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FXUS65 KSLC 252201 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 401 PM MDT Thu Apr 25 2024

SYNOPSIS
An active pattern will continue into the weekend, resulting in valley rain and mountain snow along with a cooling trend.

.SHORT TERM (Through 12Z/6AM Sunday)...The short term period of the forecast will be characterized by cool, unsettled conditions as an area of low pressure moves through the region. As of early Thursday afternoon, increasing large-scale ascent was acting on numerous low-level boundaries/cold pools as well as terrain to force scattered showers and thunderstorms across northern Utah and SW Wyoming and isolated showers and thunderstorms across southern Utah. This activity will continue to be capable of producing gusty outflow winds, frequent lighting and brief, locally heavy rainfall along with small hail. Deterministic and high-resolution models are in good agreement with showing a diminishing precipitation trend through the evening as instability wanes.
However, large-scale ascent and westerly upslope flow will keep showers going, particularly over/near terrain overnight.

On Friday, a deepening trough across the Great Basin will eject several shortwaves across Utah and SW Wyoming, resulting in a gradual development of widespread precipitation. Some of this precipitation will be briefly, locally heavy. Guidance supports the highest probability for heaviest precipitation from sunrise Friday morning through afternoon across northern Utah, with central Utah favored during the afternoon into early evening and finally southwest Utah from late morning into the early afternoon. 24 hour probabilities for 1" or greater QPF is maximized across northern Utah and SW Utah near the Nevada border, where these probabilities are in the 40-50 percent range. This will lead to further rises on area creeks/rivers/streams as long as standing water on roadways. Ingredients for central and southern Utah flash flooding are rather marginal, with 300-400J/kg of SBCAPE, marginal shear and PWATs in the 125-150% of normal range, though LCLs will be dropping as low-level moisture quality improves. However, if a cell or group of cells trains across a vulnerable area, we can't rule out an isolated instance of flash flooding. Flash flood probabilities, albeit non-zero, are very low, or 10% or less on Friday.

Late Friday through Saturday, models are in good agreement showing a closed low tracking from near the UT/AZ border Friday night and into Colorado through Saturday. This will place all but eastern Utah within a broad deformation zone, with widespread precipitation continuing within this region through mid afternoon Saturday. Snow levels will initially begin in the 8000-8500 foot range this evening before falling to 7000 to 7500 feet late Friday night through Saturday, when temperatures areawide will fall to around 10F below seasonal normals for late April. This will lead to persistent, widespread snow at high elevations, and as a result have issued Winter Weather Advisories for the Wastach Mountains south of I-80 and the Western Uintas, where 25th to 75th snowfall forecasts are generally in the 8 to 16 inch range for this event.
We can't rule out isolated 12" amounts across the Tushars and Brian Head as well, but have opted not to include these farther south mountain zones in a headline at this time given the limited geographical extent and limited impacts. Should amounts increase both spatially and in magnitude, may need to consider adding these areas to the existing Advisories.

.LONG TERM (After 12Z/6AM Sunday)...The second half of the weekend will be much drier than the first, as the Friday and Saturday storm system will have become a closed low and have tracked to the east.
Clearing will continue from west to east, although a trailing shortwave with remnant moisture will allow for convective updrafts, mainly around higher terrain in the Bear River Range, Wasatch, and Uinta Mountains. These will be scattered, with isolated to scattered thunderstorms as well. Minimal rain, or snow above roughly 8000 feet, is forecasted with showers tapering off into the evening.
Temperatures will be around 5-10F warmer than Saturday.

A low amplitude trough will track to the north or graze southwest Wyoming and northern Utah Monday with zonal flow further south.
There is good agreement from GEFS and EPS members on scattered showers and isolated thunderstorms, primarily for the mountains of northern Utah. Rain and snow accumulation will be light, with showers diminishing later in the day. Temperatures will be roughly 5- 10F warmer than normal.

There will be less shower coverage Tuesday, as flow becomes zonal for southwest Wyoming and northern Utah with a longwave trough approaching the PacNW. Dry conditions are likely Wednesday for southwest Wyoming and northern Utah, as that trough will bring drier southwest flow. Temperatures will warm to the 70s and 80s for most valleys, around 15F warmer than normal.

Ensembles are in good agreement that the longwave trough will not dig into Utah through at least Thursday, so a warm and likely more windy day is likely. There is more uncertainty later in the week on the trough and on forcing, cooling, and precipitation that could push into Utah.

AVIATION
KSLC...Showers and thunderstorms are expected this evening with potential for briefly gusty winds and reduced visibility. VFR conditions are forecast in the near term with ceilings possibly decreasing to MVFR this evening. A break in conditions are possible for a few hours tomorrow morning. Ceilings are then expected to drop again by the afternoon hours under additional rain showers.

REST OF UTAH AND SOUTHWEST WYOMING
Scattered showers and thunderstorms are forecast throughout the evening, bringing the potential for briefly gusty winds, reduced visibility and lowered ceilings. Otherwise, VFR conditions will prevail. Continued rain, or even snow showers for BCE, are forecast overnight and throughout the day tomorrow. Widespread MVFR to IFR conditions are forecast as a result. The exception will be SGU where VFR conditions are expected throughout the period.



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

WY...None.

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