Flash Flood Warning 125, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
226 PM MDT Sun Oct 2 2022

.SYNOPSIS...Unsettled weather will continue across portions of
southern and eastern Utah through Monday. Upper level ridging
will build into the region Tuesday and persist through much of
the remainder of the week.

&&

.SHORT TERM...Water Vapor satellite imagery this afternoon shows a
broad circulation center impacting much of the Western US,
centered over Eastern Montana with bands wrapping around
cyclonically as far south as Northern Arizona. A shortwave trough
can be picked out over along the Utah/Nevada border near Wendover,
sliding southeastward. In advance of these feature, a number of
towering cumulus stretch from around Mesquite Nevada to the
Uintas. Area radars show heavy rain as the main threat with these
storms along with small hail. A number of Special Weather
Statements and a Flash Flood Warning have been issued thus far
today over our area.

Showers and thunderstorms continue to become more numerous across
southern and eastern Utah this afternoon in response to the
increasing divergence aloft ahead of a shortwave pivoting around the
upper-level low centered near the WY/ID border. Within this area of
ascent, low-level southwesterly moisture advection is helping to
increase instability while mid-level lapse rates also continue to
steepen. This is resulting in a broad area of about 1000 J/kg of
MLCAPE across southern and eastern Utah ahead of a surface front
draped through central Utah. In addition, bulk shear values are
increase to 30-35 knots along and ahead of the front, with fairly
straight hodographs through the mid-levels. Given this, left and
right splitting supercells are expected, and have been observed on
radar so far, with some upscale aggregation on colliding cold pools
to mulicell clusters expected. With the initial round of convection
aided by orographic ascent along the central spine of mountains of
Utah, these storms will evolve eastward across eastern Utah through
this afternoon, with the main threats being large hail, strong winds
and locally heavy rainfall which could result in flash flooding.
Into this evening, along the boundary stalled across west-central
Utah, another round of showers and thunderstorms may redevelop
with additional training of storms into the evening as the
shortwave interacts further with the stalled boundary. The 12Z
HREF membership favor a Cove Fort to Fish Lake to Goblin Valley
line southward to around Panguitch for a heavy rain threat. While
there are less threats in the area, we will have to watch the
Halfway Hill Burn scar and also Marysvale vicinity which has had a
number of debris flow events in recent year.

A few showers may linger overnight from around the Sevier Valley
southeastward to Lake Powell. Going into tomorrow, the deeper
moisture will be shunted to south of I-70 as one final shortwave
trough pushes through northwest flow aloft. This wave will be
trailing the parent upper-level closed low which at this point
will be moving into North Dakota. Drying can be expected north of
I-70, while afternoon convection will fire over the higher terrain
south of I-70 and generally east of US-89. Interrogating HREF
fields via the DESI tool shows that while simulated cores do not
look particularly strong, they will be taking a favorable down-
drainage trajectory for most rec areas from around Capitol Reef
southwestward to the Paria drainage. Thus, one more day of flash
flooding threat is in store for Capitol Reef / Grand Staircase /
Glen Canyon, with some threat for all southern Utah mountains due
to the general convection threat. Drier air moving in Monday
night will finally bring an end to any thunderstorm threat.

.LONG TERM (After 12Z/6AM Tuesday)...Long term period looks to
have a much less active pattern setting up. An amplifying ridge will
be over our area for the period. North to northwest flow aloft
indicates subsidence. With the ridge and northwest flow aloft, much
calmer conditions are expected. Temperatures are expected to be 5 to
10 degrees above normal. Overall this is shaping up to be a
beautiful week as far as temperatures and weather go to enjoy the
fall colors.

At the end of the week, the pattern appears to be shifting to a more
active one. Sixty percent of all ensemble members have a shortwave
that moves through the area this upcoming weekend. This increases
the chance for precipitation as PWAT values for central and
especially southern Utah range between 0.60" and 0.80". The best day
for precipitation chances appears to Sunday at the moment. The other
40% of ensembles have the ridge axis shifting either eastward or
westward and also have the ridge weakening slightly. While there is
a decent chance that the calm and nice weather will continue into
next week, the trends for a more active pattern have been increasing
slightly over the past couple of days.

&&

.AVIATION...KSLC...VFR conditions will prevail for the valid TAF
period. Northwest flow is expected to continue until roughly 03-04z,
when light southeasterly flow will prevail. Northwesterly flow will
return around 17-18z.

.REST OF UTAH AND SOUTHWEST WYOMING...Scattered showers and isolated
thunderstorms will continue to develop across much of central and
southern Utah through at least 03z. Additional showers and
thunderstorms are expected through roughly 08-09z between I-70 and
KBCE and east of I-15. Periods of gusty and erratic winds are
possible near thunderstorms. MVFR to IFR VIS is possible within
heavy rainfall. Northern UT and southwest WY will remain dry, though
a light, isolated shower may reach as far north as KHCR before
sunset.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

SHORT TERM...
LONG TERM...
AVIATION...

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