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.
&&
.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.
&&
.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.
&&
.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.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&
$$
Whitlam/Kruse
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NWS SLC Office Area Forecast Discussion