Flash Flooding and Damaging Winds in Utah, September 13 to 15, 2022

Area Forecast Discussion
National Weather Service Salt Lake City UT
338 AM MDT Tue Sep 13 2022

.SYNOPSIS...The remnants of Tropical Storm Kay will move across
Utah and southwest Wyoming today, resulting a wet day across much
of the area. Showers and thunderstorms will be widespread across
the area again on Wednesday before the primary threat of
convection shifts to northern Utah by Thursday. Coverage of
showers and thunderstorms will gradually decrease as the weekend
approaches.

&&

.SHORT TERM (Through 12Z Thursday)...High pressure is centered
over Colorado this morning, putting Utah and southwest Wyoming
under a southerly flow aloft. This is drawing tropical moisture
northward, with satellite derived PW values as high as 1.0 inch
over southwest Utah. This moisture will only increase during the
day today as shortwave energy, including the remnants of Tropical
Storm Kay, makes its way across the area. By the afternoon PWs
of 1.0 inch or greater should be common across the entire forecast
area, with the remnants providing additional instability for
convective development. This is a somewhat unusual setup for Utah,
as it is a bit uncommon to get tropical remnants moving through,
so am anticipating a heightened risk for flooding. Limiting factor
with scenarios such as this is always cloud cover, which will be
ample and could act to inhibit convective development and make
this more of a steady rain. However, areas directly impacted by
shortwaves within this remnants will see the greatest development,
with the CAMs indicating the risk of this is highest over
southwest Utah, particularly during the mid to late morning. Going
Flood Watch for Lower Washington County and Zion NP looks good and
have added southwest Utah to the watch due to the potential for
urban flooding in Iron county, with many CAMs showing high totals
for Cedar City and Enoch.

During the afternoon, shortwave energy from the remnants of Kay
will shift into northern Utah, heightening the threat of
convective development. CAMs have been consistent in showing the
greatest threat of mid to late afternoon convection in
northeastern portions of the forecast area, where several active
burn scars exist over the higher terrain, including Bald
Mountain/Pole Creek, Dollar Ridge, East Fork, and Trail Mountain.
Continue to have some concerns about just how heavy the rainfall
will get in this area with the cloud cover, but given the
sensitivity of many of these burn scars, am highly concerned about
the possible development of debris flows and have added a Flood
Watch to this area.

The tropical remnants are expected to make their way out of the
area tonight/Wednesday morning. However, the associated moisture
will remain in place and instability should be enhanced by
additional shortwave energy moving in from a trough to the west.
Though it looks like best coverage of precipitation will be over
central Utah, given the amount of moisture it is looking like
another active day for much of the area. Associated cloud cover
will keep high temperatures a bit below seasonal normals for both
today and Wednesday.

.LONG TERM (After 12Z Thursday)...Long term period
continues to show a longwave trough setting up over the area with
shortwaves moving through this larger scale system. The period
begins with a shortwave moving through the area Thursday into Friday
morning. This shortwave will provide ample energy and coupled with
tropical moisture, PWATs are expected to be between 0.75" and 1.00",
will provide good chances for showers and thunderstorms especially
over northern UT and southwest WY. This shortwave will leave behind
excess energy to aid in the development of more showers and
thunderstorms during the day Friday for northern UT and southwest
WY. Southern UT looks to begin drying out Thursday and continue
drying out for the rest of the period. However, an isolated terrain
induced storm will be possible throughout the period.

By Saturday another shortwave is expected to move through and effect
northern portions of the CWA. This will give another chance at
showers and thunderstorms as PWAT values remain around 0.75". Moving
on into Sunday, a low pressure system is expected to move onshore
and get closer to our CWA. With the positioning of this system,
guidance shows a chance of gusty winds Sunday afternoon for
southwest UT. Otherwise isolated precipitation chances will be
possible for northern UT and southwest WY.

There is a decent amount of uncertainty at the tail end of the
forecast period. First and foremost is whether or not a closed low
forms as part of this next system early next week. About 65% of all
ensemble guidance points to this outcome. Another area of
uncertainty is the location of this potential closed low or next
system. By Saturday, all ensembles agree on the amplitude and
location of the system. For the rest of the forecast period however,
about 55% have the system to our west and 45% have the system
overhead or to our north. This overall uncertainty will have an
impact on whether or not we see temperature or precipitation changes
early next week.

Overall, expect a wet period for northern UT and southwest WY and
drying for southern UT. Temperatures for the forecast period are
expected be slightly below average with highs in the upper 60s to
upper 70s for the northern portion of the CWA and mid 70s to around
90 degrees for the southern portion.

&&

.AVIATION...KSLC...Southerly winds expected to stay for the forecast
period. Scattered showers will move in around 14Z and scattered
thunderstorms will be possible later in the afternoon around 23Z.
The latter will have the chance for thunderstorms as well. CIGS will
mainly stay around 7000ft which may cause mountain obscuration.
While VFR conditions are expected for the beginning part of the
period, MVFR conditions will be possible as either cloud decks lower
or the VIS drops when under a storm. Gusty and erratic outflow winds
will also be possible with these storms.

.REST OF UTAH AND SOUTHWEST WYOMING...Scattered showers and
thunderstorms will be possible across the CWA for the forecast
period. These showers and storms will impact southern and central UT
first before impacting northern UT and southwest WY during the mid-
morning hours. Any of these storms will have the potential to
produce heavy rain and gusty and erratic outflow winds. CIGS are
expected to be around 5000ft to 7000ft. While MVFR conditions are
expected, MVFR conditions will be possible under these storms
depending on the rainfall rate and if CIGS drop below 3000ft.

&&

.FIRE WEATHER...The remnants of Tropical Storm Kay will move across
Utah today, bringing abundant tropical moisture and ample
instability. As a result, numerous to widespread showers and
thunderstorms will develop over much of Utah today. The focus this
morning will be over southwest Utah, with the threat of
precipitation spreading northward during the day with the movement
of the remnants. Wetting rains will be likely with any storms that
develop, with an elevated threat of flash flooding, particularly
over southwest Utah and for the burn scars of eastern Utah.
Wednesday will be another wet day for much of Utah before the
threat shifts into northern Utah for Thursday. By the weekend,
showers and thunderstorms will become more isolated.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flood Watch through late tonight for UTZ111>113-117-122>124.

WY...None.
&&

SHORT TERM/FIRE WEATHER...Traphagan
LONG TERM/AVIATION...Cecava

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