Severe Thunderstorm Warning 15, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
430 AM MDT Wed Jun 29 2022

.SYNOPSIS...Thunderstorm activity will continue through the week,
with the main threat being gusty winds for northwest Utah and
heavy rainfall potential for central and southern Utah.
Some drying is expected Friday through the weekend with
increasingly breezy conditions developing by Sunday for the
western half of the forecast area.

&&

.SHORT TERM (Through 12Z/6AM Friday)...Latest analysis depicts a
weakening ridge across the Desert Southwest whose northern extent
has become flattened by a trough propagating eastward across
eastern Washington, Oregon and the Idaho Panhandle. This process
is resulting in the development of broad southwest flow across the
region. A round of height falls is in progress across northern
Utah thanks to the influence of this trough. A weak attendant cold
front will develop today across northern Utah and remain in place
through Thursday before weakening. A couple of jet streaks will
propagate eastward near and to the north of northern Utah and SW
Wyoming through Thursday night, providing modest large-scale
ascent. While all of this is happening, theta-e/moist advection is
underway as deeper moisture oozes northeastward through Thursday,
especially for areas along/east of I-15 and along/south of US-6.
A fairly sharp moisture gradient will be in place, with much drier
conditions across northwest Utah through the period. PWATs are
forecast to range from near to slightly below normal across NW
Utah to 175% to briefly 200% or normal across south-central Utah
this evening, to illustrate the sharpness of the gradient.

In terms of sensible weather, high-based showers with an isolated
thunderstorm are currently ongoing across portions of the state
(extreme northwest and SE portions are the exception early this
morning), seemingly tied to the mid-level moisture advection and
height falls. Shower/thunderstorm activity will become more
organized once again late this morning/early afternoon,
especially over the higher terrain of central and southern Utah as
well as the Uintas. Modest steering flow in the 15-25kt range
will promote general northeastward propagation of convection.
Heavier cores will once again be capable of producing locally
heavy rainfall. The storm motions will limit residence time of
cells over a particular location; however training of cells could
become problematic and increase flash-flood potential for burn
scars, normally dry washes, slickrock, steep terrain and slot
canyons. Areas on the northern and western fringes of deeper
moisture will have a greater microburst/gusty thunderstorm outflow
wind threat once again today owing to high values of DCAPE. An
analysis of HREF QPF thresholds suggests the greatest potential
for heavy rainfall today resides across the central mountains
stretching northeastward towards the Book Cliffs, however heavy
rainfall is possible anywhere along/east of I-15 and along/south
of US-6 as well.

Shower/thunderstorm activity will tend to diminish in coverage
and intensity as we head through Wednesday evening, however think
at least isolated showers/storms will linger through the night,
especially along/east of the central and southern mountains where
moisture advection and modest large-scale ascent continues. On
Thursday, both large-scale ascent and steering flow wanes during
the afternoon as the Northern Rockies trough begins to weaken.
Expect showers/storms to once again initiate over higher terrain
areas during the late morning and early afternoon. CAPE profiles
are more robust on Thursday, owing to higher moisture quality
forecast to be in place by then. Deeper moisture (PWATs of 175% of
normal across SE Utah) and slightly lighter steering flow that
gradually trends westward by Thursday evening signals a slightly
higher flash flood risk across southeast Utah, especially for
places such as Capitol Reef and surrounding slot canyons. HREF QPF
through the DESI viewer suggests that the stronger cores (high-
end of the QPF distribution) will be capable of producing 1-1.25"
of rain, with the ensemble mean indicating 0.25-0.50" of rainfall
beneath the heavier cores. A Flood Watch may be needed across SE
Utah, but have held off for now due to a general lack of low-level
moistening and relatively high bases.

.LONG TERM (After 12Z/6AM Friday)...Ridging is likely to be pushed
far enough east that monsoonal moisture will be more limited Friday.
Scattered showers and thunderstorms are expected mainly for the
eastern third of the CWA. Precipitable water values will top 0.8" in
this portion of the state. Outflow winds and flash flooding near
stronger thunderstorms will be the main threats.

