Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 230336
Area Forecast Discussion
National Weather Service Salt Lake City UT
936 PM MDT Wed Mar 22 2017
.SYNOPSIS...A Pacific storm system will move east across the
Great Basin tonight through Thursday. High pressure will return
for Friday, followed by another storm system this weekend.
.SHORT TERM (Through 00Z Saturday)...The axis of the advancing
upper level trough sits squarely over Nevada this evening. Precip
has expanded rapidly across southern Utah during the evening, with
embedded thunderstorms noted across mainly south-central Utah.
Synoptic-scale lift is increasing across the south as dynamic
lift supported by fairly strong deep layer moisture and cold
advection moves into the area with the trough. Thunderstorms will
likely persist for several more hours as the left exit region of
the upper jet resides across south-central through east-central
Up north scattered convection appears to be slowly increasing in
areal coverage again mid-evening. This activity will likely
continue to trend upward in areal coverage and intensity through
early Thursday morning as the axis of the upper trough works
slowly across the area. Orographic enhancement will likely become
a player late Thursday morning through mid-afternoon as the air
mass will remain quite moist and unstable and the mean flow aloft
briefly turns to the northwest in that short window. A more
northerly flow aloft accompanied by a gradually drying and
stabilizing air mass should bring an end to most if not all
terrain-tied convective showers by early evening.
High pressure aloft will spend Friday over Utah, moving east of
the state by the end of the day. Lingering moisture and abundant
insolation could generate some terrain-based buildups, though none
of this is expected to generate more than brief and somewhat
isolated showers through the afternoon.
.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.
.Hydrology...Concern through Thursday is expected rainfall across
the north which could take many rivers already running near
bankfull to flood stage Thursday. The models have trended lower
with overall QPF across this area, and at this time anticipate
rivers to remain just below flood stage, although they 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
Thursday evening and allow the flows in the area rivers to
gradually lower heading into the weekend.
.AVIATION...The moist southerly flow ahead of the upper level
trough just to the west of Utah will maintain light precip at the
terminal through Thursday morning. VFR conditions will prevail,
though ceilings will run around 5 kft. A weather disturbance
moving north across western Utah later tonight could bring a
period of heavier precip and short periods of MVFR ceilings around
10z through 16z. The upper level trough axis will move through
the area early in the afternoon, leading to increasing northwest
winds with slowly lifting ceilings and lingering light showers
until late in the afternoon.
UT...Winter Weather Advisory until 11 AM MDT Thursday for UTZ007>010-
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