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FXUS65 KSLC 262159 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 359 PM MDT Fri Apr 26 2024

SYNOPSIS
An active pattern will continue through the weekend, resulting in valley rain and mountain snow along with continued cool conditions. Modest warming is then anticipated into midweek with a weak, grazing system brining a few showers to northern Utah and SW Wyoming on Monday afternoon.

.SHORT TERM (Through 12Z/6AM Sunday)...Early Friday afternoon analysis indicated the presence of a strengthening trough across the Southern Great Basin, with an organized shortwave across southern Nevada and jet axis rounding the base of the trough across Arizona and Southern California. These features are presenting themselves quite nicely on mid-level WV this afternoon.
This is placing western portions of Utah and eastern Nevada within a favorable region of upper diffluence/large-scale ascent.
Farther north across northern Utah, a lingering low to mid level circulation (what appears to be the upper low coming together)
continues to serve as a focus for shower and thunderstorm development and maintenance. Instability is quite modest across northern Utah this afternoon, with fairly equally modest PWATs in the 100-150% of normal range. However, SLC 12Z sounding revealed a deep, saturated layer up to the tropopause and a modest, tall, skinny CAPE profile. As a result, we're able to take advantage of the available moisture and squeeze out widespread showers with locally heavy cores and occasional lightning.
Finally, over southeast Utah, breaks in the clouds have allowed for surface-based instability/buoyancy development. At the same time, large-scale ascent is increasing here as well. As a result, we're seeing a gradual increase in shower/thunderstorm development across this area. The column is dryer here and mid- level flow is a bit stronger in advance of the low, so convection is faster moving with more limited residence time. Main hazard through the night will be local, brief heavy rainfall leading to standing water on roadways and urban/poor drainage areas, especially where slow-moving and training cells exist. Can't rule out a very low (10% or less) chance of a flash flood instance across vulnerable locations of central and southern Utah (slot canyons, slickrock, normally dry washes) if heavy rain cores train across a vulnerable area.

Tonight, an upper low is forecast to track eastward across southern Utah, with the low tracking into Colorado by Saturday afternoon. As a result, a well-defined TROWAL/deformation zone is forecast to set up from southern Wyoming south and west across western Utah (primarily along and west of the terrain), and this feature is forecast to persist through the morning hours.
Persistent, widespread light to moderate precipitation will accompany this feature. By afternoon, model consensus suggests this activity will shift eastward and be located primarily over the terrain and adjacent valleys of central and northern Utah while shrinking in size with an attendant diminishing precipitation rate trend. Snow levels will primarily be in the 7500-8000 foot range, however expect snow levels to fall down to around 7000 feet in heavier showers. Expecting additional accumulations across the northern mountains in the 4-8 inch range, 6-12 inches across the central and southern mountains for the duration of the event. In moist, northwest upslope flow Saturday night, expect snow showers to persist across favorable terrain of the central and northern mountains through the night. Saturday morning appears to be a great time to take care of some indoor chores.

.LONG TERM (After 12Z/6AM Sunday)...Shortwave energy will rotate into southwest Wyoming and northern Utah Sunday as the main storm system pushes into the northern plains. Conditions will be drier throughout, but there will be isolated to scattered rain and mountain snow showers, specifically for the Bear River Range, Wasatch, and Uinta Mountains. Mountain snowfall will generally be less than one inch. Showers will taper off Sunday evening, with synoptic flow becoming zonal as a trough digs into the PacNW.

There is good model agreement that the trough will dig far enough south Monday to bring isolated to scattered showers to similar locations as Sunday. A weak baroclinc zone will track into northern Utah later Monday, but as the main trough lifts, that front will not bring much cooler conditions or track all the way through Utah.

Tuesday will be largely dry between the aforementioned trough and another that some ensemble guidance, roughly 60% of members, have a similar track for. There is more uncertainty with that, as other ensemble members have zonal or southwest flow with another trough to the west. The forecast Wednesday is uncertain with those different scenarios on precipitation and temperatures. National Blend of Models 25th to 75th percentile for high temperatures for northern Utah ranges near 20F with uncertainty on the trough.

Temperatures Sunday will be slightly cooler than normal, increasing to slightly warmer than normal Monday and Tuesday with more uncertainty the next day.



AVIATION
KSLC...Lowering cloud bases and increasing shower activity will be the name of the game today as a storm system arrives in Utah. Expecting vicinity showers through a majority of the day, with the most likely period to see showers impacting the terminal from 20-01Z. During this time, there is a 30-40% chance of thunderstorms moving over KSLC. CIGs will remain around 4-5kft AGL through the day, lowering to around 3kft AGL with passing showers.
Expect variable winds through the morning with light northerlies prevailing after 15Z.

Rest of Utah and Southwest Wyoming...An active weather day with widespread showers and isolated to scattered thunderstorms is expected across Utah and southwest Wyoming during the day today. VFR conditions will prevail outside of showers with CIGs generally in the 4-5kft AGL range. Within showers, expect MVFR CIGs . At higher elevation terminals (e.g. KEVW/ KBCE) expect periods of rain-snow mix to bring MVFR to IFR due to VIS restrictions.

SLC WATCHES/WARNINGS/ADVISORIES
UT...Winter Weather Advisory until 6 AM MDT Sunday for UTZ111-112.

Winter Weather Advisory until 6 PM MDT Saturday for UTZ113-117- 125.

WY...None.

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