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FXUS65 KSLC 281104 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 404 AM MST Tue Nov 28 2023

SYNOPSIS
High pressure will remain in place across the region, bringing persistent valley inversions across northern Utah. A series of weather disturbances will bring a long period of precipitation to northern Utah beginning Friday and lasting through the weekend and into early next week.

.SHORT TERM (Through 12Z Thursday)...Worsening valley inversions will remain the main story for the short term forecast...though the end is in sight in the beginning of the long term forecast period. Early morning upper air and satellite analysis indicates amplified upper level ridging across the Interior West with a slow moving upper level low off the Pacific Coast. The southern jet remains active along and south of the US/Mexican border.

While much of the state is seeing clear skies early this morning, persistent and stubborn stratus remains over the western Uinta Basin. Looking at model soundings, it looks difficult to fully erode this stratus through at least Wednesday afternoon. Looking at the HRRR 90th percentile (closest matching guidance to the evolution of the stratus over the last 12-24 hours), would also support the earliest window for stratus to erode would be potentially Wednesday afternoon. Pockets of stratus or fog have also developed near Bear Lake.

As far as valley inversions, model soundings support valley inversions continuing through at least Wednesday afternoon, if not into the early long term forecast period. A gradual cooling of the mid-levels is forecast to begin this afternoon, continuing into Wednesday but 00Z guidance has backed off on sufficient cooling to ventilate the valleys, especially north of Provo. Utah DEQ continues to forecast yellow/moderate conditions for most northern basins through Wednesday.

The previously mentioned upper level low will gradually weaken today and then shift across Arizona and New Mexico later Wednesday into Thursday. This system has shifted a bit further south with the 00Z guidance, thus reducing the threat of precipitation across southern Utah. There is a significant portion of the guidance envelope that would suggest isolated showers at best Wednesday evening into Thursday morning, so trended the official forecast further south, with lower PoPs.

.LONG TERM (After 12Z Thursday)...Remnants of the low passing across Arizona will be followed by another wave on Friday and then a potentially more impactful system featuring an inland penetrating atmospheric river for the weekend.

As the low continues eastward into New Mexico, another wave will move through Utah shortly afterwards, bringing minor snow accumulations to higher terrain across the forecast area on Friday...likely only a few inches of snow. H7 temperatures will steadily decline to below -10C, with a few valley snow showers possible Friday morning.

The weekend storm is looking a bit more interesting, thanks to the combination of deep column moisture, mid-level dynamics, and strong H7 flow contributing to local orographic enhancement. We can characterize this storm into roughly two phases. The first phase on Saturday/Sunday features a trough passing through our forecast area with increasing moisture across northern Utah from an inland penetrating atmospheric river. Unlike some atmospheric rivers, this first wave of moisture is not expected to raise snow levels by much, with even the NBM 75th percentile snow level still below valley floors. Strong westerly flow combined with good mid- level forcing will help to produce plentiful precipitation across northern to central Utah, especially in mountainous areas favored by westerly flow (i.e. Ben Lomond Peak).

The second phase could feature another atmospheric river, pushing in additional moisture from the West over a building West Coast ridge. However, the synoptic forcing will be lacking. Building subsidence may limit QPF on Monday, though westerly- to northwesterly-favored areas could still see good orographic enhancement. Snow levels are likely to rise with this push, with ensemble mean H7 temperatures reaching -3C to -5C by Monday afternoon. Any residual valley showers may transition back to rain.

Beyond Monday, the aforementioned ridge is likely to build over the Great Basin, but the overall pattern is looking to remain progressive.

AVIATION
KSLC...VFR conditions will likely prevail, with light, southeasterly winds transitioning to light and northwesterly around 18-19z.

REST OF UTAH AND SOUTHWEST WYOMING
Most sites will see VFR conditions and light, diurnally-driven winds. However, high-end IFR CIGs will continue over the Uinta Basin. Low stratus clouds are also forming over a few isolated areas, and may impact CIGs at KHCR (80% chance) and KLGU (25% chance) in the early morning hours.

SLC WATCHES/WARNINGS/ADVISORIES
UT...None.
WY...None.

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