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FXUS65 KSLC 050950 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 350 AM MDT Sun May 5 2024

SYNOPSIS
Gusty southerly winds will diminish behind a cold front moving through the state throughout the day.
Valley rain and mountain snow will accompany this front. Most of this precipitation stays confined to northern Utah. Northern Utah remains active and colder than normal through Wednesday morning.

.SHORT TERM (Through 12Z/6AM Tuesday)...A powerful cold front has moved into northwestern Utah, as of 3am. Temperatures are very warm ahead of the front with mid to upper 60s stretched across most Utah valleys with temperatures falling ~20-30 degrees behind the front. Additionally, there is about a 30-40 degree dewpoint depression at most pre-frontal weather stations indicating how dry the lower levels are ahead of the front. This will prevent most precipitation from reaching the surface until after the front passes. Even though showers ahead of the front are not resulting in precipitation reaching the surface, they are acting to enhance southerly wind gusts up to 60+mph in some locations by mixing down 50kt winds aloft. These brief and isolated gusts will continue until the front passes.

Frontal passage will occur along the Wasatch Front between 8-10am with central and southern Utah later this afternoon. Surface winds will transition to westerly/northwesterly behind the front, but winds aloft will maintain a southerly component until Sunday night. This will focus the heaviest precipitation along areas that do well in southerly flow, but it will also limit precipitation for northerly facing aspects and valleys that tend to get shadowed in southerly flow. Hi-res guidance seems to be picking up on this idea as QPF totals are lower compared to the lower resolution guidance. But once flow does transition to westerly/northwesterly overnight Sunday snow will increase in intensity across areas that do well in this flow regime. Snow will remain off and on across the terrain of northern Utah through Tuesday morning as moisture remains in place. The convective potential will be relatively low for this time of year as instability is meager, but some isolated convective activity can't be ruled out. Snow levels remain mostly unchanged from previous forecasts as areas below 5000 feet should remain mostly rain with some snow mixing in along the highest bench locations.

Shortwave ridging will move into northern Utah late Monday and bring a lull in precipitation overnight Monday. However, a shortwave will move through Tuesday morning and reinvigorate precipitation once more in the form of valley rain and mountain snow. All in all, QPF totals for valley locations of northern Utah through Tuesday morning will generally be 0.25-0.75" with locally higher amounts up to 1" across areas that do well in southerly and westerly flow. Snow totals will be between 8-16" with locally higher amounts 24"+ across the northern mountains.

.LONG TERM (After 12Z/6AM Tuesday)...At the start of the long term period of the forecast, model solution space remains locked into the idea of persistent negative height and temperature anomalies across the Northern Rockies that challenge records for the time of year, both aloft and at the surface. The largest surface temperature anomalies across Utah and SW Wyoming are between Monday afternoon and Thursday morning, with the highest negative EFI values Wednesday morning. Digging into the details a bit, a progressive, reinforcing longwave trough is forecast to be traversing eastward across the Northern Rockies on Tuesday, maintaining cold, cyclonic flow across most of Utah and SW Wyoming. As a result, conditions will be favorable for persistent snow showers across the mountains of northern Utah. Lake effect probabilities have dropped slightly, and are now in the 20-30 percent range from just after midnight Tuesday morning through midday Tuesday. Nonetheless, the ingredients are in place to once again support lake effect/lake enhanced activity. With W-WNW flow in place through the boundary layer, locations from Ogden to Bountiful and perhaps as far south as North Salt Lake will be favored for lake enhanced activity. Another lobe of vorticity will drop down the backside of the impressive longwave trough, acting to once again reinforce the negative height anomalies across Utah and SW Wyoming on Wednesday. This will mean a third morning of enhanced lake effect/enhancement probabilities Wednesday morning. Unlike Tuesday morning, boundary flow trajectory is more out of the NW, favoring locations from Bountiful to SLC. Going back a day, wanted to mention that lake-induced CAPE values on Tuesday are impressive, reaching nearly 1400J/kg with equilibrium heights up to 18,000 feet.
Given this, it's not out of the question that some lake effect/enhanced activity could contain lightning.

