textproduct: Salt Lake City

KEY MESSAGES

- Mild conditions exist through the early afternoon ahead of an incoming storm system. Mountain snow and perhaps a few scattered rain showers across lower elevations of northern UT begin this evening.

- Mountain snow continues through Friday morning with the bulk occurring Thursday AM. Orographic showers continue following the passage of a cold front on Thursday.

- Following the passage of a cold front early Thursday AM, lake effect snow is expected to develop for the SL Valley and perhaps the Tooele valley Thursday. Lake effect snow may also persist into Friday, though uncertainty remains.

- High pressure builds in following this system and will last through the remainder of this forecast, perhaps even beyond. Unfortunately, this would favor the development of valley inversions.

DISCUSSION

Southwest flow aloft continues to establish itself throughout the majority of the day today as a shortwave trough traverses across the Desert Southwest along with a longwave trough diving southeast across the PNW. Southwest winds will mix down to the surface keeping conditions warm and mild across the forecast area ahead of deteriorating conditions this evening.

This afternoon/evening, both aforementioned troughs will gradually phase together over time and with eastern extent, with the resultant large longwave upper trough diving south toward AZ. This trough evolution will enable markedly colder air compared to any other system we have seen thus far this season, bringing temperatures at 700mb ranging from -15C to -17C, resulting in our first legitimate shot at lake effect snow for the Salt Lake Valley and perhaps the Tooele Valley this season.

Initially, ascent ahead of the parent trough diving down from the PNW is forecast to remain weak. This will generally result in higher elevations seeing higher PoPs through most of today given orographic enhancement, though isolated valley showers across northern UT remain possible this afternoon and evening (~50% PoPs or less). Given antecedent warm conditions, snow levels will begin around 5500- 6000ft ahead of an approaching cold front.

On Thursday morning, a cold front will sweep across northern UT ushering in northwest winds. A noteworthy increase in PoPs will accompany this as the frontal boundary provides somewhat stronger ascent as it pushes through. Across lower elevations, this precipitation will generally occur as rain though mixed wintry precip can't be ruled out entirely. Within this northwest flow pattern comes more terrain induced snowfall across northern UT, bringing mountain snowfall to all of our mountains, with potentially significant mountain snowfall across northern UT, and valley snowfall across the majority of the state.

Concerning valley snowfall, amounts remain quite similar to last nights forecast given some uncertainty remains with where the lake effect band will ultimately set up and when northwest winds will switch to northerly. Currently, the ensemble mean snow accumulation at KSLC remains around 3-4" with a range spanning from nothing to over 10". Hi-res guidance now reaches the event, though leave much to be desired as the HRRR and RRFS accumulations at KSLC sit on the extreme low end of forecast accumulations with generally less than ~0.5". Despite the hi-res models hanging on the low end, lake effect snow guidance continues to indicate that northwesterly flow hangs on through the majority of Thursday into Friday morning. If this scenario were to occur, the higher end of the forecast snow accumulations may be realized in some areas locally. Unfortunately, it still remains uncertain *when* the northwest winds will clock to northerly, and which parts of the SL and Tooele valleys will see the most snowfall from this event.

Regarding mountain snowfall, how long northwest flow hangs around is also quite important for the overall totals. Locations in the Upper Cottonwoods continue to show quite the range amongst ensemble members ranging from 8" to perhaps in excess of 20" locally due to this uncertainty. Otherwise, most of our other northern UT mountains will see generally 6-12" of snow from this event, though the Bear River range could also see amounts in excess of 20" across higher elevations. For our central and southern UT mountains, generally 4- 8" is expected with locally higher amounts around 12" possible at Brian Head and the Tushars.

Unfortunately, forecast confidence continues to increase regarding a notably dry period following this system as a ridge of high pressure builds in. With weak flow aloft expected, and perhaps snowfall existing on the valley floors, the development of valley inversions appears quite likely. The overall strength of the inversion is heavily dependent on just how much snowfall the valleys see and position of the ridge overhead, yielding some uncertainty at this time.

REST OF UTAH AND SOUTHWEST WYOMING

Predominant VFR conditions can be expected through the day with general south to southwesterly winds. North of I-80 rain showers this afternoon may temporarily reduce CIGS into the MVFR range. A cold front will cross northern Utah and southwest Wyoming later this evening through tonight, dropping snow levels to the valley floors and resulting in IFR to LIFR conditions in light to moderate snow. These conditions will be most common after 06Z. Across southern Utah VFR conditions will continue through tonight.

SLC WATCHES/WARNINGS/ADVISORIES

UT...Winter Weather Advisory from 2 AM to 11 PM MST Thursday for UTZ102>105-107-108.

Winter Weather Advisory from 5 PM this afternoon to 11 PM MST Thursday for UTZ110.

Winter Weather Advisory from 11 PM this evening to 11 PM MST Thursday for UTZ111>113.

Winter Weather Advisory from 5 AM to 11 PM MST Thursday for UTZ117-125.

WY...None.