Wanship, UT Weather

Key messages

- Tropical moisture moving into the region on Friday will lead to scattered showers and thunderstorms capable of producing heavy rainfall and flash flooding, mainly across southwest Utah.

- A strong atmospheric river will interact with a potent northern stream trough Sunday into Monday, resulting in widespread moderate to heavy precipitation across the region.

- Cooler air moves in behind a strong cold front that will bring snow levels as low as 6500 feet with accumulating mountain snow.

Discussion

Our region is currently under the influence of dry southwesterly flow courtesy of a longwave troughing pattern off the west coast. This trough is expected to deepen along the coast on Friday as a northern stream cutoff low digs southward, introducing more moist, southerly flow aloft that will tap into what is currently Tropical Storm Vance. Concurrently, a shortwave is expected to eject northeastward through the region around the eastern periphery of the parent trough, introducing modest amounts of instability that will aid in the development of widely scattered showers and thunderstorms throughout the day. Storms are expected to develop mid to late morning first across southwest Utah, with coverage expanding northward through the day as moisture fills into the region. Particularly across southwest Utah, where deepest moisture and instability is maximized, flash flooding will be a risk. Current high-res guidance shows a bullseye of heaviest precipitation concentrated around Zion NP and the southern mountains, with a 75% chance of accumulations exceeding 0.3-0.8" Friday into Saturday morning, and a 25% chance for accumulations to exceed 0.8-1.3" in these areas. Elsewhere, accumulations are expected to be on the lighter side, between a few hundredths of an inch upwards of 0.25".

There will be a relative lull in storm activity on Sunday as subsidence behind the exiting shortwave fills in. Given some lingering moisture, there is some potential for some more isolated shower activity in the afternoon and evening, but overall conditions remain dry for holiday festivities.

On Sunday, the aforementioned northern stream cutoff low will phase with a Pacific low and shift inland. Confidence is increasing for this system to bring widespread moderate to heavy precipitation between Sunday and Monday morning. Two distinct areas of heaviest precipitation are evident in recent guidance: one across southwest Utah, attributed to the associated atmospheric river that will bring deep moisture and instability across southern Utah, and one across northern Utah, attributed to a period of heavy precipitation accompanying a cold front passage on Sunday. QPF amounts have increased overall, with even low-end guidance indicating likely chances (75%+) for widespread precipitation accumulations between 0.5-1" between Sunday and Monday morning. Accumulations across southwest Utah, the Wasatch Front, and all Utah mountains will likely see accumulations closer to 1.5-2" with localized areas seeing upwards of 3".

The cold front associated with the northern stream trough will usher in a much cooler airmass late Sunday into Monday morning. Snow levels will gradually decrease from 9500 feet late Sunday to around 6500-7000 feet early Monday. As such, accumulating mountain snow is expected through this period, with generally 3-6 inches expected with upwards of 12-18 inches along the higher elevations.

Drier conditions quickly build in on Monday as the weather system exits to our east, with much cooler conditions expected in its wake. High pressure is expected to prevail across the region through at least the middle half of next week.

Aviation

KSLC: VFR conditions under a veil of high clouds will persist through the TAF period. Southeast winds will transition to northwest during the 19-20Z timeframe, with a 20% chance this wind shift occurs as early as 18Z. Winds will return to southeasterly around 03Z.

REST OF UTAH AND SOUTHWEST WYOMING: VFR conditions are expected at all terminals through tonight. South to southwest winds will develop this afternoon before diurnal winds return shortly after sunset. An increase in moisture Friday morning will bring a chance of showers and thunderstorms to KCDC and KSGU after 15Z Friday.

Hydrology

Setting up for another active late week/weekend as another round of significant rainfall is expected to impact portions of central and all of southern Utah. Two primary periods of heightened rainfall potential exist, with the first materializing across namely southwestern Utah Friday afternoon and evening. Noting increasing PWAT trends streaming north across western AZ attm, with peak values reaching some 2-3 standard deviations above normal by this time tomorrow. Driving this is an amplifying upper level trough off the Pac coast, and an ejecting short wave translating into SoCal northeastward across Utah Friday afternoon. Although available PWAT won't be quite to the extent of points south (AZ) across southwestern Utah tomorrow, values are still expected to push near .80" which, coupled with MUCAPE values peaking between 400-700 J/KG will drive scattered convection capable of heavy rainfall and the potential for flash flooding in prone areas such as slot canyons, normally dry washes, and within/downstream of recent burnscars. Near to record high near surface soil moisture values for this time of year will further aid the efficiency of runoff and exacerbate impacts where and downstream of where the stronger cells form.

A relative downtick in heavy rain potential exists for Saturday, but the primary precipitation producer will be moving into the area Sunday. The aforementioned broad upper trough off the Pac Coast is expected to phase with a strong short wave embedded within a strengthening southern tier jet. Anticipated set-up will allow a land falling atmospheric river (AR) to penetrate inland into southern/central Utah producing widespread moderate to locally heavy rainfall across the Arizona strip, Grand Staircase, and southern/central mountain regions, including National Parks and Recreation Areas within these areas. Rainfall amounts of 1-2" are likely with this, and given antecedent conditions, will efficiently runoff the landscape promoting widespread rises for all waterways, including streams, creeks, normally dry washes, slot canyons, and rivers across the region.

Watches, warnings and advisories

UT: None. WY: None.