Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 222152

Area Forecast Discussion
National Weather Service Salt Lake City UT
352 PM MDT Wed Mar 22 2017

.SYNOPSIS...A 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 week. Another storm system will
cross the region during the upcoming weekend.


.SHORT TERM (Through 00Z Saturday)...Diffluent southwesterly flow
aloft prevails across the forecast area this afternoon, downstream
from an upper trough situated along the Pacific Coast. A surface
frontal boundary snakes across northern Utah stretching from near
Dugway east across far northern Utah county, downstream into far
southwest Wyoming south of Evanston.

Several embedded waves are noted within the deeper southwesterly
flow, one of which is currently lifting across northeast
NV/northwest UT and has induced another round of showers and
thunderstorms across the north. A few of these have produced small
hail, however with the increase in convection instability has
been waning and the strength of the overall convection has been on
the downward trend.

Meanwhile convection has been rapidly increasing across southern
Utah this afternoon, as the region comes under the nose of a
strong upper jet rotating around the base of the Pacific Trough.
This is also associated with the nose of a modest subtropical
moisture plume spreading across the Desert Southwest.

As the evening wears on, expect the convective activity across the
north to temporarily wane as the associated wave eventually lifts
away from the region, while the convective activity continues to
expand across the central and eastern portions of the forecast
area. Into the overnight hours the southern activity is expected
to shift east and eventually out of the area, while the frontal
boundary across the north is expected to become active once again
as the main Pacific Trough axis moves inland. As this axis passes
east of the area by Thursday morning, should see precipitation
become more orographic in nature along the I-15 corridor. Went
ahead and issued a Winter Weather Advisory for the higher terrain,
with road impacts expected above 7000 feet through Thursday

Of additional concern is potential for rainfall across the north,
where many rivers are already running near bankful. The models
have trended lower with overall QPF across this area, and at this
time anticipate rivers to remain below flood stage, although will
require close monitoring.

Mid level ridging is forecast to build across the Great Basin late
Thursday into Friday, which should bring a quick end to
precipitation by Thursday evening.

.LONG TERM (After 00Z Saturday)...The ridge that is over the CWA on
Friday continues to shift east Friday night, with a deepening
Pacific trough starting to invade the Great Basin. Similar to the
Wednesday/Thursday system, the best upper level energy digs south to
the Arizona/Utah border, but decent low level moisture and cold
advection help to focus precipitation over Utah during the day
Saturday. Have kept POPs high through this period, with temperatures
likely dipping below seasonal normals.

The system quickly exits to the east Saturday night, with another
shortwave ridge shifting over the area Sunday. Progressive remains
the buzz-word of the forecast, as that ridge shifts east Sunday
night and the next trough moves into the area Monday or Monday
night. That said, model agreement begins to break down quite a bit
after 12Z Monday, with significant differences in the timing and
structure of the incoming trough.

Of particular note is the disagreement in how cold the incoming
airmass is, with the Canadian and ECMWF advertising a decent cool
down that could bring snow down to the valley floors by Tuesday,
while the GFS cuts off the storm to our southwest and keeps the
airmass over Utah mostly mild. For now, have kept POPs high through
Monday and Tuesday; there is of course lower confidence in the
temperature forecast, which for now has been kept fairly seasonal.


.AVIATION...VFR conditions will continue at the SLC terminal through
the evening with showers/thunderstorms developing near the terminal.
Ceilings should trend toward low end MVFR or high IFR after 06z as
heavier showers develop towards the morning hours. Northerly winds
should prevail through the TAF period, though may become erratic at
times due to nearby thunderstorms.


UT...Winter Weather Advisory until 11 AM MDT Thursday for UTZ007>010-




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