Damaging Winds in Utah, September 30, 2023

Area Forecast Discussion
National Weather Service Salt Lake City UT
526 AM MDT Sat Sep 30 2023

.SYNOPSIS...A storm system pushes into the region today with dry
and windy conditions prevailing through the late afternoon and
moisture increasing significantly thereafter. Cooler and wetter
weather will remain in place through early next week before a
ridge of high pressure attempts to rebuild, bringing a period of
warming and drying conditions.

&&

.SHORT TERM (Through 6PM Monday/ 00Z Tuesday)...A storm system
currently located over the western Great Basin will continue to
fill into the forecast area over the next 24 hours, bringing
significant changes to sensible weather across the region. Through
much of the day today, a large majority of the forecast area will
remain in a dry and windy pre-frontal environment, with the
strongest winds anticipated across the western half of Utah. Peak
wind gusts in this area will approach 45 mph, with isolated areas
near high terrain features experiencing gusts near 50 mph. Have
continued existing Wind Advisories across portions of western Utah
with no changes.

As we move later into the evening, an initial baroclinic zone will
combine with a stout mid-level disturbance and kick off a period
of moderate to heavy precipitation across western Utah. This
baroclinic zone will act as a focus for precipitation through the
remainder of the overnight hours, progressing eastward gradually
into the early morning hours on Sunday. Initially, snow levels
will start around 10,500ft, but will drop toward 8,500-9,000ft by
early morning Sunday. Some of the highest terrain of northern
Utah, especially the Uinta Mountains, will likely begin to see
snow accumulations by the early morning hours on Sunday,
potentially upwards of 8-10 inches in the highest terrain of the
Uinta Mountains. Given warm antecedent road temperatures,
expecting limited road impacts through the higher mountain routes
in these areas (i.e. Mirror Lake Highway, SR-190, and SR-210). For
this reason, have opted not to issue winter weather headlines for
the time being. In fact, by the time snow levels drop low enough
to begin potential accumulations on roads, the main forcing for
precipitation will have ejected to the north, cutting off strong
potential for additional accumulations.

A relative lull in precipitation is expected during the day on
Sunday, though areas of rain showers and high elevation snow
showers will continue across the northern third of Utah and
southwest Wyoming as mid-level instability increases with the
progression of the upper low. With the colder airmass in place,
should see daytime high temperatures 10-15 degrees cooler than
today across the forecast area. Finally we'll be back to more
fall-like weather.

Moving through Monday, a secondary, loosely defined baroclinic
zone will move through the region, bringing even cooler
temperatures into the region as well as another round of
precipitation. Flow will transition from southerly to typical
post- frontal northwesterly wind, favoring upslope regions along
the I-15 corridor. Further expansion of the upper cold pool will
help to kick off convective showers across much of the area, with
most of the action anticipated across the northern two-thirds of
the area. Additionally, with even colder temperatures filling in
overhead, high temperatures across the northern two-thirds of the
area will decrease by upwards of another 5-10 degrees.

.LONG TERM (After 6PM Monday/ 00Z Tuesday)...A cutoff low associated with a
longwave trough will continue to be impacting the area at the
beginning of the extended period. By late Monday afternoon, the
trough axis will move through Utah and the upper level flow will
switch to the northwest. After this trough axis moves through,
southern Utah will dry out while northern and central Utah will
maintain the low level moisture through Tuesday. This low level
moisture combining with a shortwave that will impact the area on
Tuesday will continue the precipitation chance for these areas
through the day. Additionally, northwest flow with this moisture in
place could increase the QPF totals at higher elevations. At the
moment, additional QPF on Tuesday for most valley locations will
mostly be between .1" and .2"  while higher elevations locations
could see up to .4". Snow for Tuesday will mostly be for areas above
9000ft and will accumulate up to another inch or two at these
locations. Temperatures for Tuesday will be the coldest of any of
the days. High will be 15F to 20F below normal. This means most
valleys will see highs in the mid 50s to near 60F while Lower
Washington County will be in the low 70s.

Heading into Wednesday, the low begins to move out of the area and
Utah and southwest Wyoming begin to dry out. Ensemble guidance is
pretty consistent on the area being under the later stages of the
longwave trough before ridging returns for the rest of the week.
Cloud cover will also be decreasing as this low continues to move
further away. Temperatures for Wednesday will still be 7F to 15F
below normal however, by the end of the week, temperatures will be
near normal.

&&

.AVIATION...KSLC...Light southerly winds will continue until ~16Z
when southerly flow increases. Gusty southerly winds will continue
through the day Saturday, with gusts reaching up to 30kts. Late
Saturday evening is when scattered showers and thunderstorms will be
possible, causing winds to become gusty and erratic at times, but
staying predominantly southerly. CIGS with this precipitation are
expected to fall between 5kft and 7kft.

.REST OF UTAH AND SOUTHWEST WYOMING...Predominant southwesterly flow
is expected across much of Utah and southwest Wyoming. Gusts will
mostly be between 30kts and 35kts with some areas seeing gusts as
high as 40kts to 45kts during the morning and afternoon. Gusts will
subside as precipitation begins to enter the area late Saturday
evening. Scattered showers and thunderstorms are possible between
Saturday evening and mid-Sunday morning. Winds may be gusty and
erratic at times but predominant southerly flow is expected. CIGS
with this precipitation will be between 4kft and 7kft for most
locations.

&&

.FIRE WEATHER...A storm system will continue pushing into the region today,
bringing increased winds out of a south to southwesterly
direction alongside largely dry conditions, creating areas of
critical fire weather conditions. The strongest winds are
anticipated across the western half of Utah today, where gusts
could exceed 45 mph with afternoon minimum humidity values as low
as 15 to 20 percent. Drier conditions are expected across eastern
Utah valleys, with humidity potentially dropping into the upper
single digits and wind gusts approaching 35 to 40 mph. Moisture
will fill into the western portions of Utah through the late
afternoon/ early evening hours, with chances for precipitation
significantly increasing around sundown. A majority of the
precipitation associated with this system will fall across the
northern third of Utah, however, the remaining southern two-thirds
of the area will see upwards of 0.25 inches of precipitation with
locally up to 0.5 inches in the southern Utah mountains.

Sunday forward, a colder airmass will settle into the eastern
Great Basin region, bringing much cooler (up to 20 degrees colder)
temperatures and higher humidity values. Chances for showers
(with snow above about 9,000ft) will remain in the forecast for
northern Utah through Tuesday as the storm system and associated
moisture lingers. From about Wednesday forward, a ridge of high
pressure will gradually build back into the region which will help
to initiate a warming and drying trend heading through the
remainder of the week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from 9 AM this morning to 9 PM MDT this evening
     for UTZ102-115-122.

     Wind Advisory from noon today to 9 PM MDT this evening for
     UTZ126.

WY...None.
&&

Webber / Cecava

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