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FXUS65 KSLC 031035 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 435 AM MDT Fri May 3 2024

SYNOPSIS
An approaching storm system will bring much warmer conditions Saturday with strong afternoon winds for portions of Utah. A cold front will track into Utah Sunday. Lighter winds, cooler conditions, valley rain, and mountain snow are likely once the front pushes through.

.SHORT TERM (Through 12Z Monday)...A grazing shortwave with a weak cold front has brought showers, snow above roughly 6000 feet to southwest Wyoming and northern Utah. Drier conditions will build in through the morning from west to east as the trough axis will push downstream. Shortwave ridging will build in Friday night into Saturday, with a clear, chilly night into Saturday.

Strong warm air advection will push valley temperatures into the 70s and 80s as a longwave trough with a closed low track into the PacNW. As it digs southeast, flow aloft will increase through the day. With very dry conditions and strong lapse rates, that flow will be efficient to mix. The strongest winds will not be until later Saturday afternoon, through the overnight, and into Sunday.
The main uncertainty is with valley winds during the overnight Saturday into Sunday. That is when there is good agreement on the strongest winds aloft, with more than 50 knots at 700 mb. While conditions will be dry, increasing cloud cover and timing will work against efficient mixing. That could allow for surface winds to stay steady or decrease slightly. The most confidence for winds to hit at least advisory criteria is for southwest Utah, specifically for portions of Iron, Beaver, and Millard counties, including Cedar City, Delta, and Milford. Models have almost 100% probabilities gusts of more than 45 mph Saturday afternoon into Sunday, with more than 50% confidence for winds exceeding High Wind Warning speeds. There is lower confidence in places like lower Washington County and Zion National Park that often don't get as windy as guidance with southwest flow. High-resolution guidance will give more confidence.

Winds will drop off for those locations as a baroclinc zone pushes into Utah Sunday. There are differences in timing of several hours, but high confidence in the front pushing into western Utah during the morning and into the urban corridor late morning or early afternoon. Strong cold air advection, forcing, and a transition to northwest winds will come with the boundary.
There is a low, roughly 15% chance for thunderstorms near the boundary. Snow levels will drop from roughly 9000 feet to 5000-5500 feet after the front pushes through. The northern Utah mountains will get the most precipitation, with most of that behind the front with upslope orographic enhancement. Snow water equivalent of 1-2 inches is likely, locally higher in the upper Cottonwoods and Ogden area mountains. There will be a range of snowfall for the mountains, with generally 6-12 inches, but locally around 24 inches for those aforementioned locations. Many northern valleys will get around 1 inch rainfall from Sunday and Monday.

For far southern and eastern portions of Utah, Sunday will be a windy day in the warm sector, with similar wind speeds and gusts as western Utah. The front will push through there Sunday into Monday. Precipitation will generally be lower further southeast.

.LONG TERM (After 12Z Monday)...A common theme appears to be in store for the bulk of the long term portion of the forecast, and that is a continuation of cool, unsettled conditions. Kicking things off on Monday, model solution space is in universal agreement in their depictions of negative height anomalies in place across the PacNW and Northern Rockies, courtesy of the presence of a broad longwave trough. Closer to home, northern Utah will reside within broad cyclonic flow in the wake of a departing low and cold front, and this will maintain showery conditions across northern valleys with snow showers in the mountains. Monday morning, snow levels drop down to as low as 5000-5500 feet, low enough to support snow on benches. If we peer into the realm of the low outcome probability spectrum, we do indeed see 90th percentile/10% probability snowfall amounts of a few tenths of an inch to as high as 1-2 inches on benches. In the northern and central mountains, there is a high probability of snow showers persisting through the day. As you might imagine, temperatures will be well below early May normals Monday, checking in some 10F to 20F below seasonal normals. Brrr.

By Tuesday, slight temperature moderation is seen across extreme southern Utah while central and northern Utah see steady state temperatures and a smattering of valley rain and mountain snow showers. We once again cannot rule out snow showers down to benches Monday night Tuesday morning. Surely, lake effect snow cannot happen in May, right? Lake effect snow probabilities suggest that yes, it indeed could. Local lake effect snow probability output reaches as high as 40% Sunday night, 30% Monday morning and 20-30% Tuesday morning. An early look at lake effect snow parameters do suggest the ingredients are in place (sufficient lake sfc to 700mb delta Ts, favorable fetch, steep low to mid level lapse rates and sufficient moisture in the DGZ). How much this could contribute to totals is uncertain at this time and will require further analysis and refinement particularly as we get into the high-resolution horizon, generally within 48 hrs.

As we nudge time into the midweek portion of the forecast, we see a continuation of cool, northerly flow trajectories across the region with an easing of the cyclonic flow regime thanks to the gradual weakening/departing trough. Deterministic and ensemble solutions are in good agreement through Thursday in their depictions of this evolution. For you and me living here at the surface, this translates to a continuation of cool, albeit moderating conditions through midweek, although we can't rule out diurnally-driven showers through Thursday, particularly over terrain features of northern Utah. We do begin to see the emergence of some divergence in the solution space by Friday, with approximately 1/3 of the solutions hanging onto negative height anomalies while the remainder of the ensembles are more progressive with a ridge nudging into the area.
If the troughy scenario plays out, expect temperatures to remain 0F- 10F below normals by Friday, while the other camp of solutions suggests temperatures warming back to normal and perhaps slightly above by Friday.

AVIATION
KSLC...Scattered to broken ceilings will last through the morning. Clouds will diminish through the afternoon. Light northwest winds will last through the day.

REST OF UTAH AND SOUTHWEST WYOMING
MVFR or lower ceilings are likely for southwest Wyoming and portions of northern Utah most of the morning. Clouds will diminish through through the day there, with mostly clear conditions elsewhere. Light northwest winds will last through the day for southwest Wyoming and most of Utah.

SLC WATCHES/WARNINGS/ADVISORIES
UT...High Wind Watch from Saturday afternoon through Sunday morning for UTZ115-122.

WY...None.

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