As the monsoonal push diminishes more Saturday, showers and
thunderstorms will form more over higher elevations over eastern
Utah and drift northeast with the synoptic flow, which will be
increasing from the southwest. A trough approaching the Pacific
coast will increase the flow. Red Flag Warning conditions could be
met for some of western Utah Saturday, but more likely by the end of
the weekend.

There is good ensemble agreement in that trough moving inland either
later Saturday or early Sunday. That will allow for a strengthening
pressure gradient and wind gusts over 30 knots to mix to the
surface. Red Flag Warning conditions are likely with minimum
relative humidity values in the teens. There is high confidence in
that throughout western Utah, with more borderline Red Flag
conditions further east.

Winds will be enhanced Monday as the trough lifts northward over the
PacNW. Red Flag conditions are expected again. Ensemble members from
the EPS, GEFS, and CMCE have similar speeds Sunday and Monday, with
Sunday more likely to have slightly stronger peak winds.

Conditions are likely to stay warmer than average through the
weekend and into the following week, trending more dry.

Less wind is expected Tuesday as ridging retrogrades westward, far
enough where monsoonal moisture advection is not expected, but small
position changes in guidance could result in better moisture
advection.

&&

.AVIATION...KSLC...Southeast winds are expected through the morning,
with a transition to northwest flow around 18Z. Scattered showers
and thunderstorms are possible after 20Z, but dry conditions will
allow for erratic outflow winds. Minimal precipitation is expected.
Shower and thunderstorms chances will diminish through the evening.
Winds will transition to southeast around 03Z.

.REST OF UTAH AND SOUTHWEST WYOMING...Scattered afternoon showers
and thunderstorms are expected throughout southwest Wyoming and
Utah. With very dry conditions, erratic outflow winds will be the
main threat with not much precipitation expected. Southwest winds
will gust near 30 knots in southwest Wyoming and southern Utah, but
nearby showers or thunderstorms could alter direction and enhance
speed. Shower and thunderstorm chances will decrease from 03-06Z.

&&

.FIRE WEATHER...A ridge of high pressure has moved to the south and
east of the state and has undergone some weakening. Meanwhile, a
system moving east through Northern Rockies has dragged a weak
cold front across northern Utah, which will remain in place
through Thursday before diminishing. This will leave southwest
flow in place across the state from today through at least the
early portion of next week. Increased moisture will move up from
the south today and tomorrow, providing scattered mainly wet
showers and thunderstorms, with an isolated, dry storm possible on
the northern and western edges across far northern and western
Utah. The heaviest shower and thunderstorm activity is once again
expected mainly along/east of I-15 and along/south of US-6 today,
with the axis of greatest thunderstorm coverage shifting slightly
eastward on Thursday. Light showers and isolated thunderstorms
this morning will increase in coverage this afternoon, with
locally heavy rainfall expected both this afternoon/evening as
well as Thursday afternoon/evening. Showers and storms will be
capable of producing heavy rainfall, frequent lighting and
gusty/erratic outflow winds, especially across northern and
western Utah on the drier fringes.

As we head into Friday, slightly drier air will move into the
state from the west, restricting showers and thunderstorms to
the eastern portion of the state, with isolated to locally
scattered coverage expected. A mix of wet and dry activity
remains expected.

From this weekend into early next week, an area of low pressure
remains on track to approach the Pacific Northwest and essentially
remain parked here into early next week. At the same time, a ridge
of high pressure is expected to build across Texas and across areas
near the four-corners. This will cause an increase in southwesterly
winds, especially from late this weekend into early next week across
western portions of the state. The combination of gusty southwest
winds in the 25-35 mph range with locally higher gusts and
minimum humidity in the high single digits to low teens may lead to
a wide area of critical fire weather conditions. Farther east,
chances for isolated showers and thunderstorms are expected to
continue for eastern portions of the state into the weekend.

&&

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

ADeSmet/Wilson

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

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php

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