Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 220957

Area Forecast Discussion
National Weather Service Salt Lake City UT
357 AM MDT Wed Mar 22 2017

.SYNOPSIS...A strong and significantly colder Pacific storm
system will move east across the Great Basin through Thursday,
followed by a return of high pressure for the end of the work


Water Vapor Satellite shows a weakening closed low lifting toward
British Columbia. Meanwhile jet energy is approaching a
disturbance at the base of the trough off of the southern
California coast. 400-200mb MDCARS wind observations place a
100-140kt cyclonically curved jet from the eastern Pacific, over
central California and Nevada. GOES/SLC 00Z RAOB/HRRR indicate
that precipitable water values vary from 0.20"-0.30" mountains to
0.50"-0.70" western valleys.

The combination of a weak cold front across the north and waves
ejecting out of the trough to the west are causing showers across
northern Utah and southwest Wyoming early this morning.

During the afternoon instability increases modestly per SREF
guidance. Diffluence aloft increases with the approach of the
trough, while the exit region of a cyclonically curved upper level
jet arrives. Coverage of showers and thunderstorms will increase
to include much of the region. Given the increasing instability/moisture,
dynamics and lapse rates around 8C/km, some of these storms may
be strong or severe. Effective shear will likely exceed 30 kts.
SPC has placed much of the region as a marginal risk for severe
weather today.

Other storm characteristics will also come together as the storm
progresses through the area. Southern Utah will see a the brunt
of the cold core aloft Thursday. There is a strong height
fall/rise couplet as well as a deepening low to near 1000mb
(albeit a weaker feature than seen yesterday). Finally evidence of
potential vorticity seen at 500mb and as low as 700mb.

Steeper lapse rates and better dynamics will support locally
heavy precipitation across southern Utah. In addition, a second
area of heavier precipitation associated with the deepening
cyclone looks to target the west desert perhaps into the Wasatch

Snow levels look to fall from 9kft today to 6.5kft or slightly
lower Thursday and Thursday night. Significant snowfall is
possible in the mountains. However higher totals will likely be
spotty due to the convective nature of the precipitation. Showers
may continue into Thursday night with northwesterly orographic
flow remaining favorable.

Height rises, anticyclonic flow and warming aloft will end the
threat of showers by Friday.

Gusty southwest winds prevail today across portions of southern
and central Utah. Tomorrow expecting gusty northwest winds where
post frontal winds are favored, across the west desert, southwest
Wyoming, western Uinta Basin and Castle Country.

A progressive weather pattern will continue through the long term
as the ridge on Friday quickly gives way to another trough which
will cross the area on Saturday. Global models are still in
relatively good agreement regarding the strength and timing of
this trough. Widespread precipitation is expected, mostly during
the day Saturday, as the trough crosses the area. Snow levels are
expected to remain generally above 6000ft.

A shortwave ridge is forecast to return to the area by late Sunday
afternoon, but models diverge quite significantly after that. The EC
brings another fast-moving, open-wave trough through the forecast
area on Monday while the GFS`s pattern is more amplified, resulting
in a slower progression of this next storm and causing it to close
off with the low center tracking across southern Nevada and northern
Arizona Tuesday through Wednesday. The EC does bring a similar
closed low to the area, but close to a day later. In addition, with
the airmass in the EC already a bit cooler than the GFS due to the
Monday storm, the EC remains cooler than the GFS through midweek.

Forecast confidence remains low beyond Sunday, especially since
model spread has been increasing the past couple of days. Have kept
some general low to midrange PoPs and near climo temperatures in the
forecast pending better model agreement, but the pattern supports
unsettled weather regardless of the details.


Rain showers will continue to develop near and over the SLC
terminal through much of the day. However, there may be a brief
lull in showers for a few hours around mid-morning. Otherwise,
expect mostly VFR conditions, although CIGS may fall below 6000ft
especially this afternoon as rain becomes heavier. Light south
winds are expected after 12-14z with north winds returning by
midafternoon. However, there is a 40 percent chance that the south
winds may not materialize or may be brief.




SHORT TERM...Rogowski

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