463
FXUS65 KSLC 201029
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
429 AM MDT Fri Sep 20 2024

.SYNOPSIS...A typical early fall pattern will continue across the
Beehive State through the next week. An upper level disturbance
will bring the threat of showers and thunderstorms to portions of
southern and eastern Utah Saturday. A weak cold front will cross
northern and central Utah Monday.

&&

.SHORT TERM (Through 12Z/6AM Sunday)...Satellite imagery reveals
a closed low positioned over southern California early this
morning, which will end up dictating much of the sensible weather
through the weekend across Utah and southwest Wyoming. As the
closed low continues to make its way inland, showers and
thunderstorms will develop across far southwestern UT this
afternoon, increasing in coverage heading into the evening hours.
Gusty microburst winds will be the main threat, though any
training in storms over Zion NP will produce a minor flash flood
threat as well. Given strong dynamics with this closed low and
sufficient moisture, these showers will likely continue overnight,
slowly shifting eastward with the progression of the low.

As the low lifts into the Four Corners region Saturday morning,
precipitation chances will expand further north, staying mostly
east of I-15 through the afternoon. Scattered showers and
thunderstorms on Saturday will result in a heightened flash flood
risk for prone areas in slot canyons or near recent burn scars,
particularly for areas further east, with mean QPF ranges between
0.1-0.25" across southern UT. As the low exits the area late
Saturday, precipitation chances will largely dissipate.

The low passing just to our south will also result in gusty
easterly canyon winds along the Wasatch Front and Cache Valley
Friday night/Saturday morning, with the highest gusts expected in
the most prone areas in Davis County. HREF max gusts top out
around 40 mph, though canyon mouths could see brief periods up to
45-50 mph.

.LONG TERM (After 12Z/6AM Sunday)...To close out the weekend,
deterministic global solutions and ensemble solutions begin the
period in good agreement regarding the large-scale pattern. We kick
things off with a closed low over/near the Colorado Front Range on
Sunday, with lingering early Sunday cyclonic flow extending westward
into extreme eastern Utah, with a very low (<15% chance) for a
shower for these extreme eastern areas Sunday. Otherwise, Sunday is
expected to be precipitation-free with temperatures just a shade
below seasonal normals. The journey into the long term portion of
the forecast is a short one before model spread is introduced, with
deviating solutions arriving as early as Monday. First, we`ll dive
into where solutions are aligned: there is agreement that a ridge
will build offshore of the western CONUS with a trough diving down
the periphery of the ridge across the Rocky Mountains Monday and
Tuesday. Two forecast scenarios are in play regarding the evolution
of this trough. There is a subset of the ensemble solution space (as
well as the deterministic ECMWF) that closes a low off directly over
Utah on Monday, with the low lingering around through Wednesday.
This would result in cool, showery conditions for the start of the
workweek. This outcome is represented by a minority of the
solutions, and interestingly the NBM is more reflective of this
solution, with mentionable precipitation across the southern half
of Utah on Monday and closer to the Arizona border Tuesday. The
alternate scenario is one with a progressive trough which would
maintain seasonable temperatures and dry conditions for the early
portion of the workweek.

Ensemble spread then decreases (with an associated increase in
forecast confidence) for the mid to late period, with consensus
showing positive height anomalies gradually building across the
Great Basin, resulting in a gradual warming and drying trend through
the remainder of the week. While there are a few ensemble members
that want to bring a trough through the northern tier and knock down
the ridge, the overwhelming consensus maintains positive height
anomalies/ridging across the region, resulting in stellar weather to
kick off the official start of the Autumn season.

&&

.AVIATION...KSLC...VFR conditions are expected through the period,
with southeasterly drainage winds becoming northwest around 17Z.
Cumulus buildup expected over terrain during the afternoon, with a
very low (<15% chance) for an isolated afternoon terrain-based
shower. Winds most likely to remain northwest Friday night, but
confidence in wind direction is low due to the emergence of an
easterly gradient over the region.

.REST OF UTAH AND SOUTHWEST WYOMING...Area of low pressure is
forecast to track just south of Utah through the period, bringing
scattered showers and thunderstorms to southwest Utah beginning
during the late afternoon hours. Elsewhere, afternoon terrain-based
cumulus will develop into isolated showers, especially over central
Utah. Any shower/storm will be capable of producing gusty/erratic
winds.

&&

.FIRE WEATHER...A low pressure system will slide across the Desert
Southwest Friday and Saturday, lifting northwards across CO
Saturday night. With this system, expect showers and thunderstorms
to develop over southwestern UT Friday afternoon, then spreading
eastward overnight and into Saturday morning across much of
southern and eastern UT. Gusty outflow winds are likely with
these storms. The highest chances for wetting rains are over the
southern mountains on Saturday, particularly between Bryce Canyon
and Capitol Reef. Expect improving overnight recoveries through
Sunday night. A mostly dry cold front will push through northern
and central UT Monday afternoon and evening, though there is a
small chance for some showers across southern UT. High pressure is
then likely to build, producing warmer and drier conditions
heading into midweek.

&&

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

$$

Cunningham/ADeSmet

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