Additional snow accumulations in the 48 hrs preceding 12Z Thursday morning are in the 6-12 inch range across Utah's northern mountains, with locally higher amounts in the Logan and Ogden area mountains as well as the Upper Cottonwoods. During the overnight and early morning hours, snow levels will be cold enough to support snow down to valley floors, and locations away from the Great Salt Lake shoreline (with its moderating influence on sfc and boundary layer temperatures) could see anywhere from a trace (25th percentile) to a few tenths of an inch of snow (75th percentile). It's worth noting that the 95th percentile values paint 1-3 inches across the Northern Wastach Front with 4 inch amounts over benches, representing a low probability outcome.

Temperatures will be some 10F to 20F below normal through Wednesday for highs. The coldest morning appears to be Wednesday morning, with widespread 20s forecast for many western, eastern and central Utah valleys, including the Tooele and Rush Valleys. We're currently forecasting temperatures near 32F in Eastern Box Elder County, the Cache, and outlying areas of the Wasatch Front. Folks will once again need to take steps to protect tender/sensitive vegetation.

Ensembles are in good agreement in depicting gradually rising mid/upper level heights from Thursday through next weekend. At the surface, we see gradually warming temperatures during this period, with temperatures rising to near to slightly above normal by next Sunday. What's interesting is that the Canadian and GFS global, deterministic solutions continue to latch onto the idea of maintaining low pressure across the Great Basin through next weekend, maintaining cooler, unsettled conditions, but not as cool as what we are in line to experience early this week. This unsettled pattern has representation from about 25% of the solution space.
While the most likely scenario supports warming, dry conditions, do know that about 1/4 of the solution space suggests we'll hold onto unsettled conditions. Stay tuned to see how this trend evolves over the next few days.

AVIATION
KSLC...Active period as a winter-like storm will be in place through midweek. Strong, gusty S winds to around 40kt will continue through about 14Z. Incoming cold front will begin to cause a wind shift to the SW around 14Z and then to the W-NW by 15Z and finally NW by 18Z with gradually decreasing speeds. Winds off the surface, mainly above 2kft, remain S-SW until late afternoon, possibly resulting in some LLWS. By late morning, shower chances become increasingly likely ahead of cold front, though strong south flow may initially yield some downsloping off terrain, abating precip to an extent. Widespread precip will develop after the frontal passage by early afternoon with MVFR conditions fairly likely during times of heavier precip, with periods of IFR possible (with nearly a 20% chance.) Trailing wave will bring an uptick in precipitation tonight, with temperatures cold enough to allow snow to mix in at times.

REST OF UTAH AND SOUTHWEST WYOMING
An anomalous winter-like system will be in place across the region through early week, resulting in active TAF period along with persistent periods of mountain obscuration, especially across northern Utah. Strong southerly winds will precede a cold front as it marches eastward across Utah today, exiting into Colorado this evening. Showers ahead of the front will aid in mixing, with occasional gusts to 50kts reaching the surface at times. Precipitation chances become further elevated through the morning and afternoon as cold frontal boundary moves through. MVFR conditions likely in heavier precip, with IFR possible at higher elevation terminals where snow may mix in, with the best chance of this across SW Wyoming. Strongest winds/gusts will begin to subside following the frontal passage, with directions largely trending more west to northwesterly. Periods of rain and snow showers then persist across northern Utah and SW Wyoming into Monday morning.

FIRE WEATHER
Unseasonably cold conditions will be in place across the area through midweek, especially across northern Utah. Significant snow for the time of year will fall across northern Utah's mountains. A warming and drying trend will then commence by Thursday, with gradual warming and drying expected to continue into next weekend. Isolated showers may persist through the end of the week, especially over northern Utah terrain. Much lighter winds will be in place across the area from Wednesday through next weekend.

SLC WATCHES/WARNINGS/ADVISORIES
UT...Wind Advisory until 6 AM MDT early this morning for UTZ101-102- 105-122.

Winter Storm Warning from 6 AM this morning to noon MDT Tuesday for UTZ110>112.

Wind Advisory from 6 AM this morning to 10 PM MDT this evening for UTZ114-120-121-126>131.

High Wind Warning until noon MDT today for UTZ115.

WY...None.

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