Damaging Winds in Utah, September 19 to 21, 2023

Area Forecast Discussion
National Weather Service Salt Lake City UT
359 PM MDT Tue Sep 19 2023

.SYNOPSIS...Another round of showers and thunderstorms is in store
for northern Utah and southwest Wyoming tomorrow. A deeper, colder
storm system pushes into the region late in the week, bringing a
round of rain and high elevation snowfall to the northern two-
thirds of the forecast area. Cooler temperatures prevail Friday
with a warming trend heading through the weekend.

&&

.SHORT TERM (Through 12Z/6AM Thursday)...Lingering mid and upper
level moisture aided by increasing mid-level lift has helped to
produce scattered showers and thunderstorms in the northeastern
portions of the CWA this afternoon. Coverage of these storms has
been increasing gradually over the Utah/ Nevada border region as
an approaching shortwave trough has provided additional lift into
the environment. A dry low level environment has allowed for some
gusty winds with these showers, though surface based observations
have not shown any gusts exceeding 40 mph. That said, the threat
of an isolated microburst still remains in place through the
remainder of the evening, particularly for northern Utah and
southwest Wyoming.

The aforementioned shortwave trough, currently located over
southwestern Idaho, will continue to shift into the region through
the evening, extending the period in which we'll see showers over
the forecast area. As was the case with today, showers will be
confined to northern Utah, favoring areas of higher terrain
through the overnight period.

Tomorrow, moisture will be pushed slightly farther northward as
deep layer flow turns more southerly, bringing a drying trend to
portions of northern Utah. That said, lingering mid and upper-
level moisture will remain in place and bring another round
of showers and isolated thunderstorms to northern Utah and
southwest Wyoming. A mid-level cutoff low will phase into the flow
pattern over the forecast area late in the day, which may act to
bring a round of showers and thunderstorms to the central and
southern Utah mountains through the late afternoon and evening.
At this time, not anticipating any significant threats with these
storms, but as was the case with today, isolated microburst winds
will be possible given how dry the low level environment is. The
phasing of this cutoff should produce enough lift during the late
evening and overnight period to keep areas of showers going across
northwest Utah as we head into Thursday, where a deeper, colder
system will push into the area.

.LONG TERM (After 12Z/6AM Thursday)...A cool, fall-like storm will
bring several impacts to our area Thursday/Friday, followed by quiet
and dry weather with a gradual warming trend into next week. Impacts
with the late week storm will include gusty winds over the southern
2/3rds of Utah Thursday, potential for stronger thunderstorms
Thursday, widespread precipitation later Thursday into Friday from
roughly Price northward, a dusting of snow at the high elevations
(mainly Friday), the coolest temperatures yet this season Friday,
and a potential for the first freeze of the season for several
valleys with agricultural interests.

Early Thursday, a deep upper-level closed low over Eastern Oregon is
progged to set up strong southwest flow aloft over the Great Basin
while pushing an initial mid-level baroclinic zone across the
northern Great Basin with a band of widespread light to moderate
precipitation. Ensembles are in good agreement that this initial
round will largely dissipate as it approaches our area, perhaps
brushing far northwest Utah but unlikely to reach the I-80 corridor.
Instead, widespread showers develop Thursday from around I-80
northward as the center of the upper-level low slightly retrogrades
and the aforementioned initial wave washes out. There is a low
chance of stronger thunderstorms during this period in the same
area, with plentiful shear and marginal instability (depending on
cloud cover). Strong southerly winds will develop Thursday over
southern and central Utah. There is a moderate chance (40-50%) of
wind gusts exceeding 45 mph, mainly west of I-15 and south of US-50.

By late Thursday, the parent low begins to shift southward while a
stronger shortwave trough rounds its base and pushes into our area.
The majority of ensemble members are in agreement with the
development of this stronger wave and resultant broad precipitation
developing from around Delta to Price northward. Cold air advection
with this wave will begin to drop snow levels, potentially as low as
7,500' feet in far western Utah to 8,500' in the Uintas and Wasatch
based on the NBM 10th and 25th percentile snow levels. Overnight
Thursday into Friday, broad cyclonic flow sets up over the region
with potential for additional showers. Depending on cloud cover,
Friday's highs will likely be the coolest yet this season, e.g. low
to mid 60s for the Wasatch Front. Latest guidance shows a 50/50
chance of a half inch of precip from around Davis County northward,
and a likely chance (>60% chance) of a tenth or more from Utah
County northward. Higher elevation snow amounts do not appear
impressive as of yet (due to the more robust baroclinic zone not
reaching most of our area and instead relying on cold air advection
showery air mass), though will continue to monitor trends.

Late Friday into Saturday the upper-level low finally pushes east as
it weakens. There is a chance of lingering showers, especially over
the higher terrain, though the latest guidance favors rapid drying
by dusk Friday. Attention then turns to clearing skies Friday night
with a cooler air mass overhead (-3C to 2C range at 700MB).
Depending on how exactly how cold the air mass materializes and how
quickly we clear in the wake of the low, this could mark the first
freeze of the season for several valleys with agriculture interests
including the Sanpete, Sevier, Castle County, and perhaps the Cache.
Will continue to monitor trends for this freeze potential.

Quieter weather and a warming trend are then in store for the weekend
into early next week, with temperatures returning to normal by
Sunday and even a few degrees above normal Monday.

&&

.AVIATION...KSLC...There is high uncertainty regarding winds at KSLC
through the afternoon and evening. Southerly winds are more likely
to prevail this afternoon, though there is a 25% chance that outflow
produces northwesterly winds before 04z. Around roughly 04-06z, it
is more likely that winds will go northwesterly (60% chance).
Thunderstorms are likely to remain to our south until around 02-04z,
when there is a 20% chance of thunder impacting KSLC. Light,
southeasterly winds will prevail overnight.

REST OF UTAH AND SOUTHWEST WYOMING...Scattered showers and
thunderstorms will continue through roughly 06z, with most activity
north of KDTA, and all activity north of KBCE-KCDC. A few isolated
showers may linger overnight across northern Utah. Any showers that
develop may be capable of producing periods of gusty and erratic
outflow winds.

&&

.FIRE WEATHER...Lower level moisture will lift out of the northern
portions of the area, introducing a slight drying trend for the
central and northern Wasatch, as well as the Uinta Mountains.
Otherwise, all other areas of Utah are likely to see minimal
changes in humidity trends tomorrow. The lingering moisture in the
mid-levels will help to pop another round of afternoon showers
and isolated thunderstorms, mainly across the northern two-thirds
of Utah. That said, isolated showers and a slight chance for a
thunderstorm will extend through the southern Utah mountains.

By Thursday, a colder storm system will begin pushing into Utah,
bringing a stout increase in south to southwesterly flow across
the region. As these winds increase and the cold front moves
slowly into northern Utah, portions of west-central, southern, and
eastern Utah will likely experience winds exceeding 25 mph and
humidity below 20 percent. For northern Utah, an area of
precipitation will push through the area on Thursday, producing
wetting rains and high elevation snowfall above 9,000-10,000ft.
Friday will feature cooler temperatures and higher humidity,
particularly across northern Utah, in the post-frontal
environment, however, the weather patten will begin to favor a
warming and drying trend heading through the weekend.

&&

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

SHORT TERM...Webber
LONG TERM...Van Cleave
AVIATION...Cunningham
FIRE WEATHER...Webber

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