Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 171645

Area Forecast Discussion
National Weather Service Salt Lake City UT
1045 AM MDT Sat Mar 17 2018

.SYNOPSIS...A slow-moving storm system will bring cool and
unsettled weather for the weekend. High pressure aloft will return
for the first half of next week and bring a warming trend to the
region before another storm system approaches by the end of the



An active weather day is unfolding across Utah and southwest
Wyoming as a shortwave trough pushes in ahead of a deeper longwave
trough over the Western US and a surface cold front approaches.
Over southern Utah, gusty south winds of 25-35 mph are being
reported. Over northern Utah and southwest Wyoming, a slowing
baroclinic zone and surface cold front have brought areas of snow,
with accumulating snow at benches and higher. Cams and reports
indicate a couple inches have fallen on benches around the Wasatch
Front, while cams indicate snow sticking to untreated roads above
about 5,000`. Snow cover on I-80 from Evanston to First Divide
today prompted an extension of the current winter weather advisory
to cover Uinta County Wyoming.

Warming and thinning cloud tops on IR imagery along the UT/NV
border in vicinity of the shortwave trough progged to move
overhead today suggest that this shortwave is weakening on our
doorstep. Thus, an initial band of precipitation this morning
co-located with the baroclinic zone pushing through northern Utah
is weakening at the moment from south to north. Although this band
will continue to decay as it moves eastward, high-res models
suggest destabilization from colder air moving in aloft will
trigger additional scattered showers this afternoon, with the
most coverage in vicinity of the aforementioned baroclinic zone.
Expect snow and graupel with these showers. Calibrated
thunderstorm guidance from SPC even indicates a low- end threat of
thunderstorms, mostly from Utah County through the Uintas.

By this evening, the stronger primary trough axis upstream s
expected to quickly shift east into the Great Basin, and push an
associated shortwave trough through the area with the best forcing
focused over southern Utah. This will reinvigorate the mid-level
baroclinic zone already present over central/southern Utah and
bring another period of snow to generally the southern half of the
state, especially east of I-15. Will need to monitor for
potential expansion of the current winter weather advisory in
place over the mountains to perhaps cover valleys to the east,
especially higher spots such as the San Rafael Swell.

Updated the forecast to bring in more recent 12Z guidance for

.Previous discussion...issued 412 AM MST...

.SHORT TERM (Through 00Z Wednesday)...Early morning water vapor
imagery and objective analysis depict an upper low spinning across
central Oregon, with a couple of shortwave features rotating
through the larger scale flow. One such wave is currently ejecting
across the eastern Great Basin, while an upstream wave is noted
digging just off the Pacific Coast. An upper jet associated with
the ejecting wave extends from eastern NV into north central Utah,
with the left exit region residing across northwest Utah. The
attendant low level baroclinic zone associated with this wave
remains along the UT/NV border, but has begun to shift east over
the last hour or two.

Regional radar imagery shows showers continuing across northern
Utah in association with the left exit region of the
aforementioned jet, while the ejecting wave has pushed the surface
boundary into northwest Utah, where a secondary band of
precipitation is currently spreading east. This front is expected
to make its way into the Wasatch Front by mid morning, however as
the parent wave lifts away from the region convergence along this
boundary may weaken with time, and many of the hi-res models are
only producing spotty precipitation with this feature. Have
maintained high PoPs through the day however QPF may wind up being
on the light side. Daytime heating this afternoon along with cold
air aloft will also bring a slight chance of thunderstorms.

Attention then turns to the upstream wave along the Pacific Coast,
which is forecast to turn inland and cross the lower Colorado
River Valley tonight, before ejecting into the central/southern
Rockies Sunday. Large scale ascent is expected to increase across
central and southern Utah tonight and linger into the day time
Sunday, bringing periods of snow to areas above roughly 4500 feet.
Precipitation should wind down Sunday afternoon as the parent wave
moves away from the region. Meanwhile a weaker upstream wave will
maintain a chance of snow showers across the north Sunday.

Currently have Winter Weather Advisories in effect for the higher
terrain south of I-80 and this looks reasonable. May need to
expand these into the central and eastern Valleys for tonight
depending on how the upper wave evolves as it crosses the region.

A relatively cool airmass will linger across the region Monday
keeping temperatures below climo. Low level southwesterly flow is
forecast to develop Tuesday as mid level ridging builds across the
region. This should yield a warming trend across the forecast area
pushing temperatures back near climo.

.LONG TERM (After 00Z Wednesday)...High pressure looks to remain
parked over the forecast area Tuesday night before shifting east
Wednesday morning, allowing a southwest flow to develop. Models
indicate that this flow will draw tropical moisture into Utah and
southwest Wyoming and embedded shortwave energy will allow for the
development of precip Wednesday into Thursday. Latest EC is about
twelve hours slower with the arrival of the best moisture compared
to the GFS, and thus delays the arrival of the most widespread
precip. With this difference in timing, have broadbrushed the POPs
for the midweek period at this time.

The reign of southwest flow is expected to end with the arrival of
the next Pacific storm system Thursday night into Friday. EC is a
bit stronger and colder with this system compared to the GFS,
bringing with it 700mb temps as low as -14C to northern Utah,
compared to -10C or -11C in the GFS. All guidance confines the
coldest air and best associated precip to northern Utah with the
passage of this trough and have indicated that in the grids.
Regardless, this should bring a noticeable cooldown compared to the
very mild temperatures in the southwest flow.

With the weaker system, the GFS brings in a more zonal flow for
Friday night into next weekend, allowing for a bit of a rebound in
temperatures. Meanwhile, the EC closes off the trough over the
central United States, keeping the influence of the trough (and thus
much colder temperatures) over the forecast area. Have lowered temps
for day seven to account for this solution.


.AVIATION...Winds at the SLC terminal have shifted to the
west- northwest with the passage of a cold front. As the
precipitation tapers off through 19z, winds may become light and
variable this afternoon. Northwest winds are expected to prevail,
with a 30 percent chance for brief southeast winds after 04z.
Periodic MVFR conditions in low cigs and vis with passing snow
showers possible through 21z near the airport.


UT...Winter Weather Advisory until noon MDT Sunday for UTZ009-010-517-

     Winter Weather Advisory until midnight MDT tonight for UTZ008.

WY...Winter Weather Advisory until noon MDT Sunday for WYZ021.



Van Cleave/Dewey

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