Flash Flood Warning 104, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
433 AM MDT Mon Aug 31 2026

.KEY MESSAGES, Issued 1046 PM MDT Sun Aug 30 2026...

- A moderate risk (level 3/4) for excessive rainfall is in effect across
  portions of eastern and southern Utah Monday.

- A flood watch for flash flooding remains in effect across much
  of central and southern Utah through Monday 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.

- 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 Friday into
  next weekend.

&&

.DISCUSSION, Issued 1046 PM MDT Sun Aug 30 2026...
Convection has largely waned late this evening
across the area, with most of Utah and southwest Wyoming seeing
some relief from the rainfall during the overnight hours. Far
southeastern Utah, largely near Lake Powell, may still see some
shower and isolated thunderstorm development tonight and
during the morning hours as a weak shortwave trough passes across
the Four Corners. Uncertainty still remains with the timing of
this wave, which will ultimately determine the coverage/intensity
of afternoon convection across far southeastern Utah. Earlier
model runs suggested showers moving into the area by 9AM which
would likely result in more limited afternoon instability.
However, recent model trends show this a bit later, coincident
with daytime heating/instability.

Across the rest of the area, widespread thunderstorms will develop
during the afternoon, owing to anomalous moisture in place
coupled with broad-scale dynamics. The aforementioned shortwave
trough moving through the Four Corners will move in sync with a
separate shortwave trough crossing western Utah from southwest to
northeast, providing plenty of lift and deep layer shear,
especially when considering upper-level diffluence already in
place. Thunderstorms will form as this shortwave lifts, first
entering the St. George vicinity late morning and becoming
widespread by mid-afternoon as instability peaks.

While outflow winds are also possible along the leading edge of
these storms (especially over west-central and northwest Utah),
the environment will be very favorable for flash flooding,
notably across southern and eastern Utah and over sensitive areas
such as slot canyons and near recent burn scars. This flash flood
threat is especially high given recent rainfall across much of the
state. High-res guidance suggests storms will be capable of
producing rainfall rates potentially in excess of 1-2"/hr, with
average return intervals (ARIs) exceeding 10-20 years across much
of eastern and southern Utah...locally higher in places.

Conditions trend much drier after Monday, and even drier following
the passage of another shortwave trough mid-week. However, this
break may be short-lived. Model guidance suggest moisture of
tropical origins returns by the weekend, coupled with another
disturbance from the PacNW. Plenty of uncertainty still remains,
but this could yet again increase the flash flood threat heading
into next weekend.

&&

.AVIATION, Issued 1046 PM MDT Sun Aug 30 2026...

KSLC...There is around a 40% chance of light north to variable winds
for a period overnight. Southerly winds likely to increase by mid
morning Monday. Monday afternoon will see convection chances
increase through the afternoon, especially after around 21Z to 04Z
Tuesday. Storms will be capable of brief periods of heavy rain,
lightning, and gusty erratic outflow winds. Additionally, stronger
storms will be capable of brief categorical VIS reductions to MVFR
or so, as well as lowered CIGS.

REST OF UTAH AND SOUTHWEST WYOMING...Isolated showers and
thunderstorms will be possible overnight, especially across central
Utah. Winds will generally favor a diurnally normal direction. on
Sunday, southerly flow will increase as will cloud cover. Afternoon
showers and thunderstorms will once again become more widespread
through the afternoon hours, especially along and east of Utah's
high terrain. Like previous days, these storms will be capable of
periods of heavy rain, lightning, gusty erratic outflow winds, and
small hail. Additionally, the stronger storms will be capable of
periods of decreased VIS to around MFVR or so, as well as some
lowered CIGS. In general, coverage of showers and thunderstorms will
begin to wane late evening into the overnight hours.

&&

.FIRE WEATHER...A significant push of moisture alongside a low pressure system
will bring widespread showers and thunderstorms across a majority
of Utah today, with high probability of wetting rains (50 to 85
percent) across the high terrain of central and southern Utah. In
concert with the widespread rainfall and thunderstorm activity,
high humidity is expected areawide, with very few locations
falling below 30 percent, and many struggling to fall below 40
percent.

A drying airmass will begin to push into the region on Tuesday,
initiating a drying trend through at least Thursday. While humidity
will remain relatively high on Tuesday, a more substantial drying
is expected into the middle of the week, with minimum afternoon
humidity falling back into the teens across the western half of
Utah. By Thursday, afternoon humidity in the teens is expected to
spread into the lower elevations of eastern Utah. An active area
of low pressure on the west coast will bring an increase in
southwest winds alongside this drying trend, bringing elevated to
near critical fire weather conditions across a portion of west-
central and southwest Utah on Wednesday and Thursday afternoon.
For the high terrain across the state, very little, if any, shower
and thunderstorm activity is expected. Moisture looks to return
by the end of the week and over the weekend, bringing increasing
chances for showers and thunderstorms, as well as an increasing
trend in humidity.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flood Watch through this evening for UTZ116>121-123>131.

WY...None.
&&

PUBLIC...Cunningham
AVIATION...Warthen
FIRE WEATHER...Webber

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

Preliminary Local Storm Report
National Weather Service Salt Lake City UT
837 AM MDT Mon Aug 31 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0800 AM     Flash Flood      14 WNW Big Water        37.11N 111.91W
08/31/2026                   Kane               UT   Official NWS Obs

            429 cfs flood wave reported by the gauge at
            US-89.

&&

CRK