NWS Salt Lake City, UT wrote its last area forecast discussion before the first event at Mar 28, 3:47 AM MST, 7 hours 43 minutes earlier.
Area forecast discussion, Mar 28, 3:47 AM MST, 7 hours 43 minutes before
Area Forecast Discussion
National Weather Service Salt Lake City UT
447 AM MDT Tue Mar 28 2023
.SYNOPSIS...Southwesterly flow will bring a temporary warming and
drying trend today. The next storm system will begin to impact
the region by Wednesday, bringing strong winds, potentially
moderate to heavy mountain snow, and valley rain transitioning to
snow on Thursday.
&&
.SHORT TERM (Through 12Z/6AM Friday)...Mid level ridging centered
across the forecast area will slide east this morning, allowing
southwesterly flow aloft to overspread the eastern Great Basin by
this afternoon. This will help erode the cold and stable low level
air mass in place across much of Utah, and allow temperatures to
warm into the upper 40s/low 50s across most valleys. The
exceptions will be the Cache and Bear River Valleys, as well as
the western Uinta Basin, where shallow cold air will likely remain
entrenched through this evening.
The next in a seemingly endless stream of upper troughs is
currently anchored around an upper low spinning just off the
Oregon coast. This low will dig south along the California coast
through Wednesday, before turning inland and slowly crossing the
Intermountain West Wednesday night through Thursday. As this low
drops south, southwesterly flow downstream will continue to
strengthen across the Great Basin, resulting in windy conditions
across the western valleys Wednesday afternoon and evening. Have
gone ahead and issued a Wind Advisory for these locations, and
although there is some potential High Wind Warning criteria is
reached, increasing cloud cover during the afternoon may limit
mixing of stronger momentum to the surface.
The surface front/low level baroclinic zone associated with the
approaching trough will lift across the forecast area Wednesday
evening through Wednesday night, accompanied by a band of
precipitation. This band will be enhanced somewhat by remnant
moisture from a decaying AR spreading around the southern end of
the Sierra. This precip will initially favor areas who do well in
SW flow, with snow levels near 7500 feet at the onset, falling to
near the benches of the I-15 corridor by late Wednesday night.
The upper trough will slowly cross the region Thursday
afternoon/evening resulting in another period of widespread
precipitation with snow levels in the 4500-5000 foot range. This
will be followed by a window of favorable orographics within a
moist and unstable northwest flow late Thursday through Thursday
night. This will favor the I-15 corridor and adjacent terrain. At
this time have held off on winter headlines for the higher terrain
given the onset of impactful snow holds off until Wednesday
evening/night, but will be needed as the event draws closer.
.LONG TERM (After 12Z/6AM Friday)...Global deterministic and
ensemble members are in excellent agreement to start the long term
forecast period, with the consensus depicting a broad area of low
pressure across SE Idaho or south-central Wyoming Friday morning,
resulting in cold northwest flow (700mb temps -9C to -11C) across
the area. Some semblance of a shortwave is seen embedded in the flow
dropping south across northern Utah during the morning hours. If
this setup seems familiar, you're correct as a similar setup was in
place Monday morning across the region. The difference with this
Friday morning setup is that it is not forecast to be quite as cold
as Monday morning (when -16C observed on the 27.12Z KSLC sounding),
so the lake effect/enhanced potential is low (~20% chance) at this
time. However, given the presence of cold, cyclonic flow and quasi-
stable NW flow, the Friday morning period could be productive for at
least those portions of the northern mountains that do well in such
a flow regime, such as the Upper Cottonwoods. Indeed, 6-hr snowfall
probabilities of 6" of snow Friday morning are in the 30-40% range
across the Ogden area mountains, and in the 20-30% range across the
Sessions and Cottonwoods. This activity is expected to gradually
wane through Friday afternoon as cyclonic flow and moisture
gradually wanes.
Deterministic and ensembles are then in reasonably good agreement
through the weekend, depicting a transient, shortwave ridge axis
cresting across the area Saturday. This would lead to a slight
moderation in temperatures and dry conditions.
Par for the course for this season, active weather then quickly
returns as a trough digs across the PacNW Sunday night into Monday.
Consensus then suggests this trough will continue to dig into the
Great Basin into early next week. Clusters are all very closely
aligned through day 6, then begin to exhibit some minor differences
by day 7. Differences appear to revolve around how quickly the
trough amplifies. Regardless, the underlying theme will be a return
to unsettled conditions to end the weekend and begin next week,
complete with additional rain and mountain snow. Given 70% of
clusters indicate the presence of an anomalously cold, deep low
across the Great Basin, widespread precipitation and below average
temperates will result. In fact, latest probabilities for measurable
snow at SLC are already medium (~50%) in the Monday-Tuesday
timeframe. It is too early to be able to dive into the details
regarding specific impacts, but any hope for the emergence of
springlike conditions is not on the agenda in this forecast period.
&&
.AVIATION...KSLC...VFR conditions expected through the period with
increasing high and mid level clouds as a transient mid level high
pressure ridge moves east across the area. As the ridge does slide
east, southerly flow will increase this afternoon, with S winds in
the 15-25kt range expected to develop between 17-18Z. Some
uncertainty regarding whether or not winds will decrease around
sunset. Currently, most likely scenario suggests they will decrease
around 02Z, but a low, or 30-40% chance exists for S gusts to carry
through through the evening. Very low chance for an afternoon wind
shift to the NW.
.REST OF UTAH AND SOUTHWEST WYOMING...Shortwave ridge axis moving
across the area today will bring benign conditions, although patchy
fog has developed in the Cache and Bear River Valleys this morning.
This should mix out towards sunrise as increasing mid and high
clouds move in from the west. Otherwise, VFR conditions expected
through the period.Increasing southerly winds expected to develop
Tuesday afternoon, with speeds reaching 15-25kts across the more
wind prone terminals in a southerly flow regime.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from noon to 9 PM MDT Wednesday for UTZ101-102-115-
122.
WY...None.
&&
Seaman/ADeSmet
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