Special Weather Statement, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
349 PM MDT Mon Sep 27 2021

.SYNOPSIS...A cut-off upper level low will gradually shift
eastward across the Desert Southwest today. A strong cold front
will cross the state Tuesday into early Wednesday bringing
unsettled conditions and much colder temperatures. A gradual
warming trend is expected Thursday into next weekend.

&&

.SHORT TERM (Through 12Z/6AM MDT Thursday)...
A closed upper level low can be seen centered over eastern New
Mexico early this afternoon, while a neutrally tilted trough
approaches the PacNW region. Much of northern Utah remains
sandwiched in a col, or a region of relative lull, in between
these two features. The northern fringes of deeper moisture are
sufficient enough across southern Utah this afternoon to aid in
the development of isolated to widely scattered shower and
thunderstorm activity, when combined with the aid of terrain and
daytime heating. Showers and thunderstorms are drifting ever so
slowly off to the south and west early this afternoon, and a heavy
rainfall threat remains possible under the strongest cores. Gusty
outflow winds will also remain a threat with any shower and
thunderstorm activity. CAPE values are modestly higher when
compared to yesterday, owing to slightly better moisture quality.
Given the weakly sheared environment, updraft maintenance will
remain a challenge once convection propagates off of terrain,
except where cold pools briefly combine. Given increasingly
large- scale lift owing to the approaching trough, shower and
thunderstorm activity may have some semblance of staying power,
and may persist until closer to midnight if not just beyond,
while exhibiting gradual weakening.

Meanwhile, an approaching low pressure trough will continue to
propagate eastward across the Northern Rockies through Tuesday.
This will drag an attendant surface front across northern Utah and
SW Wyoming during the morning. The front will continue to sag
southward through the day, clearing central Utah during the
evening, and finally crossing southern Utah by late Wednesday
morning. As the trough axis crosses the Northern Rockies, it will
begin to shear out, and as such the most robust dynamics to help
force precipitation with it. An exception will be a trailing
shortwave and jetlet which are forecast to dive into southern
Nevada by late-day Tuesday and track eastward across southern Utah
through Wednesday. These features will interact with the
antecedent frontal boundary and aid in the development of
showers across central Utah as we head through Tuesday morning,
and especially by afternoon. Lingering instability will aid in the
development of isolated showers as well. The leading edge of this
activity will slowly sag southward through southern Utah Tuesday
night. With the combination of the slowed forward progression of
the front and dynamics associated with the trailing trough
features, shower generation is expected to continue Wednesday,
mainly along and south/east of an Evanston to Provo to Beaver to
St. George line. This activity should gradually diminish
Wednesday night with the loss of both daytime heating and
dynamics.

Temperatures will drop about 10 degrees from today to Wednesday
behind the front, and 10 to 20 degrees across northern Utah behind
the front. Temperatures will be cold enough to promote frost and
freeze conditions in many northern valleys and high elevation
valleys both Wednesday and Thursday morning. This may necessitate
a freeze headline for the Cache Valley both mornings. Temperatures
should remain sufficiently warm to prevent frost in the SLC metro
as well as the Utah valley.

Flow throughout much of the column will gradually become
southwesterly in the pre-frontal airmass, and a consequence of
this will be gradually increasing smoke concentrations across
central and northern Utah per the HRRR smoke model. Smoke
concentrations are forecast to gradually lessen through Tuesday
afternoon in the post-frontal northwesterly flow-driven airmass.

.LONG TERM (After 12Z/6AM MDT Thursday)...
As the trough axis moves east of the forecast area, models
indicate a high likelihood that the southern edge will split and
form a closed low over the Arizona/New Mexico border by Friday
morning. While models are uncertain as to the depth and
progression of the cutoff low, what is clear is that this feature
should remain far enough east that little moisture will be
advected into the region under its influence. Thus, PoPs are
appropriately low, with a slight chance of mountain showers in the
southeastern portion of the forecast area Thursday through
Saturday afternoons. No accumulating precipitation is expected and
increased cloud cover with virga is more likely than any
impactful weather.

