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FXUS65 KSLC 201027 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 427 AM MDT Mon May 20 2024

SYNOPSIS
A storm system will bring unsettled conditions to much of Utah and southwest Wyoming through this evening, and leave cooler air in place across the region Tuesday. The pattern will remain active through the remainder of the week, with the next cold front pushing through the region late Wednesday into Thursday.



.SHORT TERM (Through 12Z/6AM Wednesday)...An upper trough axis extends from the northern Rockies of ID/MT southward into the Great Basin of NV early this morning, with an upper jet located along the downstream side of this trough from northern Utah into north central Wyoming. A surface front bisects the forecast area, extending from roughly Green River to just north of Cedar City.
Showers persist along and just north of this surface boundary across west central Utah. Most hi-res guidance largely dissipates this area of showers early this morning, before redeveloping at least scattered convection in the vicinity of this boundary this afternoon as it pushes southeast. Any storms developing immediately along the surface front as it pushes south and east this afternoon will pose a threat for strong microburst winds as indicated in CAM guidance. Ahead of the boundary warm, dry and windy conditions can be expected across areas from the San Rafael Swell southward into the Grand Staircase. Away from any convection this afternoon, winds will gust into the 30-40 mph range with localized areas perhaps approaching advisory criteria (45 mph).

Meanwhile further north, the entrance region of the upper jet is interacting with the trailing low level baroclinic zone, resulting in an area of banded precipitation from the northern Wasatch Front/Salt Lake County westward to near Ibapah. These mesoscale bands will persist through the morning, bringing swaths of precipitation up to 0.25 inches, while areas outside of the bands will see little if any precipitation. As the upper trough beings to swing east this afternoon, model guidance focuses the greatest precipitation coverage across the Wasatch Plateau, western Uintas and western Uinta Basin with in a region of jet coupling along the low level baroclinic zone. Snow levels falling below 8000 feet late today into this evening will result in some accumulation across the higher elevations before precip winds down.

The upper trough will move east Tuesday leaving a seasonably cool airmass across the forecast area. Max temperatures Tuesday afternoon will generally run 6-10F below climo across the forecast area. Afternoon highs will only reach low to mid 60s across most valleys of northern, central and southwest Utah, while temperatures reach the low 80s around St George. Moisture along the back side of the departing trough will maintain a chance for showers across northern Utah, mainly along and east of the Wasatch Crest.

.LONG TERM (After 12Z/6AM Wednesday)...On Wednesday, a broad longwave trough will extend from the PacNW on into the Midwest, with the local forecast region sitting beneath largely zonal flow to maybe very weak shortwave ridging. Through the day, a more marked and compact shortwave impulse along with a cold frontal boundary will dig southward from the PacNW. Most guidance suggests this feature will only begin to impinge upon the northern fringes of the forecast region by the afternoon/evening, yielding a modest uptick in isolated to scattered shower activity. The majority of the region will remain dry, with afternoon high temperatures fairly close to climatological normal for mid to late May.

Wednesday night on into Thursday, the shortwave will graze the northern half of the forecast region and push the cold frontal boundary southward through Utah and southwest Wyoming. Overall, combination of forcing/moisture doesn't look all too impressive, and while a slightly more organized band of precipitation will be possible along the front, generally just expect isolated to scattered shower chances to continue to creep southward into central Utah or so, with the boundary likely stalling somewhere near the I- 70 corridor during the day. Probably the more noticeable impacts will come in the form of gusty prefrontal winds across southern and eastern Utah (afternoon gusts ~25-40 mph), and much cooler temperatures behind the front. To the latter point, afternoon highs behind the front on Thursday are expected to run about 7 to 15 degrees below normal. Around 80% of ensemble members support some sort of trailing impulse moving through on Friday, but with the boundary washing out and limited overall support, mostly anticipate some isolated afternoon diurnal showers over some of the high terrain. Additionally, places which saw frontal passage the prior day will see temperatures rebound several degrees for afternoon highs as the airmass moderates.

Northern stream remains active moving into the weekend, but some uncertainty is noted on the exact flow evolution. Similar to the Thursday system's progression, another somewhat compact shortwave trough and associated frontal boundary will dig towards the northern Rockies from the PacNW. The amplitude of the trough will largely dictate how far south impacts spread, which is made further complicated by potential for some weak phasing with the southern stream (effectively deepening the trough a bit more). As of this forecast package, around 35% of ensemble members show a deeper trough or weak phase scenario, with remaining membership depicting a flatter pattern and more of a grazing system. In both scenarios, nothing at the moment shows much in the way of substantial precipitation chances or cold air intrusion, so forecast includes near to slightly below normal temps Saturday and Sunday along with some isolated to scattered precipitation generally north of the I-70 corridor.

Deterministic and ensemble guidance show good agreement that Monday into midweek should see a ridge start to build into the area, resulting in drier and warmer weather. That said, Memorial Day does still show a bit of uncertainty with about 20% of ensemble members keeping some modest lingering influences from the departing trough, which would allow for maybe some isolated diurnal showers and a slower warmup. Will want to keep an eye on the forecast for any outdoor plans, but trend is at least favorable so far.

AVIATION
KSLC...Trough and associated cold front move through during TAF cycle. Shower activity will tend to be scattered, though a more consistent banded feature appears likely somewhere along/near the front. If this impacts the terminal, most likely window currently seems to be between ~12-18Z. Generally expect BKN to OVC VFR cloud cover, but around a 10-20% chance of MVFR conditions possible in any heavier precipitation. Precipitation chances begin to decrease and become more isolated Monday evening into the overnight hours. Winds favored to remain NW, with occasional gusts, primarily Monday afternoon.

REST OF UTAH AND SOUTHWEST WYOMING
A system will progress into the region and push a cold front southward as it does so. For northern terminals, will see scattered showers continue to develop as a result, likely along with a band of more consistent showers along/near the cold front. Largely anticipate conditions to remain VFR with BKN to OVC CIGS, but around a 10-20% chance of MVFR conditions will be possible within any heavier precipitation. At southern terminals, scattered shower/thunderstorm chances increase by the afternoon, with subsequent frontal passage expected likely by late Monday evening. For areas behind the front, a more northerly component to the wind direction is favored.

FIRE WEATHER
A cold front stretching from roughly Vernal to Cedar City will remain nearly stationary this morning, before pushing through southern and eastern Utah this afternoon and evening. Ahead of this front a warm, very dry (RH less than 15 percent), and windy airmass will reside across areas mainly east of Highway 89. As this front pushes through these areas later this afternoon and evening, scattered showers and thunderstorms accompanied by strong outflow winds can be expected. Any rainfall with these storms will remain less than one tenth of an inch.

To the north of the front, a cooler airmass will reside across northern and west central Utah. As the parent upper level storm system associated with this front crosses the area, showers and occasional thunderstorms are expected, with the greatest coverage across the Wasatch and western Uintas.
With the cooler airmass in place, snow levels will fall this afternoon and evening allowing for accumulating snow above 8000 feet. This snowfall will be greatest across the Uintas.

This cooler airmass will reside across the state Tuesday, while unstable conditions associated with the passing low pressure system maintain a chance of showers across the north.
A brief warming trend can be expected Wednesday, before the next cold front pushes into northern Utah late Wednesday afternoon and evening, then through the remainder of the state Wednesday night into Thursday. Any precipitation with this midweek system will remain confined to northern Utah.

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

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