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FXUS65 KSLC 302303 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 403 PM MST Thu Nov 30 2023

SYNOPSIS
Unsettled conditions will develop through the weekend as a series of storm systems impact the area, bringing potentially significant snow, especially for northern Utah. High pressure is expected to return to start the upcoming work week.

.SHORT TERM (Through 00z Monday)...Early afternoon satellite and H5 analysis indicates a broad long wave trough encompassing much of the CONUS, with its western periphery extending upstream across the eastern pacific. A strong upper level jet can be noted off the PacNW coast
trends encroaching southeastward
with the first of a series of short wave troughs embedded impinging on the Oregon coast attm. A sharper short wave is just upstream of this feature, with orientation near 145W. These waves are expected to translate across the northern Great Basin and into namely northern/central portions of the forecast area Friday, and again Friday night, bringing light to at times moderate snow to valleys/mountains alike...with snow most widespread during each of the mid level axis passages (Friday afternoon and again later Friday night). Anticipating our first somewhat widespread potential for low elevation road snow/slush impact along the Wasatch Front given cold antecedent conditions at precip onset, namely late Friday into Saturday morning as the aforementioned trailing wave crosses the region. Have issued winter weather advisories for the northern valleys due to this potential, even though accumulations should remain on the light side, at least initially.

The mode of this upcoming unsettled period begins to shift later Saturday, and especially leading into Sunday, as a transition from consistent (but modest) cold advection from these first two waves phases neutral, then into fairly stout warm advection for the remainder of the event (Sunday on into Monday morning).
Coincident with this is decent IVT penetrating inland and across northern Utah within an anticyclonic northwesterly flow aloft, this remnant from the atmospheric river impacting areas upstream.
Favorable jet support aloft coupled with overrunning of cold pools in place, and upglide coincident with this moist WAA, will maintain heavy at times precip Sunday into the overnight hours across northern Utah. Heavy mountain snow with increasing density trends can be expected across the central/northern Wasatch on into the Bear River mountains yielding our first decent thump this early winter (20-30" storm totals, 1.5-2.5" SWE)...have issued a winter storm warning has been issued for these mtn ranges as such.
Have also issued a warning for the Wasatch Back as favorable flow/overrunning is expected to produce high enough rates to maintain isothermal snowfall through Sunday, even when temps aloft warm enough to not usually support it. Storm totals of 12-18" is not out of the question for the Liberty/Eden areas, lesser amounts further south into the Coalville/Park City regions.

The aforementioned warm advection will eventually pose a p-type challenge with lower valley snow transitioning to more of mixed precip and eventually rain along the Wasatch Front first, then the Cache. The challenge with this is primarily tied to timing of this eventual occurrence, and additional snow accumulations will be directly tied to the switchover. In large, expecting a south to north transition with the Salt Lake/Utah Valley areas potentially mixing by Saturday night, the northern Wasatch Front sometime Sunday morning, and finally the UT/ID border region and Cache sometime Sunday afternoon. Still, uncertainty exists with this timing, as PI rates will heavily influence maintenance of isothermal snowfall potential, or lack thereof. Expiration of the winter weather advisories are largely tied to this anticipated timing, but could easily see some areas transitioning away from snowfall sooner...especially south of Ogden. Future shifts will keep tabs on this.

Winter weather advisories were also issued for the western Uintas and central mountain for modest snow accumulations during this period, with expirations occurring sooner across the central mountains due a the moist WAA period becoming more focused across the north in time.

Lots of details/subtleties were needed to be dissolved in this short term forecast...and I'm looking forward to seeing how it all plays out in time. Regardless, it will be a net positive for building on our early season mountain snowpack for many reasons! This said, if planning on getting out in the backcountry, make sure to keep tabs with the latest observations and forecasts from the Utah Avalanche Center as this winter season is ramping up.

.LONG TERM (After 00Z Monday)...Beginning of the long term picks up at the tail end of this weekends storm. Some valley rain showers and mountain snow showers could continue into the morning hours on Monday. Not much is expected in the way of snow accumulation, maybe a few inches during this time period.

After this storm moves out of the area by Monday morning, there continues to be good agreement in both deterministic and ensemble guidance that a ridge will be back in place through the middle of the week. With this strong subsidence in place, shown by the ensemble mean 700mb temperature of +3C by Wednesday, inversions will return, especially in valleys that have snow cover. Compared to the current inversion conditions, the ridge seems to be stronger as forecasted temperatures are between 5F and 10F above normal. These temperatures though would be affected by snow cover and inversions if one or both of these are in place. Towards the end of the week, guidance is honing in on another trough moving through the Great Basin area that could get rid of any inversions in place.

AVIATION
KSLC...MVFR conditions owing to a low stratus deck and hazy conditions will gradually transition to periodic VFR conditions this evening/overnight as CIGs and VIS rise ahead of tomorrow's storm. VIS will remain between 5-6SM as inversions don't completely clear overnight, and there is ~30% chance VIS goes <3SM briefly overnight. Winds will remain light and variable overnight with a rogue light snow shower possible (~10% chance).

REST OF UTAH AND SOUTHWEST WYOMING
Across northern Utah, a low stratus deck and hazy conditions will gradually improve overnight as our next storm approaches. This will transition MVFR conditions to periodic VFR conditions as CIGs and VIS rise, but mountain obscuration will persist. Snow could begin across northwestern Utah as early as 12Z. Winds will remain light and variable. Across southern Utah, mainly VFR conditions with brief periods of reduced categories owing to isolated mountain snow showers. CIGs <7kft will keep mountain tops obscured.

SLC WATCHES/WARNINGS/ADVISORIES
UT...Winter Weather Advisory from 11 AM Friday to 11 AM MST Sunday for UTZ103-104-107-109.

Winter Weather Advisory from 11 AM Friday to 5 AM MST Sunday for UTZ105-106-113-117.

Winter Storm Warning from 11 AM Friday to 5 PM MST Sunday for UTZ108.

Winter Storm Warning from 11 AM Friday to 5 AM MST Monday for UTZ110-111.

Winter Weather Advisory from 11 AM Friday to 5 AM MST Monday for UTZ112.

WY...None.

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