Flood and Hail in Utah, May 22 to 26, 2023

Area Forecast Discussion
National Weather Service Salt Lake City UT
326 PM MDT Sun May 21 2023

.SYNOPSIS...Above normal temperatures will continue across the
region through the next seven days. Moisture remains in place and
will continue to lead to the development of showers and
thunderstorms each day, especially over the high terrain and
adjacent valleys.

&&

.DISCUSSION (Now through Sunday May 28)...
The main weather impacts of note through the next week remain:
* The spring runoff continues with elevated flows and some
  flooding on area rivers, with temperatures running concisely
  around 5-10 above average helping to continue snowmelt
* Smoke from Canadian Wildfires creating hazy skies and some
  areas of reduced air quality through early this week
* Isolated flash flood potential across southern Utah through
  Monday, then a marginal (1 of 4) risk of excessive rainfall
  across northern Utah Tuesday - due to daily shower and
  thunderstorm chances trending northward early this week

In the near term today into Monday:
A messy weather pattern continues with a broad ridge across the
western CONUS with an ill-defined low undercutting the ridge near
the Four Corners. This afternoon, on the western periphery of that
low, water vapor satellite imagery reveals southeastward
propagating wave across eastern Utah which is helping to advect
slightly high PW air westward into Utah, while also supporting
convective development across the central Utah mountains and San
Rafael Swell area. This wave will meander southwestward and is
expected stall in place this evening over eastern Utah, while
helping to enhanced the mid-level northeasterly flow over
southern/central Utah. Overall expect this to continue to support
convective development southeastward this afternoon. Mesoanalysis
paints the best MLCAPE across southeastern Utah, however with very
little shear and only about 5 knots of steering flow. Thus storms
are expected to remain pulsey and chaotic with short-lived
updrafts that redevelop somewhat randomly along outflow boundaries
from previous storms. This lack of organization with marginal CAPE
and PW values only around 0.5 to 0.6 in should keep the flash
flood potential very isolated and limited to off chance that
thunderstorm happens to pop up right over a very sensitive area.

On Monday the flow turns around more westerly as the ridge moves
downstream to the east and heights aloft subtly fall across Utah.
This will also advect in higher PW air from the west-southwest
pushing values locally above 0.6 to 0.8 across much of Utah. The
light westerly flow with increased moisture and instability may
better favor locations like Capitol Reef for an isolated flash
flood threat on Monday. In addition the westerly flow with
moisture advection may also support thunderstorms developing over
the higher terrain in Nevada with storm evolution into western
Utah later Monday afternoon and evening, as well as localized
backbuilding of storms across southwestern Utah which could
increase the flash flood risk.

Otherwise, slightly above normal temperatures continue across the
region with Canadian Wildfire smoke remain in place with very
little flow to push the smoke out of the area. The increasing
westerly flow Monday may start the process of removing the smoke,
but with the flow it will likely still take some time, but should
start a slow improving trend for early in the week.

Tuesday into Wednesday:
A trough over the Pacific Northwest will swing a shortwave through
northwestern Utah late Tuesday which will eventually push a
stalled frontal boundary into northern Utah late Tuesday
afternoon/evening. With a frontal boundary, and added right
entrance region jet support over northern Utah, there will be
added synoptic support for a bit more widespread coverage of
showers and thunderstorms across northern Utah. Enhanced CAPE and
shear with synoptic support could support some stronger
thunderstorms with low-end severe potential, although timing will
be critical, as a slower wave evolution would bring the better
forcing into the evening hours as instability wanes, which would
reduce the threat. However, a marginal threat of excessive
rainfall exists, especially as showers thunderstorms could train
along the boundary into Tuesday night leading to higher totals.
With already swollen creeks and rivers due to runoff, will need to
watch this potential closely, however there are still many
details that need to be nailed down as we approach the event.

For the second half of next week:
Ensemble clusters generally keep a trough to our west with varying
amplitudes and exact placements. However overall for our sensible
weather this will bring a drying trend across southern Utah by mid
week, and more slowly to northern Utah later in the week as drier
air is advected into the region from the southwest. Daily shower
and thunderstorm chances will continue across northern Utah, but
with a decreasing coverage and intensity as moisture wanes.

&&

.AVIATION...KSLC...Continued VFR conditions expected with sparse
clouds and limited convective potential through the evening. A
return to southeasterly drainage flow overnight is expected.
Enhanced southerly flow aloft has a low probability of delaying the
shift to northwest flow tomorrow (Monday) morning. Smoke should
continue to reduce in concentration through the next 36 hours.

.REST OF UTAH AND SOUTHWEST WYOMING...Generally expecting VFR
conditions across the area with a low probability of drops to MVFR
in/near showers and thunderstorms. Limited heavy rain potential will
keep VSBYs above IFR even if storms pass through terminals.
Convective activity will fizzle after 02Z tonight and resume late
morning on Monday across southern Utah, especially in the vicinity
of high terrain. Smoke should continue to reduce in concentration
through the next 36 hours.

&&

.FIRE WEATHER...
Scattered showers and thunderstorms will continue to develop
daily, particularly across southern and central Utah today with
localized wetting rains in downpours. The moisture shifts
northward on Monday spreading high humidity and chances for
showers and thunderstorms across the entire state. The moisture
will remain in place and concentrate across northern and central
Utah on Tuesday, while some initial drying sneaks into southwest
Utah by later Tuesday. Shower and thunderstorm chances will
increase particularly across northern Utah in proximity to a
stalled frontal boundary with the potential for a little more
widespread wetting rain for far northern Utah. From Wednesday
through the remainder of the week, southern and portions of
central Utah will trend drier with dry southwesterly flow in place
ahead of a low pressure to our west. There remains some
uncertainty with how long moisture will remain in place across
northern Utah, but there will be potential for daily isolated to
widely scattered showers and thunderstorms particularly over the
northern Utah terrain through the remainder of the week - although
an overall decreasing trend is expected.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flood Watch through Thursday afternoon for UTZ107.

WY...None.
&&

PUBLIC...Church
AVIATION...Wessler
FIRE WEATHER...Church

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http://weather.gov/saltlakecity