Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 301041

Area Forecast Discussion
National Weather Service Salt Lake City UT
441 AM MDT Fri Jul 30 2021

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)...
Early this morning a decaying mesoscale convective complex is in
place over northern Utah, with a discernible MCV in KMTX radar
imagery nearly over Strawberry Reservoir. This is driving the
continued shower activity with embedded thunder across portions of
northern Utah and southwest Utah early this morning. Overall
shower/thunderstorm strength has been diminishing, coincident with
increasing CIN and decreasing CAPE in mesoanalysis and warming
cloud top temperatures. Thus expect scattered showers and isolated
thunderstorms will continue through the dawn and mid morning
hours but will diminish by mid to late morning. Otherwise, another
decaying MCV is present in southern Utah, with a circulation
apparent just north of KICX, however this is only generating a few
very light showers in southern Utah that will continue to
diminish early this morning.

The forecast area will remain on the western periphery of an
elongated ridge axis along the Rocky Mountain spine from Colorado
into northwestern Montana today through the weekend. This will
continue deep southerly monsoonal flow across the entire region
through the weekend. Today, the main focus for convection appears
to be across southwest Utah where convection will fire over the
higher terrain and generally move off the northwest across
southwest Utah including Iron and Beaver Counties. However,
southeasterly low level flow, will also favor back building of
storms along southeast aspects of the southern mountains and the
Grand Staircase area. In addition, models are in decent agreement
on storm initiating in northeast Arizona, and with enough
instability and low-level moisture in place across southwest
Utah, could propagate into southwest Utah later this afternoon and
evening, including the St George, Zion and Hildale/Colorado City
areas. Thus have added a Flood Watch for this whole southwest
corner of the state today, as we will continue storms capable of
very heavy rainfall as we have seen over the last several days.
Otherwise, convection will likely re-initiate over northern Utah
mountains and southwest Wyoming again this afternoon, as long as
we get sufficient clearing behind the morning MCV to help
destabilize the airmass. However convective coverage over the
lower elevation valleys, including the Wasatch front does not
promise to be particularly widespread.

Diurnal thunderstorm chances will continue on Saturday, although
there will be a general lack of synoptic organization and thus
expected storms to be terrain driven and only moving into nearby
valleys. Thus not expecting particularly widespread coverage into
the lower elevations. Sunday continues to look like the better day
for a bit more numerous to widespread shower and thunderstorm
potential as finally a shortwave moving by to west in the
southerly flow helps to slightly lower our 500 mb heights, as well
as subtle 700 mb trough axis passage and a weak jet streak right
entrance region adding to the low-level convergence and upper
level diffluence over the area.

.LONG TERM (After 12Z Sunday)...
Models continue to support a pattern change in the extended
forecast period with drier air coming next week. This will be
preceded on Monday with a weak shortwave trough moving into the
ridge. This trough will mostly just graze northern Utah on Monday,
but should provide some large-scale lift to focus convection more
across northern Utah, although plenty of moisture remains in
place for at least scattered convection elsewhere.

Behind the trough, the high pressure is progged to re-center itself
over southern Nevada. This will bring a drier northwest flow to
Utah. This drying trend will reduce precipitation chances Tuesday,
although cannot rule out some isolated to scattered convection due
to lingering low-level moisture.

The ridge will slowly shift eastward trough the rest of the week.
However, most deterministic and ensemble members show a somewhat
elongated ridge, suggesting a relatively dry southwest flow across
the area. With the ridge in place, however, temperatures should
trend warmer with slightly above normal maxes by late week.


KSLC...Showers with a few thunderstorms over and in the vicinity
of the SLC terminal will continue through around 12-13z. Erratic
winds will be possible with the stronger storms, otherwise winds
will likely prevail from the east to southeast through much of the
morning hours, then trend more south to southwest during the
afternoon. Another round of showers and thunderstorms will be
possible this afternoon, most likely after 21z. CIGS may briefly
fall to or just below 6kft in the heavier showers.

Rest of Utah and southwest Wyoming...Showers and thunderstorms will
continue through around mid-morning mainly across northern Utah and
southwest Wyoming. Showers and thunderstorms will then increase
during the afternoon across southern and central Utah, with some
storms spreading into northern Utah and southwest Wyoming later in
afternoon. Erratic outflow winds are the primary aviation concern,
followed by the potential for brief locally heavy rain. Due to the
heavy rain, storms will be capable of MVFR visibilities and may
obscure terrain at times. Outside of storms, VFR conditions will


A moist air mass will be in place across the region through the
weekend and into early next week. Widespread showers and
thunderstorms will be prevalent over the mountains each day with
scattered showers and thunderstorms over the valleys. Locally
heavy rain will accompany most of these storms with wetting rains
expected. RH values will be elevated through the weekend with min
values >20% today but rise to >35% by Saturday and Sunday. By
Tuesday, the moisture will be abating and RH values in the valleys
should approach 20-25% and into the upper teens by Wednesday. In
addition, shower/thunderstorm chances will retreat to only a
slight chance in the Uinta mountains by Wednesday.


UT...Flash Flood Watch from noon MDT today through this evening for




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