NWS Salt Lake City, UT wrote its last area forecast discussion before the first event at Apr 25, 11:57 AM UTC, 1 hour 13 minutes earlier.
Area forecast discussion, Apr 25, 11:57 AM UTC, 1 hour 13 minutes before
Area Forecast Discussion
National Weather Service Salt Lake City UT
557 AM MDT Sun Apr 25 2021
.SYNOPSIS...A trough will strengthen along the West Coast today
which will result in increasingly strong southwesterly flow across
western Utah valleys. The associated cold front will gradually work
its way across western Utah tonight and the rest of the state Monday
resulting in widespread precipitation.
&&
.SHORT TERM (THROUGH 12Z WEDNESDAY)...A southwesterly flow aloft
remains in place over Utah this morning ahead of the closed low
currently situated off the PacNW and NorCal coast. Meanwhile, a
surface trough, which moved into the area yesterday, extends across
west-central to northeast Utah. On the north side of this trough,
high-based showers are evident on radar imagery. As the pattern
amplifies through the day, southerly flow will continue to
strengthen across much of southern and central Utah. Increasing low-
level convergence into the boundary north of surface trough will
then cause precipitation to fill in more, but the bulk of
precipitation is expected to remain north and west of Utah through
the afternoon hours given the slow progression of the positively-
tilted trough.
Of greater concern through this afternoon will be the strengthening
south winds south of the surface trough as H7 flow increases to 35kt
across southeast Utah and 50kt across the southwest and the surface
pressure gradient tightens. Already, some southwest Utah sites have
seen winds mix down with gusts over 45 mph already reported
overnight at Cedar City. Many southern and central Utah valleys
remain under a Wind Advisory, with a High Wind Warning for the
typical windier area of the western valleys between Cedar City and
Delta. Even in areas across southern, central, and eastern Utah that
are not under any wind highlights, winds will still be breezy to
windy.
Precipitation will begin to slowly spread south and eastward into
Utah this evening, but it will likely be late tonight when the upper
jet arrives before it really gets going across much of northern
Utah. Moisture is still expected to be plentiful given an
atmospheric river with IVT anomalies of over 5 standard deviations
with 30-yr return intervals in both the NAEFS and ECMWF streaming
into Utah. The boundary will continue to slowly shift east and south
across the area through the day tomorrow, with precipitation
becoming more widespread. Expect valley rain and mountain snow
through the day. By tomorrow night, the airmass will cool enough
that the rain may mix with or briefly change to snow in some
valleys. Otherwise, windy conditions will continue across southeast
Utah into at least the midafternoon hours tomorrow ahead of the
boundary.
The upper low will continue to slowly move across the desert
southwest on Tuesday, but subtle differences in the models regarding
the orientation of the low makes things a bit more complicated. The
GFS brings precipitation into northern Utah in conjunction with a
deformation axis while showers trend downward across the south.
However, the EC solution maintains the H7 trough axis (by this time,
it will be more of a shear axis) across southeast Utah while the
northern deformation axis remains north and west of Utah. So while
the GFS focuses precipitation across the north on Tuesday, the EC is
more focused across the southeast. Looking at cluster analysis of
GEFS, EPS, and Canadian ensemble members, a larger percentage of
members are more in line with the EC, so have trended the forecast
in that direction.
The low will finally start shifting into New Mexico Tuesday night,
allowing high pressure to nudge into northwest Utah with a more
stable airmass spreading into the area. Overall, northern and
portions of western Utah will be quite wet owing to the atmospheric
river. GEFS ensemble means indicate storm totals of just under an
inch at SLC (which is a bit less than the deterministic GFS). This
is fairly in line with what the NBM has, although it is still worth
mentioning that the SREF means are still coming in lower at around a
half inch. The northern mountains will see significant snow with up
to a foot of storm total accumulation expected. The heaviest rates
will be tomorrow morning, which may result in significant travel
impacts over the higher passes. A Winter Storm Watch remains in
place until the day shift has the opportunity to coordinate with
partners.
.LONG TERM (After 12Z Wednesday)...A winter-like trough expected to
bring widespread impacts across the forecast area will continue
south and east over the NM/TX region at the beginning of the
extended period, allowing a high amplitude ridge to begin building
into the region. Ensemble means continue in great agreement of this
feature dominating the forecast from Wednesday through at least
Saturday. Because of this, expecting warm and dry conditions to
prevail across Utah and Uinta County, Wyoming Wednesday through
Saturday.
As 700mb temperatures begin to warm on Wednesday, should see surface
temperatures recover to right around seasonal norms. This high
amplitude ridge will build over the western U.S. through the
remainder of the work week, causing a rapid warming trend across the
area. By Thurdsay, daytime high temperatures are anticipated to rise
to ~5-10 above average. Temperatures continue to warm through the
end of the work week, with highs stretching to ~10-20 degrees above
average by Friday. Thankfully, this warming trend will likely
stabilize by Friday as ensemble means favor the ridge cresting
overhead. Turn to Friday evening and guidance begins favoring a
flattening of this ridge as a Pacific trough begins to move inland.
This trough is currently progged, with fairly high confidence, to
deepen over the region towards the end of the weekend. As this
trough settles, an up tick in winds can be expected, and may bring
fire weather concerns to portions of the forecast area. Precip
chances increase through Sunday, especially across northern Utah and
southwest Wyoming. Not getting into to much detail with this lead
time, but starting to anticipate a cool down at the very least, with
less certainty in the precipitation impacts.
&&
.AVIATION...VFR conditions expected to prevail over the KSLC
terminal through the valid TAF period, with minimal operational
concerns expected. CIGs are anticipated to linger around 10kft AGL
through the day, with 8kft AGL CIGs possible in nearby showers.
Light west to southwesterly flow will remain in place through the
morning hours, becoming gusty out of the northwest in the afternoon.
&&
.FIRE WEATHER...Gusty winds and low humidities continue bring
critical fire weather conditions today across all zones where fuels
have been deemed critically dry. Winds will slowly decrease from
west to east late tonight into tomorrow. That being said, portions
of southeast Utah will likely continue to see critical fire weather
conditions through at least mid to late afternoon tomorrow. As such,
have extended the Red Flag Warning for those zones.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Red Flag Warning until 9 PM MDT Monday for UTZ489-498.
Wind Advisory from 11 AM this morning to 10 PM MDT this evening
for UTZ116-121-123-124-128>131.
High Wind Warning from 11 AM this morning to 10 PM MDT this
evening for UTZ115-122.
Red Flag Warning until 9 PM MDT this evening for UTZ482-495-497.
Winter Storm Watch from late tonight through Monday evening for
UTZ110>112.
WY...None.
&&
Cheng/Webber
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