Severe Thunderstorm Warning 36, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
926 AM MDT Fri Jul 22 2022

.UPDATE...Shortwave approaching the California coast this
afternoon will bring increasing shear to northern Utah locations,
with 30-40 kts of bulk effective shear possible with a moderately
unstable environment. Convective allowing models are increasingly
bullish on potential coverage of convection this afternoon and
into this evening. Though the dry lower levels will limit the
amount of rain with these storms, there is a higher than usual
potential of gusty microburst winds late afternoon through mid
evening. Have updated the forecast to increase coverage of
POPs/thunderstorms into the isolated category for much of northern
Utah and southwest Wyoming into the early evening. No additional
updates planned.

&&

.SYNOPSIS...Dangerously warm temperatures will last into the
weekend throughout southern Utah with scattered afternoon showers
and thunderstorms for much of Utah through the weekend and into
next week.

&&

.SHORT TERM (Through 12Z/6AM Sunday)...High pressure transitioning
to near the Utah and Arizona border has resulted in less monsoonal
moisture advection compared to earlier in the week. That will be the
case through the day, with most showers and thunderstorms initiating
over higher elevations southern Utah. With dynamics and moisture
limited, storms aren't expected to propagate much off any terrain
for this portion of Utah.

A shortwave trough off the northern California coast will build
inland through the day, enhancing a southwest to northeast
pressure gradient. With dry conditions, mixing will be efficient,
so surface wind gusts peaking over 25 mph are expected for much of
western Utah and many exposed locations through the afternoon.
Minimum relative humidity values will be around 15% for many
valleys, resulting in elevated fire weather conditions.

The aforementioned shortwave will push up to 40 knots of 0-6 km
bulk shear into northern Utah during the later afternoon and
evening in an environment with steep lapse rates and marginal
CAPE. High resolution models bring isolated to scattered showers
and thunderstorms with this increased forcing. With how dry
conditions are, the primarily threat with any showers or
thunderstorms in this portion of the state will be gusty outflow
winds from roughly 3-9 p.m.

Temperatures will be in record territory for southwest Wyoming
and northern Utah, with most places expected to be within 4-5F of
their record high. Further south will be have similar temperatures,
but not quite as close to records, with headlines for those warm
temperatures in effect.

Slight cooling is expected Saturday as the shortwave trough pushes
northward and flattens the ridge slightly. This will drop
temperatures a few degrees. Increasing moisture is expected, but
showers and thunderstorms will generally stay in the higher
elevations of southern Utah. More monsoonal moisture advection is
expected later in the weekend and into next week.

.LONG TERM (After 12Z/6AM Sunday)...High pressure ridging that
has been centered south of Utah for several days will move towards
the Four Corners by Sunday before briefly shifting across the
Southern Plains early next week. This will open the door to the
return of monsoonal moisture on Sunday, although this initial
surge of moisture will translate to a low probability of heavy
rainfall. As moisture continues to increase on Monday and Tuesday,
the risk for thunderstorms capable of producing heavy rainfall
will continue to increase. Ensembles are in good agreement in
depicting the most anomalous moisture moving into SW Utah, including
St. George, Cedar City, Zion, Bryce Canyon south to near Kanab. PWAT
anomalies are forecast to approach 200% of normal by late Monday and
early Tuesday here. At St. George, for example, nearly 100% of
ensemble membership indicates PWAT increasing to 1" by late Monday.
Heaviest cells during this time will pose a flash flood risk. The
moisture gradient will be impressively tight on the northern edge of
the monsoonal moisture surge, with locations generally along/north
of I-80 forecast to be near to slightly below average in terms of
PWATs, per ensemble means. Seventy-five percent of clusters are
suggesting QPF will land above the ensemble mean on Monday across
portions of southern Utah, with this trending down to about 55% by
Wednesday. This suggests room for current forecast QPF to increase
during this timeframe.

The increase in moisture will force a cooling trend across
central and southern Utah. By Tuesday, highs are forecast to cool
to near climatological normals across central Utah, and dropping
to as much as 10 degrees below normal for southern Utah. These
values will ultimately be dependent on the amount of daytime cloud
cover and precipitation. A sharp contrast is evident, with
temperatures across northern Utah forecast to top out as much as
10F above normal. It does appear to be the tale of two regimes
this week...warm and dry north and cooler and wetter south.

