Area Forecast Discussion 
FXUS65 KSLC 292149
AFDSLC
Area Forecast Discussion
National Weather Service Salt Lake City UT
349 PM MDT Thu Jul 29 2021
.SYNOPSIS...A monsoonal moisture surge will remain across Utah
and southwest Wyoming through the weekend. This will result in an
extended period of thunderstorm chances with a heavy rain threat,
along with temperatures trending slightly below normal.
&&
.SHORT TERM (Through 12z Sunday)...Afternoon satellite and H5
analysis continue to place the area along the western periphery of
strong high pressure oriented over the western plains, and beneath
a generally light south to southeast flow aloft. Deep layer sub
tropical moisture remains in place with PWAT generally above an
inch. Some subtle drying from east has occurred across eastern
Utah, but mostly at the upper levels an in large is a non factor
given breadth in place. Have noted a rapid uptick in convection
over the last few hours with focused activity across Iron/Beaver
Counties associated with the leading edge of a return flow wave
which induced that subtle drying aforementioned. Outside of this
focused activity, terrain based thunderstorms have developed over
the mountain spine extending from the southern mountains northeast
to the Uintas.
The current wave lifting NW across southwestern Utah will continue
to induce the most widespread storm development into this
evening, with increased developed shifting north over time into
and across Millard and Juab Counties through that time prior to
gradually dissipating. A secondary feature which has been noted in
CAMs for a few runs has initiated fairly widespread Thunderstorms
over the four corners region, with activity poised to shift north
across southeastern and east-central Utah later this evening and
through the middle of the night. Elevated flash flood potential
exists with this latter wave across portions of the Grand
Staircase, Capitol Reef, and portions of the San Rafael Swell
where a Flash Flood Watch remains in effect into the early morning
hours.
For Friday a continued stream of upper level energy will be
shifting across southwestern Utah NW across Nevada expanding the
breadth of anomalous moisture northwest, while also providing
potential for more focused storm development across southwestern
Utah once again. Midnight shift will be assessing the potential
for Flash Flood Watch need across the southwest as such.
Elsewhere, the southeast is expected to be quite suppressed on
Friday in wake of tonight`s wave passage, but remnants of the
wave will likely produce more widespread storms across the north
once again. Any storm will continue to carry the potential for
locally heavy rainfall given increasingly moist lower levels and
environment in place.
More of the same for Saturday, but subtleties of weak return flow
waves and or inhibition from them remain unknown at this time.
The environment will remain primed for another active monsoonal
day however. One note on the larger scale for Friday is a slight
southerly shift and pivot of the high pressure allowing a more
focused southerly flow aloft across the area, this aided by an
upper disturbance off the SoCal coast (a feature to take note of for
Sunday).
.LONG TERM (After 12Z Sunday)...Current projections indicate that
Sunday will finally feature an upper-level disturbance of
significance to complement the pooled moisture in place and
potentially help organize and strengthen thunderstorm development, in
the form of a shortwave trough lifting northeastward across the
Great Basin on the western periphery of the long-lasted ridge to our
east. GEFS ensemble members show decent agreement in the the QPF
surface reflection of the wave, although there are some timing
differences spanning between Sunday afternoon and Monday afternoon.
The NBM QPF probabilities suggest that most guidance favors Sunday
afternoon for this additional boost for shower/thunderstorm
development, but it`s worth mentioned that there has been a trend
towards more support of the threat lingering into Monday. We will
still need to wait a few days for any level of forecast confidence
on this upper-level feature, but assuming it holds we would likely
have better confidence leading into that timeframe than recent days
in terms of locations of higher threat. This is because high-res
models often perform better when they have a synoptic feature to
track, instead of terrain-based convective buildups as we`ve had in
recent days. As has been the case all of this week, the main concern
remains elevated flash flood potential, especially around any
sensitive areas such as slot canyons and burn scars.
Temperatures look to bottom out Monday at 5-10 degrees below normal,
with the biggest drop below normal for northern Utah and southwest
Wyoming. Yes, you read that right - BELOW normal. Of note, SLC has
not had a high below 90 since June 26th, and the current forecast
finally falls below that mark on Sunday.
Global ensembles show high confidence in shutting off the
precipitation threat for at least the valleys fairly quickly, with
the only uncertainty being around a 12-hr window timing difference
of this shutoff and whether it will allow for any activity Tuesday
afternoon. This uncertainty is moreso for northern Utah and
southwest Wyoming; southern Utah should shut off either way by
Tuesday. By the middle of next week onward, convection should become
isolated to only the highest terrain (e.g. the Uintas), with more of
fair-weather cumulus buildups in the afternoons over terrain
elsewhere. This shutoff will occur as a new high pressure ridge
builds in from the southwest, bringing drier southwest flow, with
some potential for smoke, especially for northern portions of the
forecast area.
&&
.AVIATION...
KSLC...VFR conditions are expected through the TAF period with
typical diurnal wind patterns absent any outflow winds. Showers and
thunderstorms have developed over the terrain to the southeast and
southwest of the terminal this afternoon, though at this time they
are not expected to reach the terminal itself or push outflow winds
through it. Additional showers are possible overnight and tomorrow,
with a better chance of impacting the terminal itself tomorrow
afternoon.
Rest of Utah and southwest Wyoming...Showers and thunderstorms will
continue to spread northward this afternoon/evening, with more focus
into southwest Utah and less in northwest Utah today. Erratic
outflow winds are the primary aviation concern, followed by the
potential for brief locally heavy rain should a storm impact a
terminal directly. Due to the heavy rain, storms will be capable of
MVFR/IFR visibilities and may obscure terrain. Outside of storms,
VFR conditions will persist.
&&
.FIRE WEATHER...Deep layer moisture remains entrenched across the
district and will remain in place through Monday prior to a
net drying trend from the west. Daily shower and thunderstorm
activity over the mountains and surrounding valleys will remain
the norm through Saturday, with Sunday most likely to be most
active day of the group as an upper level wave approaches from
the west, then lifts through Nevada into the overnight hours.
The bulk of these thunderstorms will be capable of locally heavy
rain and wetting rains. Regarding RH, values will remain quite
elevated through Monday prior to the drying trend. Min RH values
will remain >30% in large through that time.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flash Flood Watch until 3 AM MDT Friday for UTZ128>131.
WY...None.
&&
Merrill/Van Cleave
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http://weather.gov/saltlakecity
For frequently asked questions about the Area Forecast Discussion
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