Flash Flood Warning 115, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
1254 PM MDT Mon Aug 31 2026

.KEY MESSAGES, Issued 1117 AM MDT Mon Aug 31 2026...

- Numerous showers and thunderstorms capable of producing heavy
  rain this afternoon and evening will result in a heightened risk
  of flash flooding for the southern two-thirds of Utah, where a
  Flood Watch for flash flooding is in effect through this
  evening.

- The threat of flooding is highest across slot canyons, normally
  dry washes, slick rock areas, recent burn scars as well as
  areas that have already received very heavy rainfall this
  weekend including the Sanpete and Sevier Valleys, Castle
  Country, and the San Rafael Swell.

- After some modest drying for the middle of the week, there is an
  increasing probability that deep moisture associated with a
  tropical cyclone in the Eastern Pacific will spread northward
  into Utah ahead of a Pacific disturbance for Labor Day weekend.

&&

.DISCUSSION, Issued 1117 AM MDT Mon Aug 31 2026...
High pressure remains in place over the central
United States this morning, with a trough moving onshore over
California and a stronger trough moving into the Dakotas. In
between these features, Utah and southwest Wyoming are under a
southerly flow that is drawing deep monsoonal moisture northward.
Satellite derived PW values remain around 1.0 inch over southern
Utah, with values predominately in the 0.7 to 0.9 range elsewhere,
with the exception of far northern Utah.

Showers and thunderstorms have been bringing heavy rainfall across
portions of southern Utah through the morning, with many of these
cells producing flash flooding. The threat of flooding will only
increase and expand this afternoon as the California trough moves
across Utah, providing additional moisture and instability.
Increased coverage of convection will only increase a risk of
flash flooding that has been elevated all weekend, as indicated by
the rare moderate (level 3 of 4) risk of excessive rainfall
indicated bu the Weather Prediction Center. Additionally, many
areas, especially eastern Utah, have already seen heavy rain,
leading to saturated soils, further increasing the risk. With this
in mind, have expanded the going Flood Watch for flash flooding
to include the Book Cliffs, Uinta Basin, and west central and
southwest Utah.

The California trough is expected to exit the area by tomorrow
morning, with another low approaching the northern California
coast. Guidance indicates this will allow the southerly flow to
advect drier air into the area, at least over western Utah
beginning tomorrow afternoon, with the drier air becoming more
widespread Wednesday into Thursday as the second trough moves
across the Pacific Northwest into the Dakotas. For Tuesday,
guidance overall looks to be unusually quick with diminishing the
potential of thunderstorms and thus flash flooding, so have
increased POPs given recent trends.

The drying trend looks to be short lived, unfortunately, with a
lot of the longer range guidance indicating the southerly flow
increasing Friday into the Labor Day weekend with the approach of
another Pacific Northwest trough. This would allow a tap into an
even more deeply moist airmass from a tropical system, a rather
substantial influx of moisture and something we are keeping a very
close eye on.

&&

.AVIATION, Issued 1212 PM MDT Mon Aug 31 2026...

KSLC...Showers/thunderstorms will develop mid afternoon over the
Oquirrhs and begin to impact the terminal vicinity with a 30% chance
of gusty winds and towering cumulus. Southerly winds will become
established by mid-afternoon, gusty at times. There is then a 70%
chance that a more organized and widespread area of showers with
embedded thunderstorms will move through the Wasatch Front including
KSLC between 23Z and 02Z, bringing a greater threat of gusty and
erratic winds as well as heavy rain bringing a low chance of MVFR
visibilities and lower cigs. There is a threat of wind gusts as high
as 45 kts with this late afternoon/evening round of thunderstorms.
Quieter conditions are expected overnight with lighter southeasterly
winds.

REST OF UTAH AND SOUTHWEST WYOMING...Afternoon showers and
thunderstorms will develop across the majority of the region today,
initially over southern Utah (including KSGU and KCDC) through the
early afternoon, higher terrain through the mid afternoon, and
finally the majority of the Wasatch Front by late afternoon and
early evening. There is a 70% chance that the late-afternoon and
early evening thunderstorms over the Wasatch Front from Nephi
northward becomes a large area of rain with embedded strong
thunderstorms. Any storms will be capable of gusty and erratic winds
in addition to very heavy rainfall reducing visibility to at least
MVFR in addition to lowered ceilings. This activity should push
northward out of the area and come to an end by 03-06Z, with light
southerly winds persisting overnight into tomorrow.
Shower/thunderstorm coverage tomorrow looks to be more limited as
drier air moves into the region.

&&

.FIRE WEATHER...Showers and thunderstorms are continuing to
develop and will only increase in coverage across the entire
state into the evening as a low pressure system crosses the area
in an already moist airmass. This will bring a high chance (50 to
85 percent) of wetting rain, particularly for the higher terrain
of central and southern Utah. Relative humidities will also remain
on the high side, with very few locations falling below 30
percent. Drier air will start to move in from the west on Tuesday,
but this should not be noticeable for much of the area until
Wednesday into Thursday. This is likely to push minimum relative
humidity values into the teens across western Utah Wednesday into
Thursday. In combination with breezy southerly winds due to a low
along the Pacific coast, this could bring area of critical fire
weather conditions. The period of dry weather should be short
lived, however, with guidance increasingly indicating a
substantial surge of tropical moisture into Utah over the weekend.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flood Watch until 9 PM MDT this evening for UTZ113>131.

WY...None.
&&

PUBLIC...Traphagan
AVIATION...Van Cleave
FIRE WEATHER...Traphagan

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

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