Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 252304
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
404 PM MST Thu Feb 25 2021

.SYNOPSIS...
The first in a series of weather systems will move through
northern Utah tonight through early Friday. A stronger storm
system will increase coverage and intensity of snow late Friday
into Saturday. Dry conditions are expected to return on Sunday as
the storm exits the region.

&&

.SHORT TERM (Through 12Z Sunday)...
Mid-level wave seen in GOES water vapor imagery late this
afternoon over the Pacific Northwest will push through northern
Utah tonight through early Friday morning. Increasing west-
southwesterly flow ahead of this storm system will increase snow
showers over the northern Utah mountains, mainly after midnight,
with a transition to moist northwesterly flow through mid-Friday
morning. This will bring a period of mountain snow with some light
accumulations. Meanwhile valley locations will be starting out
quite dry ahead of this initial storm system, as evidenced by the
current dew point of 5F at SLC. Top down moistening with only
modest dynamics will mainly yield cloud cover, virga, and
flurries or very light snow showers. This initial wave exits to
our east by mid morning Friday, with just some lingering
orographic snow showers over the northern Mountains through
midday.

However, the next storm system comes quickly on the heels of the
first, with flow backing around to the southwest and warm
advecting again Friday afternoon. Expect to see mountain snow
redevelop by Friday afternoon, first near the Idaho border, but
then developing southward down the Wasatch and western Uintas. A
sharp cold front passage is then expected with this storm system
Friday late afternoon and evening from northwest to southeast.
Expect a brief window of gusty winds, along with a band of
convective snow and graupel showers, with some possible rumbles of
thunder, to accompany the cold front passage into central Utah.
Behind the cold front passage, moist, unstable northwesterly flow
will remain in place. Expect a brief lull in the subsidence behind
the cold front passage, but orographics will quickly dominate the
forcing overnight for northwest flow favored mountains (including
the Cottonwoods). There is potential for lake enhanced snow off
the Great Salt Lake, especially late Saturday night into Sunday
morning as the 700 mb temperatures fall to near -16C with the GSL
around +3C. The cold pool aloft favorably extends up to 500 mb
under the trough axis, along with ample mid level moisture. The
main question remains how moist the low-levels will remain behind
the cold front, with the (always aggressive) NAM BUFKIT profiles
showing ample low- level moisture, while the GFS BUFKIT profiles
show much less favorable low-level moisture to work with. In
addition, boundary layer winds will be back from the northwest
behind the front to more westerly by Saturday morning, which would
discount organized lake effect, in favor of just some lake
enhancement along the northern Wasatch Front benches through
Saturday morning. Thus valley snowfall totals remain much more
uncertain than the mountains, but there will likely be a wide
range with lesser amounts across the valley floors, but potential
for locally higher totals long the eastern benches, particularly
north of Salt Lake City, east of the Great Salt Lake.

As this cold front pushes through southern Utah early Saturday
morning, expect to see increasing canyon winds in Washington
County, with winds peaking late Saturday morning, with gusts of
40-55 mph possible in the favored gap wind areas, once the 700 mb
northerly winds become better aligned with the surface pressure
gradient.

A secondary vort max within the main mid-level trough will rotate
through Saturday afternoon in more westerly flow aloft. Expect to
see rejuvenation of more widespread snow showers given the timing
with peak heating in a very unstable airmass up through 500 mb.
Mountain snow will continue to do well through Saturday afternoon,
with more widespread snow showers across the lower elevations.

This secondary vort max will continue to dig southward, trying to
eventually close off into a mid-level low as it moves into
northern Arizona Sunday night. Easterly winds will develop over
Northern Utah in response, but these will be shallowly driven by
the surface pressure gradient, with no cold advection nor 700 mb
support. Thus expect gusty canyon winds will develop thanks to the
easterly pressure gradient, but with no threat for widespread
downslope winds.

.LONG TERM (After 12Z Sunday)...
The long term forecast calls for a gradual warming trend for much
of the work week. With the departing trough digging south into
northern Arizona, there is concern for a weak downslope wind
response for the mouths of the canyons across Davis and Weber
counties. This downslope wind potential and the warmup will be
discussed below with a short excerpt on how the models are
handling the Pacific trough.

.(Weak Downslope Winds)...Several model outputs are showing a closed
low forming across northern Arizona/southern Nevada during the
afternoon on Sunday. Some ensemble members are showing a very weak
downslope wind signal for the highest prone areas across Davis and
Weber counties for Sunday. Parameters we analyze for these types of
events including the H7 winds, temperature advection and cross
barrier flow are all coming in weak, even amongst the ensembles
members that have been the most aggressive. Decided to add sub wind
advisory wind/wind gusts in the highest prone areas. The timeframe
for these winds will be from Sunday morning into Sunday afternoon.

.(Work Week Warmup)...Confidence is increasing for gradually warming
temperatures beginning Monday as upper level ridging moves into the
Great Basin region. Differences in the strength and orientation of
the ridge exist within the model outputs, but overall the consensus
is to have some varied degree of ridging in place Monday and
Tuesday. After highs in the 30s for Sunday, temperatures will jump
into the mid to upper 40s for Monday and Tuesday. With a low
confidence on the handling of the developing Pacific trough (more on
this below), the forecast approach is to keep a dry forecast as
ridging is still representative in the GFS/EC outputs heading into
Wednesday and Thursday. Depending on the flow aloft and surface flow
across Utah, this will dictate whether we experience a more
sustained warmup, or experience a slower warmup on Wednesday and
Thursday. Current temperature forecast approach is to gradually warm
temperatures into the 50s on Wednesday and hold these temperatures
steady heading into Thursday.

Low confidence continues to be present in how the models are
handling the Pacific trough for the middle part of the week. Large
differences in strength, trajectory and timing still need to be
worked out before an adjustment to temperatures and adding PoPs can
be adjusted to the long term forecast. Given the low confidence the
forecast will remain dry and warm throughout the long term period.

&&

.AVIATION...
Light northwest winds will continue at the SLC Terminal until the
diurnal shift around 03z. VFR conditions will persist into the
evening with any cigs remaining above 10k until near midnight.
Cigs expected to drop to 5-6kft agl by 10-12z with the passage of
a cold front, which will shift winds to the NW around 10z.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from 5 PM Friday to 9 PM MST Saturday
     for UTZ006>009.

WY...None.
&&

$$

SHORT TERM...Church
LONG TERM...Woodward
AVIATION...Verzella

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php


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