Scipio, Utah Weather Forecast Discussion

291
FXUS65 KSLC 250948
AFDSLC
Area Forecast Discussion
National Weather Service Salt Lake City UT
348 AM MDT Thu Sep 25 2025
.SYNOPSIS...A slow moving cut-off upper level low will bring the
threat of strong thunderstorms with the heavy rain, small hail,
and strong, gusty winds to southwestern Utah today. The threat of
convection will continue across southern Utah Friday, gradually
shifting north and east through the weekend.
&&
.SHORT TERM (Through 12Z Saturday)...The main story for the short
term forecast period continues to be a slow moving cut-off low and
associated moisture flux into the region. Early morning upper air
and satellite analysis indicates a quasi-omega block remains in
place across the Interior West. The previously mentioned upper
level low is off the central California coast. Yet another trough
is currently noted in the Gulf of Alaska as the northern stream
remains quite active.
With moisture advection ongoing through the day, expect
precipitable water values in excess of the 90th percentile for
this time of year to reach southwestern Utah after 18Z. By this
evening, PW anomalies will reach the 97th to 99th percentile
across southwestern Utah. So that gives us one piece of the
convective puzzle...moisture.
With increasing moisture, expect increasing instability. SBCAPE
values by late afternoon will exceed 500-1000 J/kg. This, combined
with deep layer shear at or above 35-40kts will support strong
thunderstorms with the threat of very heavy rain, small hail and
strong, gusty winds.
Convective allowing models develop convection initially over the
Pine Valleys of Utah and the higher terrain of northern Arizona,
shifting convection into southwestern Utah after 21Z or so. The
HRRR in particular continues to indicate a threat of wind gusts in
excess of 60 mph with this activity. Looking at the HREF, peak
rainfall rates of 0.50-1.00" per hour are likely. This convection
slowly moves north and east...though some training looks likely
especially across Washington County and western Kane and western
Garfield Counties. Any remaining convection should weaken and
gradually diminish after 03-06Z.
Given the set-up with anomalous moisture, strong shear, and
modest instability, both the Storm Prediction Center and the
Weather Prediction Center have outlooked southwestern Utah in a
marginal risk for severe thunderstorms as well as excessive
rainfall. With stronger thunderstorms comes the potential for
higher rainfall rates. Based on this, also upgraded the flash
flood potential for both Snow Canyon and Zion National Park to
Probable.
The upper level low will be near San Diego Friday morning. This
will shift the deepest moisture south...though with the lack of
any front, expect deeper low level moisture to remain across the
lower desert areas of southern Utah. SBCAPE values will likely be
higher Friday than Thursday, with the HREF 25th to 75th percentile
range from 500 J/kg to 1200 J/kg. Missing from Friday, however,
will be the strong deep layer shear, which will only be around 10
kts or so. Still, convective allowing models indicate at least
isolated to widely scattered convection will develop across
southern Utah after 15-18Z Friday. Given slow storm motions and
moderate instability, expect another day of isolated flash
flooding potential, particularly across southwestern Utah.
.LONG TERM (After 12Z Saturday)...The previously discussed cutoff
low is expected to become/ remain quasi-stationary by early Saturday
over the Sonoran Desert region, which will aid in the continued
northward surge of moisture into the forecast area. As moisture is
expected to begin to reach its peak over southern Utah on
Saturday, there are a small grouping of EPS members that favor
significant precipitation occurring over south central Utah on
Saturday as the moisture pairs with the synoptic forcing from the
cutoff low. Early morning forcing on the north side of the low
will bring a low-ish probability (15-20% chance) of isolated
showers across the far southern portion of Utah. As afternoon
heating adds more energy into the mix, anticipate shower and
thunderstorm coverage to expand across the southern half of Utah,
bringing continued risk of flash flooding in slot canyons,
slickrock areas, dry washes, and recent burn scars due to heavy
rainfall.
Through Sunday, a majority of model guidance is in support of
this cutoff low transitioning into an open wave as a stout
longwave trough begins to encroach on the West Coast, forcing the
low to phase with the northern stream. The additional lift
provided by the open wave translating through the region will help
to bring increased coverage of showers and thunderstorms into
central/ northern Utah and southwest Wyoming during the day on
Sunday. As moisture will be peaking with the passage of the wave,
the aforementioned threats are anticipated to expand into these
areas. Additionally, with the cooler temperatures tied to the low/
open and general cold pools produced by widespread precipitation,
will see temperatures decrease areawide on Sunday... pulling us
closer to climatological normals.
As we head through the first half of next week, the
aforementioned longwave trough is expected to push into the
interior western U.S. before lifting northward as it loses
momentum. Current ensemble forecasts are supportive of below
average mid-level atmospheric heights ending right around the
UT/NV border region, favoring increased southwesterly flow across
much of the forecast area. With the ensemble mean model output in
mind, the coldest temperatures in the forecast area are likely to
be reserved to the far western portion of Utah with precipitation
favored in the western and northern half of the area. There is
generally minor variance in the east-west location of this trough
by Tuesday, however, ~30% of models are favoring above normal mid-
level heights (warmer & drier) across Utah on Tuesday, and about
30% favoring near normal to slightly below normal heights (cooler
& wetter). The remaining 30- 40% of members fall in line with the
ensemble mean. Big picture, there is generally high confidence
that temperatures will return to near normal for this time of year
across the region, and medium confidence that we`ll see
precipitation across the northern and western portions of Utah
Monday/ Tuesday. This trough is expected to lift northward late
Wednesday/ early Thursday, ushering in drier air into Utah and
southwest Wyoming.
&&
.AVIATION...KSLC...VFR conditions are expected over the KSLC
terminal through the valid TAF period. Winds are expected to
remain light out of the south through around 19-20Z with periods
of light and variable flow between 18-20Z, then becoming
northwesterly after 20Z.
.Rest of Utah and Southwest Wyoming...VFR conditions alongside
clear skies are expected across the northern half of the forecast
area today while the southern half of the area experiences
increasing coverage of showers and thunderstorms through the
afternoon hours. Winds will largely be dominated by terrain across
the northern and eastern area while southwest Utah will see more
of a south to southwest flow. Areas near/ under passing showers
and thunderstorms are likely to see gusty and erratic outflow
winds in excess of 20 kts, with areas of 40+ kts possible.
Additionally, brief MVFR to IFR conditions will be possible under
heavy rainfall through the afternoon and evening hours across
southern Utah.
&&
.FIRE WEATHER...A slow moving upper level system will help to
bring semi-tropical moisture into southwestern Utah this
afternoon. Strong thunderstorms with the potential for strong,
gusty winds, small hail and very heavy rainfall will develop
across southwestern Utah during the afternoon and shift north and
east with time across the remainder of southwestern and south
central Utah through the evening.
As the upper level low shifts southwest, the threat for
convection will lessen somewhat Friday afternoon and evening. By
Saturday, moisture will again surge north and east across Utah,
with a more widespread threat of convection continuing through the
weekend.
A portion of the ensemble members bring a strong cold front
through Utah early to mid-week. There is a higher than average
uncertainty in the timing and strength of this cold front, but it
is worth monitoring future forecasts.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&
$$
Kruse/Webber
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NWS SLC Office Area Forecast Discussion