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FXUS65 KSLC 192114 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 314 PM MDT Sun May 19 2024

SYNOPSIS
Unsettled weather will continue through Tuesday, as the next storm system brings increasingly cooler and wetter conditions.
A series of additional weak disturbances is then expected through the remainder of the work week.

.SHORT TERM (Through 12Z/6AM Tuesday)...Water vapor satellite imagery this afternoon depicts a broad trough settling into the Pacific NW with an area of southwest flow aloft and embedded cooler cloud tops stretching inland on infrared imagery from NorCal to eastern Idaho. This southwest-to-northeast feature, associated with a baroclinic zone at the leading edge of the trough, will slide into northern Utah tonight, bringing an area of mesoscale banded precipitation by daybreak and heralding a change to much cooler and unsettled conditions.

As of this writing, a cumulus field with a few embedded thunderstorms stretches from roughly Milford to Green River, riding the edge of an existing boundary which has wobbled back and forth across the state the last 24-48 hours. Instability remains quite marginal in this area, 250 J/Kg MUCAPE per SPC mesoanalysis, with an area of higher shear over northern Utah that happens to also feature a drier air mass aloft. Not expecting anything significant from these storms, other than a dry microburst threat with a 10-20% chance of gusts exceeding 40 mph. Some CAMs are indicating that a cumulus field over northeast Nevada will send an area of convection with microburst potential across the west deserts and towards the Wasatch Front this evening, but such CAMs are currently overestimating instability so thinking this potential may be overdone with somewhere around a 15% chance of occurrence.

Mid-level cold air advection begins tonight as the aforementioned trough approaches, with a mesoscale precipitation band developing late in the overnight hours. CAMs are in great agreement that this southwest to northeast band will develop somewhere over northern Utah and southwest Wyoming, but with less agree as to exactly where. Current scenarios from CAMs place the center of the band somewhere between Ogden and Provo with a mean position over Salt Lake County. Precipitation amounts of 0.1-0.2" can be expected with this band. Elsewhere for northern Utah, cooler conditions with a few showers are in store through tomorrow morning. Meanwhile, southeast of a roughly Cedar City to Vernal line, conditions will remain drier with gusty southwest winds ahead of the front. Current NBM wind guidance is close to wind advisory criteria for south central Utah including the Grand Staircase, but remains just below. As the trough axis moves through the area later Monday into Monday night, expect shower chances to eventually reach nearly the entire forecast area, with 700MB temps dropping to around -6C and most of the area being 10-20 degrees below normal Tuesday afternoon. Monday night snow levels look to drop to around 7000 feet, with a few inches accumulation expected over the Uintas by Tuesday. Those with backcountry plans should be prepared for conditions more similar to March than May.

.LONG TERM (After 00Z Wednesday)...Long term forecast period starts with a broad longwave trough extending throughout much of the W CONUS, with an embedded shortwave impulse departing east of the area. With this feature nearby (and depending on exact timing), could maintain a few isolated showers early Wednesday mostly across N UT and SW WY, but largely anticipate things to be winding down. Following the departure of this shortwave, a brief period of zonal flow to weak shortwave ridging looks to set in.
This will result in quieter/dry conditions for most of Wednesday, as well as temperatures rebounding upwards several degrees with afternoon highs near seasonal normal.

By Wednesday evening, the next shortwave impulse will be nearing the forecast region as it digs through the longwave trough from the PacNW. Initially, anticipate this will result in an increase in isolated to scattered showers across N UT and SW WY. Thereafter through Thursday, an associated cold frontal boundary will drop southward into the forecast region, once again serving as a focus for a bit higher precipitation chances. There remains some differences in model guidance as to the exact amplitude and motion of the parent shortwave, and thus how far south this cold frontal boundary ultimately ends up progressing before stalling. Rough model consensus at this point suggests near to maybe a bit north of the I- 70 corridor or so, which if valid would limit precipitation chances across S UT. That said, a corridor of moderate H7 winds ahead of the front will likely yield gusty surface winds in the 25-40 mph range or so. Colder post-frontal conditions are also expected, with temperatures behind the front falling around 5-10F or so, back to well below climatological normal. Friday will remain a bit unsettled as the trough lingers (with ~55% of ensemble members showing a bit more of a weak trailing impulse), but generally expect a bit less activity overall. Temperatures also remain around 5-15F below seasonal normal for afternoon highs, though a good bit of spread is noted (especially areas north) due to the aforementioned split on a trailing impulse or not.

Moving into the weekend, uncertainty increases due to differences on how guidance handles another shortwave digging a similar path through the longwave trough. General ensemble consensus keeps the feature far enough northwest that locally H7 temperatures increase within deeper southwesterly flow, resulting in afternoon highs near to a bit below normal. This would also keep things a bit less unsettled, with some isolated to scattered showers, mostly diurnally driven and favored over the high terrain from central Utah northward. However, given the proximity to the shortwave and associated features, will need to keep an eye on how guidance trends in the coming days.

AVIATION
KSLC...Northwest winds expected to hold through the majority of the TAF period, with only a period of light and variable winds within the 10-13z window Monday morning. Shower chances increase with isolated potential after 03z this evening, trending scattered and more consistent than not after 16z. Gusty and erratic outflow possible with showers, most likely on Monday. VFR conditions are expected to be maintained with cigs anticipated to remain above 7kft.

REST OF UTAH AND SOUTHWEST WYOMING
A boundary across central Utah will continue to be the focus of high based showers through this evening. Some gusty and erratic outflow winds will be possible near these showers. Areal coverage of showers will increase across northern Utah and southwest Wyoming after 10z coincident with the next storm system. At this time anticipating VFR conditions, though brief reductions are possible in SW Wyoming within a rain/snow mix Monday, as well as mountain obscuration of northern Utah mountains.

FIRE WEATHER
A trough moving into the area tonight will lead to stronger southwest flow tomorrow along with an area of light to moderate precipitation and elevated RHs from roughly Delta to Vernal northward. South of this line, gusty southwest winds can be expected with continued dry conditions (RHs in the single digits to teens), especially from the Grand Staircase to the San Rafael Swell. By Monday night into Tuesday, expecting cooler and wetter conditions to settle into the entire state, with the best chance of wetting rains in the higher elevations north of US-6 and east of I-15 before the storm exits the area late Tuesday. A few inches of snow are expected above 8,000 feet, especially over the high Uintas.

Wednesday will be quieter and warmer, followed by another trough Thursday which will bring cooler conditions again along with breezy winds and a threat of showers roughly north of I-80. A similar weather pattern looks to continue into the weekend, followed by a better chance of widespread drier and warmer conditions for the last week of the month.

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

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