Severe Thunderstorm Warning 37, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
314 PM MDT Fri Jul 22 2022

.SYNOPSIS...Hot and generally dry conditions will continue into
Saturday. Deep moisture will return to central and southern Utah
beginning late Saturday and lasting through at least midweek.

&&

.SHORT TERM (Through 12Z Sunday)...High pressure is centered near
the Utah/Colorado border this afternoon, resulting in west to
southwest flow over Utah and southwest Wyoming. This has continued
to bring in some drier air to the forecast area with PWs primarily
in the 0.5 to 0.7 range except a bit higher over far southwest
Utah. The strengthening of the ridge and increasing flow has
allowed for temperatures just a bit hotter than those observed
yesterday, resulting in some record challenging temperatures.
Combined with limited overnight recovery, this is resulting in
dangerously hot conditions over central and southern Utah valley
locations, where Excessive Heat Warnings and Heat Advisories
remain in effect.

A shortwave moving into California is resulting in some enhanced
shear over portions of northern Utah, with enough instability for
the development of showers and thunderstorms. Coverage of storms
will be relatively limited but given these conditions and dry
lower levels, strong and gusty microburst winds will be possible
with any storms that develop through this evening.

The ridge is expected to weaken overnight as the California trough
moves over the top of it. As a result, temperatures tomorrow will
be ever so slightly cooler than today, keeping the threat of
dangerous heat confined to southeast Utah. Also anticipating less
in the way of thunderstorm coverage with storms primarily focused
over the higher terrain, at least earlier in the afternoon.
Guidance is indicating the potential for moisture to start to move
in the area later Saturday, which would result in better than
expected coverage of storms over southern Utah during the evening.

.LONG TERM (After 12Z Sunday)...A shift of the Four Corners high
eastward and the cessation of Pacific Northwest troughing will open
the door to increased moisture from the south beginning Sunday. WPC
has far southern Utah in Marginal risk for excessive rainfall on
Sunday. Based on ensemble PWAT and NBM 75th and 90th percentile
forecasts, rainfall potential has indeed increased for that day.
However, the balance of ensemble guidance indicates that Monday and
Tuesday will have the highest threat of heavy rainfall. Machine
learning excessive rainfall predictions do not indicate a Marginal
(at least a 5% chance within 25 mi of a point) flash flood threat
Sunday but do indicate it on Monday and Tuesday. Interestingly,
the machine learned product retains the Marginal risk into
Wednesday as well. This threat on Wednesday is consistent with a
dramatic increase in forecast PWAT over southern Utah in GEFS
forecasts over the last 24 hours as the machine learned product
uses the GEFS. The EPS is less bullish on high PWAT being retained
through Wednesday but the probability is 80% that the threat will
extend into that day.

Monday and Tuesday in particular will present a very strong contrast
in conditions across the CWA. Above average temperatures and dry
conditions (probability of precipitation <10%) will prevail in
northwest Utah, including the Wasatch Front. South of a line from
roughly Fillmore to Green River the probability of precipitation
increases dramatically, with significantly more cloud cover and
below average temperatures. It is south of that line that the threat
of heavy rainfall and flash flooding will be most pronounced.

The likelihood of heavy rain on Wednesday and later days will depend
on the presence and timing of dry northerly flow over the
Intermountain West associated with troughing in the northern plains.
If the trough is weaker and further north PWAT values similar to
Monday and Tuesday would persist through Thursday. 40% of the
GEFS/EPS membership support the persistence of anomalously wet
conditions.

&&

.AVIATION...KSLC...The primary forecast challenge this afternoon and
evening is the potential thunderstorm development in the vicinity of
the terminal and whether a storm will pass directly over KSLC. There
is a 30% chance for a thunderstorm at the terminal this evening with
temporary impacts from gusty outflow winds and the potential for a
reduction in visibility as well. CIGs will remain high based. By
02Z, southeast winds are forecast to resume and continue through the
remainder of the overnight hours accompanied by high based clouds.

.REST OF UTAH AND SOUTHWEST WYOMING...A risk for temporary
reductions in visibility are possible through approximately 03Z this
evening across much of Utah and southwest Wyoming resulting from
thunderstorms. Gusty outflow winds are possible in and around these
thunderstorms as well. The threats will diminish overnight. Light
diurnally driven wind directions are then forecast through tomorrow
morning.

&&

.FIRE WEATHER...High pressure is centered just east of the area,
resulting in hot and generally dry conditions. Enough moisture
remains for some convection over the higher terrain, and this
could lead to gusty microburst winds, particularly over northern
Utah, through the evening hours. Convection will be more isolated
as conditions stay hot and winds become breezy in some spots,
particularly over west central and southwest Utah. However, deep
moisture will start to move northward into Utah beginning Saturday
afternoon, so minimum RH values will likely be too high to support
more than isolated critical fire weather conditions. The moisture
returns in earnest to central and southern Utah Sunday as northern
Utah stays drier, and this moisture will remain in place through
at least the middle of the upcoming 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 UTZ123-
     124-128.

     Excessive Heat Warning until 9 PM MDT Saturday for UTZ131.

WY...None.
&&

SHORT TERM/FIRE WEATHER...Traphagan
LONG TERM...Jeglum
AVIATION...NDeSmet

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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