For the middle to latter part of next week, ~95% of the long-
range ensemble membership indicates that deep cyclogenesis over
the Bering Sea will induce a high amplitude pattern downstream
with ridging over the Pacific Northwest/Great Basin and troughing
east of the Rockies. The primary impacts of this pattern would be
a reduction of PWAT and lower precipitation probabilities, along
with continued hot, dry weather across the north followed by a
late week warming and drying trend across the south.

&&

.AVIATION....KSLC...Southerly surface winds expected through the
morning before the lake breeze develops early in the afternoon.
Some uncertainty as to when this will occur, as models differ
between 18-21Z. High resolution models continue to advertise
showers and isolated thunderstorms near/over the terminal between
22-02Z, with activity starting as early as 20Z. Gusty, erratic
outflow winds and lightning will be the main threats. Brief period
of BLDU also possible should strong outflow >35kts move across
the terminal. VFR cloud coverage generally FEW to SCT ~10-15 kft
expected throughout the TAF period, with the exception of
convection very near or atop KSLC. Gusty, erratic winds will
settle back into the typical SSE surface flow between 04-05Z this
evening.

.REST OF UTAH AND SOUTHWEST WYOMING...Winds at terminals should
largely follow a typical diurnal pattern through the TAF cycle
outside of any convectively-induced outflow. Isolated convection
will once again be possible this afternoon and early evening,
especially across SW Wyoming and portions of northern and central
Utah. Gusty, erratic outflow winds and lightning will be the main
threats. In regards to clouds, expecting VFR conditions to persist
with generally FEW to SCT coverage from around 10-15 kft, with
more BKN coverage where convection is more widespread.

&&

.FIRE WEATHER...A high pressure ridge will remain centered over Arizona
today before drifting east towards the Four Corners by the end of
the weekend. A pair of disturbances will move east across Idaho
today and Saturday. These will create elevated, locally critical
fire weather conditions due to wind an low relative humidity. The
most likely areas for this to occur are near the Idaho border and
SW Utah today, and portions of SW Utah and the eastern valleys on
Saturday. Poor overnight humidity recovery will be a recurring
theme for the weekend as well.

Hot temperatures will persist through the weekend across the
state, with the hottest temperatures expected today, when
temperatures will be 5 to 15 degrees above normal for late July.
The most extreme heat will be across southern Utah through the
weekend.

Isolated thunderstorms today and Saturday will be capable of
producing gusty outflow winds, especially across portions of
northern and central Utah today.

As the high pressure ridge moves off to the Southern Plains by
early next week, this will open the door to allow monsoonal
moisture to return to southern Utah by as early as Sunday. The
initial push of moisture on Sunday will translate to a slight
increase in humidity and isolated to locally scattered wet/dry
showers and thunderstorms. By Monday, however, a signficiant
increase in RH is expected across southern Utah along with an
increasing potential for scattered and even numerous wet
thunderstorms with an accompanying heavy rainfall threat. This
will continue into Tuesday. Monday and Tuesday appear to be the
two days where this potential will be highest. The ridge is then
expected to become centered over the Great Basin by midweek,
causing weak northerly flow to develop across the state. This
should lead to a period of brief drying from north to south from
the middle to latter half of next week. Enough low level moisture
may remain to promote continued, daily shower and thunderstorm
activity across southern Utah. For northern Utah, however, warm
and dry conditions are expected to be the rule through the next
week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Heat Advisory until 9 PM MDT this evening for UTZ115-116-118-119-
     122.

     Heat Advisory until 9 PM MDT Saturday for UTZ121-130.

     Excessive Heat Warning until 9 PM MDT this evening for UTZ124.

     Excessive Heat Warning until 9 PM MDT this evening for UTZ123-
     128.

     Excessive Heat Warning until 9 PM MDT Saturday for UTZ131.

WY...None.
&&

UPDATE...Traphagan
SHORT TERM...Wilson
LONG TERM/AVIATION/FIRE WEATHER...ADeSmet

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

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