theWeatherMatt: Snow set to arrive in Utah, ushering in the first day of Meteorological Winter

Several waves of moisture are setting up to bring snow across the Intermountain West region. An upper-level trough moves over the western United States on Thursday, pushing out that pesky ridge of high pressure. Unfortunately, the trough is broad and multi-faceted with a few shortwaves. As a result, mid-level cold air advection (CAA) is only weak to moderate. It lacks a strong pressure and thermal gradient, meaning winds may not be as strong as one would hope for - nor will the vertical temperature profile be unstable enough - to scour out valley temperature inversions over Utah and the Intermountain West (at least not at first). With temperatures at the 700mb level (roughly 10,000ft MSL) hovering around -7 °C to -8 °C on Thursday, and Jet-level winds well south of Utah, don't expect much improvement with the air quality in Salt Lake City until perhaps Friday (at the earliest).

In fact, due to the somewhat disorganized nature of the synoptics, current model guidance shows the earliest shot at stronger surface winds not occurring until late Friday night. The onset of these strong winds are illustrated below:

Surface Wind Gusts (knots) at 07Z (midnight MST) Sat 02 Dec 2023 [12Z 30 Nov Run]

Credit: College of DuPage Meteorology Program NeXT Generation Weather Lab

The first couple waves of moisture, however, will bring snow showers as early as Friday morning and afternoon. Perhaps Friday's cooler air at the upper levels will help lift the inversion. If not by the afternoon then most likely by the aforementioned winds Friday evening.

Snowfall forecasts still vary somewhat by model. Clusters of GEFS members are hinting at higher snowfall rates Friday afternoon (approximately 2" at SLC Int'l Airport), whereas others follow the trend of gradual snow accumulation over the course of the weekend. The GEFS, NAEFS, and CMCE averages are now in good agreement for snow amounts totaling near 3" at KSLC. But note that despite the precision of the means, there is still some uncertainty on the accuracy due to the spread and lingering outliers on both the high and low ends for KSLC. See the snow plumes for KSLC below. Note that the time on the x-axis is UTC (so subtract 7 hours for MST).

NAEFS Snow Plumes for Salt Lake City (KSLC) 00Z 30 Nov 2023 Model Run

Credit: University of Utah - Department of Atmospheric Sciences Weather Center

High elevations of Utah look to be the winner with this weekend's storms. Heavy mountain snow is expected. Totals ranging 2-5ft this weekend are not out of the question. Specifically for Alta, the GEFS mean is being the most optimistic at 55" when all is said and done. See the snow plumes for Alta ski area below. Times are UTC (subtract 7 hours for MST).

NAEFS Snow Plumes for Alta Collins (CLN) 12Z 30 Nov 2023 Model Run

Credit: University of Utah - Department of Atmospheric Sciences Weather Center

One final thought, any lake enhancement (i.e. "lake effect") snow is likely to occur once the coldest air aloft arrives on Friday evening into Saturday morning. Winds aren't very favorable (by direction) due to the more westerly track crossing over the shorter fetch of Great Salt Lake. Areas that may be impacted by lake-enhanced snowfall include Davis and Weber counties.

The local NWS office has provided the following graphical snowfall forecasts. First is the expected (50th percentile), followed by the high-end (90th percentile) amounts.

Expected (50th percentile) Snowfall Forecast: Fri AM - Mon AM (Dec 01-04)

Credit: Salt Lake City National Weather Service Weather Forecast Office