descending during the period. Winds in the boundary-layer will remain light and variable. Westerly winds ~10-20 kts at 700 mb will likely drive a channelled jet between 300-700 m AGL into the Salt Lake Valley through the Jordan Narrows from the South on Friday.

Visibility/ceiling: Low forecast skill on the fog occurrence and intensity during this period. stay tuned, as modification of current cold temperatures important for fog probabilities. Dense fog is possible during morning hours.

Winds: light and variable less than 6 kts from surface to 800 mb. 10 kts at 700 mb from west on 13 December increasing to 10-15 kts southwest by 15 December.

Cloud cover: Main concern is dense near surface fog. Large lake-air temperature differential will increase fog potential near Great Salt Lake. Deep snow cover will increase change of radiational fog in all basins.

Long-term forecast (4-7 days):

A weather system will weaken the cold-air pool late Friday into Saturday. At this time, mix-out is uncertain. It is possible that the cold-air pool remains until a front arrives on Sunday 1 January. Fresh snowfall is likely at this time as well across the region.

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.