Temperatures will rebound slowly from the chilly departures from
normal on Wednesday, and this slight warming trend will continue
throughout the weekend. This trend will take daytime highs from
around 5-10 degrees below normal on Thursday to near normal by the
start of next week. The overwhelming consensus is that ridging will
return to the Great Basin by Sunday, which would bring dry
conditions and warming temperatures across the forecast area.

There is the potential for precipitation chances to increase with a
developing trough to the north of the forecast area by late Sunday
or early Monday. At this time, this solution is a very low
probability outcome, with few enough members onboard that there is
no notable signature in the ensemble forecast clusters. However,
this will be a feature to watch as the pattern appears to remain
active beyond the extended forecast period.

&&

.AVIATION...
KSLC...Northwest winds are expected to switch to the
southeast around 03Z. Clouds will be increasing going into Tuesday
with a cold front approaching. That front is expected to move
through around 14Z, switching winds back to northwest. Gusts over
25 knots are expected after the front moves through. This will be
a relatively dry front, with only an isolated rain shower expected
through 03Z.

REST OF UTAH AND SOUTHWEST WYOMING...A cold front moving through
southwest Wyoming and northern Utah Tuesday morning will result in
gusty northwest winds reaching over 25 knots. Ahead of the front,
southwest winds gusting over 20 knots are expected in southern Utah
Tuesday afternoon. Largely dry conditions are expected through the
day. Later Tuesday into Wednesday, scattered rain showers are
expected for southwest Wyoming and much of Utah.

&&

.FIRE WEATHER...An area of low pressure will continue slowly
drift northward from the four-corners into Colorado through
tomorrow. Increasing humidity is expected across the southern
third of Utah associated with this system through this time.
Expect showers and thunderstorms to develop this afternoon and
evening and tend to move toward the south to southwest. Given the
relatively dry low levels, gusty and erratic microburst winds will
be possible along with isolated locally heavy rain under the
strongest of the storms.

A strong cold front will cross the state from the northwest to
southeast Tuesday into early Wednesday. This cold front will bring
unsettled conditions to Utah Tuesday into Wednesday, with
isolated to scattered showers expected to develop in association
with the front. Precipitation will develop gradually through the
late morning hours with precipitation becoming most widespread by
Tuesday afternoon. The best chance of precipitation during this
time is expected within a NE-SW oriented corridor from the Unitas
SW to near Callao and from Enterprise east to near Hanksville. The
southern edge of the band of showers will gradually sink
southward Tuesday evening and overnight while remaining in place
farther north. Drier air will gradually filter southward by late
Wednesday, ending precipitation chances from north to south.

Temperatures will cool 10-20 degrees from today to Wednesday
behind the front and will remain below normal through Friday.
Snow levels will lower sufficiently across the northern and
central mountains for light snow accumulations Tuesday night into
Wednesday, particularly in the High Uintas. Breezy southwest winds
ahead of the front will quickly turn to the northwest behind the
front.

An isolated shower or thunderstorm will remain possible across
portions of southern and eastern Utah during the Thursday-Saturday
timeframe as modest lingering moisture combines with a couple of
weak, passing disturbances.

&&

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

ADeSmet/Wessler/Wilson

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

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php

Special Weather Statement
National Weather Service Salt Lake City UT
610 PM MDT Mon Sep 27 2021

UTZ128-280045-
South Central Utah UT-
610 PM MDT Mon Sep 27 2021

...A strong thunderstorm will impact portions of southwestern Kane
County through 645 PM MDT...

At 607 PM MDT, Doppler radar was tracking a strong thunderstorm 10
miles east of Kanab, moving south at 10 mph.

HAZARD...Penny size hail.

SOURCE...Radar indicated.

IMPACT...Minor damage to outdoor objects is possible.

Locations impacted include...
Kanab.

This includes US Route 89 between mile markers 46 and 64.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Frequent cloud to ground lightning and heavy rain is occurring with
this storm. Lightning can strike 10 miles away from a thunderstorm.
Seek a safe shelter inside a building or vehicle.

&&

LAT...LON 3716 11251 3717 11221 3700 11220 3700 11254
TIME...MOT...LOC 0007Z 009DEG 8KT 3709 11235

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...<30 MPH

LC