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Alpine, Utah Weather Forecast Discussion

115
FXUS65 KSLC 220954
AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 354 AM MDT Mon Sep 22 2025

.SYNOPSIS...An unsettled pattern persists today along with a cooldown at many locations following the passage of a cold front. Drier conditions are then expected through midweek, with some moisture starting to return to southern Utah late in the work week.

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.SHORT TERM (Through 12Z/6AM Wednesday)...Current radar imagery depicts a line of showers and storms roughly along the I-80 corridor decaying as it moves eastward across into Wasatch Front early this morning. Another cluster of storms is located over the northern Wasatch Front and Cache Valley, which is also expected to gradually diminish as it tracks eastward through the morning. A reinforcing line of showers and thunderstorms associated with an upper- level vort max is expected to track into northern Utah around mid- morning, eventually moving off into eastern Utah through the afternoon hours. Northerly flow aloft develops tonight behind this upper- level wave, ushering in a much drier airmass and shutting off any additional convective activity areawide by this evening.

This northerly flow regime will also result in cooling aloft, with most significant cooling expected over northeast Utah and southwest Wyoming, where H7 temperatures bottom out around 1-2 C by Tuesday morning. This will yield quite the chilly morning across these areas, with the Wasatch Back and Cache Valley potentially seeing their first freeze of the season. Holding off on any headlines just yet, as temperatures are just on the cusp at this point. Regardless, it may be a good time to begin preparations on protecting any cold- sensitive vegetation.

This northerly pressure gradient is also supportive for the development of enhanced canyon winds across Washington County early Tuesday morning. Expected winds remain sub-advisory levels, with gusts 30 to 40 mph possible.

Otherwise, conditions remain dry with generally seasonable temperatures through the remainder of the day Tuesday.

.LONG TERM (After 12Z/6AM Wednesday)...As an omega block pattern sets up across the Western United States, Utah will begin the long term forecast period situated between two areas of low pressure. A cut- off low offshore near the Bay Area will gradually shift eastward through Thursday while a trough to east over the Plains shift southeastward. This combination will help to draw moisture north from Mexico...and perhaps tapping some of the tropical moisture from now Tropical Storm Narda.

By Thursday afternoon, expect precipitable water values at or above 90th percentile for this time of year across southwestern Utah. LREF 25th to 75th percentile SBCAPE values range from around 250 J/kg to 700 J/kg with deep layer shear around 20-25kts. This, combined with synoptic scale lift from the upper level low should be sufficient for deep enough convection to support at least an isolated flash flood threat. The Weather Prediction Center has outlined southwestern Utah in a Marginal Risk for excessive rainfall (at least a 5% chance flash flood guidance will be exceeded within 25 miles of a point) Thursday.

By Friday, there is a wide diversity of solutions in the track of the upper level low, as is common in omega block situations at this time scale. Some members center the upper level low near St. George (~30% of members). Some near the Nevada/Arizona border (~20% of members). Others near San Bernadino County in California (~50% of members). The closer to LA the upper level low is centered, the less of a threat of flash flooding for southern Utah. Inversely, the closer to St. George the upper level low is centered, the higher the chance of isolated flash flooding. For now, the Weather Prediction Center continues to indicate a marginal threat of flash flooding for the southern half of Utah as sufficient ensemble members support the further northeast position of the upper level low.

Depending on the upper level low position Saturday, the isolated flash flood threat could continue...but given the diversity of solutions from Friday onward, something to monitor at this point.

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.AVIATION...KSLC...VFR conditions prevail at the terminal through the period. Winds remain generally light out of the southeast this morning, shifting northwesterly after roughly 16z. There is a 30% chance for an isolated shower or thunderstorm to impact the terminal between 18-20z, which would be capable of producing a brief period of gusty and erratic winds. Otherwise, drier conditions develop for the remainder of the afternoon, with gradually diminishing cloud cover.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions to continue for all regional terminals. Showers and thunderstorms along a boundary will track into northwest Utah mid-morning, gradually moving southeast into the early afternoon. Activity along this line may produce gusty and erratic outflow winds for northern Utah and southwest Wyoming terminals until it exits the region by late afternoon. Drier conditions develop behind this boundary, with cloud cover gradually diminishing into the evening.

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.FIRE WEATHER...A band of showers with isolated thunderstorms roughly located along the I-80 corridor will continue to weaken as it tracks eastward early this morning. A secondary line of showers and storms associated with a cold front is expected to track into northwest Utah mid-morning, bringing another shot of wetting rains for areas across northern Utah. Lingering moisture across the eastern half of Utah will allow more widely isolated to scattered showers and thunderstorms to develop early this afternoon. Dry, northerly flow developing behind the cold front quickly brings drier conditions areawide by tonight, in addition to much cooler temperatures into Tuesday morning.

High pressure builds through mid-week, moderating temperatures back to normal and resulting in dry conditions across the state. An area of low pressure moves into the Desert Southwest on Thursday, with associated moisture wrapping into southwest Utah yielding increased chances for widely scattered showers and thunderstorms across the area. Precipitation chances linger across southwest Utah into the weekend, potentially spreading further north by the late weekend depending on the track of the low pressure system.

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

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Whitlam/Kruse

For more information from NOAA`s National Weather Service visit... http://weather.gov/saltlakecity

NWS SLC Office Area Forecast Discussion

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