NWS Salt Lake City, UT wrote its last area forecast discussion before the first event at May 13, 9:09 PM MST, 6 minutes earlier.
Area forecast discussion, May 13, 9:09 PM MST, 6 minutes before
Area Forecast Discussion
National Weather Service Salt Lake City UT
1009 PM MDT Sat May 13 2023
.UPDATE...As of early Saturday evening, a closed low was located
over west-central Utah, with a deformation zone and associated
precipitation wrapping around the low, stretching from southeast
Utah north and west across northern Utah. 14.00Z HREF continues to
advertise the heaviest rainfall through ~12AM will fall across
portions of the Swell, Castle Country and adjacent central
mountains, with 3-hr QPF in the 0.75"-1.25" range. Latest radar
trends and observational analysis is beginning to confirm the
emergence of heavier precipitation, as both the strength of radar
echos and extent of areal coverage is on the upswing here within
a region of low level convergence beneath increasing upper
divergence, per the latest SPC mesoanalysis. Much of the
convection in this area hasn't passed over a mesonet site with a
reliable rain gauge, but an RWIS site on I-70 indicated visibility
down to less than 1 mile in heavy rainfall, so certainly
convective cores here contain heavy rainfall. KGJX dual pol
estimates of 1 hr precipitation are in the 0.25-0.50" range.
Cannot rule out an isolated flash flood across slot canyons here.
We are less certain regarding flash flood potential for burn
scars, as much of the trail mountain scar is high elevation and
covered in deep snow, with the exception of lower elevation
south- facing drainages. Latest snowdepth analysis indicates
little snow on the bear burn scar, so will monitor that area
through this evening.
Regarding downslope winds, east wind gusts in the 40-50 mph range
are underway across SW Wyoming and the eastern Cache Valley as
expected. With the exception of the Brigham City area, where
gusts are currently around 45 mph, we have yet to see surface wind
gusts develop across the Northern Wasatch Front. This could be
due to the presence of current and recent precipitation, and this
precipitation may be altering the stability profile in such a way
to where these winds have been unable to mix to the surface.
Forecast soundings and nearby mountain observations suggest winds
have developed at ridgetop level, as snowbasin is currently
reporting gusts near 50 mph, and CAMs indicate 55kt east winds a
2kft AGL developing by 05Z this evening. Available guidance still
suggests that gusty winds will mix to the surface, however the
onset appears to be delayed, possibly until near or just after
midnight, with the exception of the Brigham City area as mentioned
above. All headlines continue as inherited and the previous
discussion follows below.
&&
.SYNOPSIS...A retrograding low pressure system over the northern
Great Basin will bring unsettled conditions through the weekend.
High pressure aloft will become established through much of next
week, bringing above normal temperatures to the region. Increased
atmospheric moisture will remain in place through much of the week
ahead, bringing isolated to scattered diurnally driven showers
and thunderstorms over the high terrain of Utah.
&&
.SHORT TERM (Through 12Z/6AM Monday)...A retrograding upper low
will continue to track westward through the next 24 hours,
increasing the magnitude of easterly low level flow across
northern Utah and southwest Wyoming later this evening through
Sunday morning. Southerly return flow will help bring a surge in
lower level moisture during the day on Sunday, helping to
destabilize the environment. With increased instability and lower
level moisture in place, we're looking at another day of shower
and thunderstorm activity over a more widespread area.
The biggest threat until we arrive at tomorrow's convection will
be the high potential for strong easterly winds across northern
Utah. Current model forecasts showing around 40 to 45 kts of
easterly cross barrier flow over the northern Utah mountains
overnight tonight. Furthermore, forecast soundings have remained
consistent with developing a significant stable layer right around
crest level of the northern Wasatch/ Bear River range overnight
which, when combined with the strong low level flow, will help to
produce strong downsloping winds, particularly in the Cache Valley
tonight through tomorrow morning. HREF ensemble probabilities of
exceeding 55 mph wind gusts in the Cache Valley tonight, mainly
along the US-91 corridor north of Logan, are in excess of 80
percent, giving high confidence in seeing headline worthy winds in
the Cache. Ensemble minimum wind gusts in this same area are only
as low as 50 mph during the overnight hours. With high ensemble
likelihood of seeing wind gusts in excess of 55 mph in this area,
have decided to issue a High Wind Warning starting at 9PM this
evening through noon tomorrow. For the northern Wasatch Front,
probabilities for seeing wind gusts in excess of 55mph are only
around 10-15 percent, however, when looking at thresholds around
45 mph, the likelihood of gusts above this threshold jump to
around 60 to 70 percent. The forecast environment favoring a near
ridgetop stable layer is also supported in this region of Utah,
however, given the higher ridgetop height, the overall chance for
a widespread downslope wind storm is a bit lower. That said, areas
near canyons will likely see some of the stronger winds tonight
(gusts approaching 45 to 50 mph). With these things in mind, have
also issued a Wind Advisory for areas from Farmington through
Brigham City. These winds are expected to ramp up near sundown,
peaking between 2 to 6AM, then decreasing gradually through the
morning hours.
