Area Forecast Discussion
Issued by NWS Salt Lake City, UT
Issued by NWS Salt Lake City, UT
969 FXUS65 KSLC 222300 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 400 PM MST Wed Jan 22 2025 .SYNOPSIS...Cool and dry northwesterly flow will persist through much of Friday. A low pressure system will spread across the Western Great Basin this weekend bringing increasing chances for unsettled conditions to Utah and Southwest Wyoming. && .SHORT TERM (Through 12Z/5AM Friday)...Late Wednesday afternoon analysis products indicated the presence of a dry, sheared shortwave propagating southward across southwest Wyoming, eastern Utah and western Colorado. Model consensus is well-clustered around the idea of dropping the wave into the Four Corners by Thursday morning. Sensible weather impacts to most of Utah and SW Wyoming associated with this wave will be limited to an increase in ridgetop winds (which have already been on a diminishing trend this afternoon) and light, upslope-induced snow showers across the northern slopes of the Uintas this evening. Heading into Thursday, there is high confidence in mid-level ridging and associated mid-level height rises across the region, resulting in dry, stable conditions. On Friday, an upstream, amplifying trough will dig into the Northern Rockies. As a result, an initial wave of mid-level height falls will be underway across northern Utah and SW Wyoming by Friday morning, with an increasing surface pressure gradient leading to a gradual increase in westerly ridgetop winds. Warm advection in advance of this trough will bring temperatures very briefly into the 5F to 10F range above normal range. .LONG TERM (After 12Z/5AM Friday)...Key takeaways for the long-term forecast are as follows: * Snowfall will begin Friday afternoon across southwest Wyoming and northern Utah. Chances for snowfall across Utah will continue to increase throughout Saturday with chances across central and southern Utah increasing Saturday morning. * Some uncertainty remains regarding how long the precipitation will last across southern Utah. Lower Washington county may see minor snow accumulations with around 10-20% chances of >1" during the weekend, with a small chance that these probabilities could extend as late as Tuesday. * Current guidance indicates lower probabilities for a downsloping/canyon wind event across the Wasatch Front/Cache Valley. Some uncertainty remains, though guidance has shifted toward a more southwest solution for the aforementioned cutoff upper low. A longwave upper trough will dive south over the Pacific Northwest Friday. On Saturday morning, ridging will slide east toward the west coast of Canada which will pinch a lobe of vorticity from the mean flow. This will develop into a cutoff upper low which is expected to meander around southern California / the Desert Southwest before joining the mean upper flow once again around midweek. The aforementioned upper cutoff low will serve to push ample moisture across Utah and southwest Wyoming coupled with just enough dynamics for some snowfall across the forecast area. Precipitation will begin across our northernmost Utah counties and southwest Wyoming before gradually increasing to the south throughout Saturday morning as more favorable moisture moves in to southern Utah. Snowfall totals across the Wasatch front may fluctuate in current forecast as the location of the low will determine if we get shadowed or not. A more northerly surface wind would be more favorable across the valley compared to easterly which also appears possible. Although, ensemble guidance has shifted the center of the low more to our southwest which would generally favor more snow across the valley compared to yesterdays forecast. A more easterly component would favor snow shadowing over the Wasatch front and perhaps bringing downsloping winds to the area. Probabilities for this appear lower at this time, though it still remains possible if the low were to shift its track once again. There still remains uncertainty regarding QPF across southern Utah as how this event unfolds relies heavily on how long the upper low meanders across the Desert Southwest. Ensemble means carry the upper low through the Desert Southwest quite slow resulting in PoPs greater than 20% generally lasting through early Wednesday this forecast cycle. This is due to upper moisture wrapping around the backside of the low into our southern and central Utah zones. Easterly flow becoming southerly throughout the day Saturday with a cold and moist airmass aloft may result in a trace snow for St. George with rain mixed in initially. Additionally, there is a 10-20% chance that St. George could also see an inch of snow from this system. && .AVIATION...KSLC...Mostly clear conditions will last through the TAF period. Light northwest winds will transition to southeast around 04Z. Light southeast winds will prevail through around 19Z when winds transition to northwest. .REST OF UTAH AND SOUTHWEST WYOMING...Mostly clear conditions will continue throughout. Gusty northwest to northeast winds in southwest Wyoming and southern Utah will diminish after 00Z. Light winds will last through the morning. Speeds will be light for most of Utah through the day, with southwest wind gusts around 20 knots for southwest Wyoming and northeast wind gusts around 20 knots for portions of southern Utah. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...None. WY...None. && $$ ADeSmet/Worster/Wilson For more information from NOAA`s National Weather Service visit... http://weather.gov/saltlakecity