Sunrise 6:21 AM   Sunset 8:39 PM Moonrise 3:12 PM   Moonset 2:50 AM

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FXUS65 KSLC 181029 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 428 AM MDT Sat May 18 2024

SYNOPSIS
An active, unsettled pattern will continue through at least the first half of next week.

.SHORT TERM (THROUGH 12Z MONDAY)...The shortwave trough the brought the latest cold front into the area is now moving across the northern High Plains while the next upstream trough is tracking down the BritCol coast. The cold front associated with the departing trough has made it to a Milford to Green River line, where it has now stalled and continues to weaken. A few showers continue to develop near the boundary, but remain very weak at this time.

Ahead of the next storm system, southwesterly flow aloft will begin to return to Utah today, with the boundary slowly lifting back to the north. Continued low-level convergence near this boundary will maintain showers through the day and into the evening hours, with increasing areal coverage as daytime heating commences. There has been a slight upward trend in surface dewpoints, indicating a slight moistening of the boundary layer. Despite this, most showers will remain high-based, producing generally light precipitation with isolated to locally scattered microburst winds. The HRRR probabilities show localized areas of 10-20% chance for winds of 40 mph or greater through this afternoon, with isolated areas of up to 40% in the Dugway Proving Grounds this evening. A few thunderstorms will also be possible with the strongest storms, as HRRR ensemble means show small pockets of up to 500J/kg of SBCAPE this afternoon.

As the next upstream trough carves into the western CONUS tomorrow into Monday, another cold front will shift south through Utah. This cold front will be accompanied by more noticeably cooler air, along with an uptick in moisture. As such, showers are expected to develop again along portions of northern and central Utah tomorrow into tomorrow night.

.LONG TERM (After 12Z/6AM Monday)...Long term forecast period begins with a trough digging into the Great Basin, and an associated cold frontal boundary slowly working southward through the local forecast region. Near to slightly ahead of the frontal boundary, ensembles generally show some agreement in PWAT values near to slightly above climatological normal. With convergence along the boundary and moderate synoptic support aloft with the trough, will see at least some scattered showers and thunderstorms along/near the frontal boundary. Thunderstorm potential will be slightly more noted in the afternoon as daytime heating yields some pockets of low end instability. Additionally, areas that remain ahead of the frontal boundary into the early afternoon (southern Utah to southeastern Utah) will see some moderate wind gusts (generally ~25-40 mph) given decent pre-frontal deep layer southwesterly flow. Frontal boundary will continue a southeastward trajectory out of the area moving into Tuesday, but region will remain unstable enough within unsettled cyclonic flow as the trough axis shifts overhead to yield continued elevated precipitation chances through the day Tuesday. Overall QPF doesn't look too remarkable on a widespread level, but will likely see some modest localized amounts where a more convective shower or thunderstorm or two move through. The highest QPF totals look to be through the Uintas, where snow levels are also forecast to drop sufficiently to result in several inches of snow for the higher elevations. While there's still a bit of uncertainty in the overall depth of the trough, post-frontal temps will drop quickly with afternoon highs in most locations around 10-15 degrees below climo.

On Wednesday, guidance offers at least some slight consensus on a quick shortwave ridge transiting the region (though ~20% of ensembles maintain a bit more troughing). If this does indeed occur, a period of quieter and more stable conditions can be expected, with temperatures quickly rebounding back closer to normal.

Remainder of the forecast period carries greater uncertainty owing to the exact positioning of a broader scale longwave trough across much of the CONUS, and associated shortwave impulses rippling through and potentially digging out higher amplitude sections of the trough. Deterministic and ensemble members have varying scenarios in regards to how deep the trough ultimately gets (and in turn any frontal passages), as well as timing. A deeper trough would yield higher chances of cooler/unsettled weather. Currently, ensembles show around 60% of membership depicting of a deeper trough across portions of the W CONUS on Thursday, and around 55% on Friday.
Ultimately though, some signal for an active northern stream remains in most guidance. Will have to monitor how things ultimately trend, but for now maintained NBM values which carry near to below normal temperatures, as well as daily isolated to scattered precipitation chances for the northern half of the forecast region where there's a better chance for the trough to impact.



AVIATION
KSLC...NW winds once again become favored after 17Z or so, and generally remain fairly light through the day. Winds expected to flip back S around 03Z thereafter, though could see an earlier flip if any shower activity south of the terminal produces sufficient outflow winds. To that end, anticipate some potential for isolated high-based showers late evening through the overnight as a weak boundary lifts north, which will carry some threat for periods of gusty erratic outflow with any showers near enough to the terminal. Anticipate some increase in cloud cover as well, but with conditions expected to remain VFR.

REST OF UTAH AND SOUTHWEST WYOMING
A fairly weak frontal boundary will lift north from central Utah towards northern Utah and southwest Wyoming from late Saturday afternoon on into the overnight hours. Anticipate some isolated to scattered high-based shower activity as it does so, which will carry some threat of gusty erratic outflow winds for any nearby terminals. With the showers more high-based in nature, generally expect VFR conditions to persist, though will see some increase in cloud cover for terminals near the front.

FIRE WEATHER
A cold front has stalled this morning from around Milford to Green River. This front will gradually lift back to the north through the day today. High-based showers continue near the boundary, and will continue to follow it north today while increasing in areal coverage during the daytime hours. Measurable rain chances have increased a bit for today, but wetting rain chances remain generally low as many of the showers will be relatively weak. That said, some of the stronger storms may result in isolated to widely scattered thunderstorms. An isolated to widely scattered threat of brief gusty and erratic microburst winds greater than 40 mph remain in place for today.

The next storm system will gradually carve into the area tomorrow through the early part of next week. An associated cold front will cross Utah tomorrow into Monday, bringing cooler temperatures and increased humidities to much of the area through at least Tuesday. A break in the weather is expected on Wednesday, with the next potential storm arriving during the latter half of the week.

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

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