Moving through the remainder of the day tomorrow, the
aforementioned increase in lower level moisture will help to bring
a further destabilization of the atmosphere across the eastern
Great Basin region. Current forecast mean CAPE values from the
HREF showing anywhere from 250 to 750 J/kg/K by tomorrow
afternoon, with noteworthy instability through the -10 to -20C
layer of the atmosphere (i.e. hail growth zone). Additionally,
forecast soundings are in support of an "inverted V" environmental
profile, signifying a relatively dry near surface environment
alongside low level atmospheric moisture. The two of these things
combined suggest that there is a moderate threat for gusty outflow
winds in excess of 40 mph from showers and thunderstorms, as well
as a threat for small hail within stronger thunderstorms. As is
the case with normal summertime convection, these storms will
initiate over the higher terrain features across Utah then drift
into adjacent valley areas. Forecast soundings suggest anywhere
from 20-40 kts of bulk shear with tomorrow's convective
environment, meaning that there will be some support for more
organized convection, especially later in the afternoon.
.LONG TERM (After 12Z/6AM Monday)...Retrograding low pressure
will be positioned in the PacNW Monday. High pressure will be
building, with the low diminishing into the mean flow early in the
week. There is good ensemble agreement that ridging will build
much of the week. Temperatures through the week will be 5-15F
warmer than normal throughout southwest Wyoming and Utah. Still,
the synoptic set up will be rather disorganized. With that, there
is more disagreement on shortwaves or closed lows that could track
to the south in a Rex Block sort of feature.
Much of the week will be dry, but daily chances for showers and
thunderstorms are expected to start around higher terrain. With
minimal wind shear, storms shouldn't last and would likely not reach
far off terrain. A shortwave or closed low that tracks into the area
would bring better chances for valley showers and thunderstorms.
&&
.AVIATION...KSLC...Westward retrograding system will continue to
result in unsettled pattern. Scattered convection persists through
the evening, with a relative decrease overnight, and a resurgence
by late Sunday morning. Convection will be capable of lightning
and gusty erratic outflow winds, in addition to degraded CIGS/VIS.
Winds also highly uncertain, modulated largely by any ongoing
nearby convection. In addition, flow setup associated with
anomalous system may once again yield periodic easterly gusts
overnight.
.REST OF UTAH AND SOUTHWEST WYOMING...An anomalous westward
retrograding system will bring scattered convection through the
evening, a bit of a downward trend in activity overnight, and
renewed convective development by late Sunday morning onward.
Convection will be capable of gusty and erratic outflow winds,
lightning, and degraded CIGS/VIS. Additionally, flow setup
associated with the anomalous system will bring another night with
periodic easterly gusts, some strong, at LGU/OGD/EVW.
&&
.FIRE WEATHER...A low pressure system will continue to move westward through
tomorrow, allowing moisture to surge along the eastern flank of
the system. Increased moisture and instability will bring another
round of widespread showers and thunderstorms to Utah tomorrow
afternoon, with the main threats being gusty and erratic outflow
winds, small hail, and lightning. Additionally, on the eastern
side of the westward departing upper low, strong easterly winds
are anticipated to develop over northern Utah on Saturday evening
through late Sunday morning, particularly along the northern
Wasatch Front and Cache Valley. For the northern Wasatch Front,
gusts in excess of 40 to 45 mph are expected. For the Cache
Valley, areas may see wind gusts exceed 60 mph overnight Saturday
and early Sunday morning.
Increased moisture on Sunday will also bring higher afternoon
relative humidity values. This moisture will remain in place
through the week ahead, however, the gradual warming trend
combined with the little change in atmospheric moisture will bring
a slow drying trend through the week. That said, minimum
humidities will bottom out around the upper teens to low 20
percent range by mid-week. With the moisture in place and abundant
afternoon heating, isolated to scattered diurnally driven showers
and thunderstorms over the higher terrain will be expected
through at least Wednesday across Utah with decreasing certainty
in showers and thunderstorms through the second half of the week.
These showers and thunderstorms will carry similar threats to
those this weekend; gusty and erratic outflow winds, small hail,
and lightning.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory until 9 AM MDT Sunday for UTZ103-104.
Flood Watch through Thursday afternoon for UTZ107.
High Wind Warning until noon MDT Sunday for UTZ107.
WY...None.
&&
ADeSmet/Webber/Wilson/Warthen
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