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FXUS65 KSLC 022311 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 411 PM MST Sat Dec 2 2023

SYNOPSIS
A decaying atmospheric river will push through the region tonight through late Sunday, bringing heavy mountain snowfall and valley rain/ snow across the northern two-thirds of the forecast area. High pressure will build back into the region early in the week with another potential storm system lined up for the late week.

.SHORT TERM (Through 00Z Tuesday)...After a brief lull in activity today following the second wave in a series of systems impacting the region, a third and final wave will push through the region tonight through Sunday. Given that this system is part of a decaying atmospheric river, there is strong potential for heavy mountain snowfall with low snow-to-liquid ratios, as well as strong winds. As of about 3PM, precipitation associated with this system is already spreading across northern Utah, and is only expected to pick up in intensity as we head through the overnight period. There have, however, been some changes to the forecast with expected snow amounts for valley areas, namely along the Wasatch Front. Previously, higher snowfall amounts were anticipated on the Wasatch Front as a brief period of snow was expected before transitioning to rain this evening. Afternoon surface observations have reveal solid warming across the southern two-thirds of the Wasatch Front, in particular the wet-bulb temperatures pushing into the mid-30s. A rain-snow mix will still be possible before warmer air forces the area between roughly Brigham City to Santaquin over to rain. For the northern Wasatch Front, wet-bulb temperatures are still right around freezing, which, when combined with a baroclinic zone skirting the northern portions of Utah, could lead to a more prolonged period of expected snowfall before the transition to rain... especially in the Cache Valley. With the above statements in mind, have opted to cancel the Winter Weather Advisories for the Wasatch Front from Utah County through Weber County, and have continued those for the Cache and eastern Box Elder County.

For the mountainous terrain, the decaying atmospheric river will help to provide an environment with deep moisture and strong winds out of the west to northwest. This combination with the addition of low level instability will lead to a period of heavy precipitation potential across the high terrain with significant exposure to the W to NW (i.e. from the Cottonwoods northward through the Idaho border). Details of this forecast are outlined in the mountain weather forecast ( www.weather.gov/slc/snow), but still expecting significant mountain weather impacts from now through at least Sunday evening.
Have allowed winter weather headlines in the mountains to continue.

The period favoring the heaviest precipitation will occur through the overnight period, with conditions gradually tapering off through the morning hours tomorrow. That said, enough flow and low level moisture will remain in place through the day to support orographically driven snowfall across northern Utah. For valley areas, precipitation in the form of rain should also continue through much of the day as trailing shortwave disturbances translate through the backside of the main trough axis.

A transition to mainly remnant mountain snowfall will be seen during the late evening on Sunday and will continue through much of Monday for the northern Utah mountains. That said, chances for seeing significant impacts related to snow will be on a steep downtrend as temperatures continue to warm overhead due to a ridge beginning to build over the area, bringing increasingly stable conditions as we head through the day.

.LONG TERM (After 00Z Tuesday)...The extended forecast period brings back mean warming and ridging by Monday evening and for the first half of the work week. This will promote valley inversions to return for susceptible areas. Models are growing in consensus on the next winter storm arriving by Thursday and bringing another statewide event that goes into the weekend.

A ridge builds into the Great Basin region by late Monday evening behind the exiting trough feature. Gradual clearing of the skies brings sun and warming temperatures throughout Utah. With the likelihood of valley inversions building into areas its likely that those lower valleys keep from their full potential in warming while the higher terrain see the greatest jump (~4-6F). Relief comes by late Wednesday into Thursday as a potential ejecting shortwave looks to brush along the UT/ID border that could bring some lingering snowfall for far northern UT and into SW Wyoming. Whether much precip happens or not it likely could be enough to mix out and cool off the inversions for SLC and northward before it gets TO bad.

Deterministic models are honing in on a Thursday arrival for the next winter storm as a longwave trough pattern looks to develop.
With its initial arrival brings clouds and the beginning of precip the best snowfall from the storm looks to kick in by Friday and into the weekend when the feature becomes more amplitudes and brings the best synoptic lift and cold aloft. NBM grid values at this point (still around 6-7 days out) bring a foot of snow up high in the Cottonwoods with surrounding high terrain 8-12" and valleys seeing a couple inches at best. These will likely change as the uncertainty in available moisture and temperatures become more clear.

AVIATION
KSLC...VFR conditions as of this writing will likely deteriorate by later this afternoon to near MVFR as the next round of precipitation moves into the area. Snow may mix in initially this afternoon and evening, but will transition to rain by mid evening with no accumulations expected. There is a 50% chance that ceilings reach MVFR conditions in heavier bursts of precipitation, becoming predominantly MVFR overnight and then lingering as such through much of tomorrow. Strong westerly winds around 30 knots several thousand feet AGL with southerly surface winds may lead to notable wind shear should surface winds slacken overnight, with a 20-40% chance of reaching low-level wind shear criteria in the TAF.

REST OF UTAH AND SOUTHWEST WYOMING
The next round of precipitation will bring widespread MVFR and mountain obscurations to the central and northern Utah from around KDTA northward late today through tomorrow. Snow levels will rise as high as 6,000' by tomorrow afternoon. Generally southerly winds can be expected near the surface for most locations, with strong westerly to northwesterly winds a few thousand feet AGL for valley floors.



SLC WATCHES/WARNINGS/ADVISORIES
UT...Winter Weather Advisory until 11 AM MST Sunday for UTZ103-107- 109.

Winter Storm Warning until 5 PM MST Sunday for UTZ108.

Winter Storm Warning until 5 AM MST Monday for UTZ110>112.

Winter Storm Warning until 5 AM MST Sunday for UTZ113.

Winter Weather Advisory until 5 AM MST Sunday for UTZ117.

WY...None.

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