Scientific Forecaster Discussion
fxus65 kslc 222300
Area forecast discussion
National Weather Service Salt Lake City Utah
400 PM MST sun Jan 22 2017
Synopsis...the active weather pattern will continue across the
region, with a series of weather systems crossing the area through
the first half of the week.
Short term (through 12z wednesday)...a 140+ kt southwesterly
oriented upper jet positioned on the southern/eastern periphery of
the deep eastern Pacific low is currently phasing overhead this
afternoon (noted in satellite imagery). Deep layer southwesterly
flow exists with 700 mb winds becoming strong and driving strong warm air advection
at the mid levels. Gusty surface winds exist across the western
valleys aided by a tightening southeast- northwest mslp gradient within a well
Models/analysis indicate strong 700 mb speed/moisture convergence
over the north currently, continuing for the next several hours
within the warm air advection. This can be noted in kmtx reflectivity signature
with rapid expansion of reflectivity from Provo north. All of this
reflectivity is indicating light to moderate snow backed up by
surface obs/cams. Hrrr indicates this zone will push across the
slc Metro area by 430-5pm allowing snow to taper, but will then
stall over the northern Wasatch front through tonight. Opted to
not issue any headlines for the Salt Lake and Tooele valleys yet
as period of snowfall looks to be limited and additional precip
should mix with rain below 5000ft by midnight, continuing through
late morning tomorrow. Advisories look solid for the west desert
and northern Wasatch front/Cache.
The most prolific moisture axis exists across southern Utah attm
and will remain so through tonight. Have noted snowfall elevations
above 4500 feet for most the day today, with mountain snow
becoming heavy as of late. Have continued to mention heavy rain in
Dixie tonight where small rises to rivers remain possible. Winter
storm warnings remain on track for the southern mountains and
staircase areas to the south where snow continues to accumulate
and will only intensify overnight.
Snow has also begun in Castle country with accums noted on Road
cams. Period of heaviest snow there will coincide with the
southerly flow through tomorrow morning. Warning remains on track.
Models continue to depict cyclogenesis over central Nevada
tonight, with further deepening of the surface low as it tracks
east-northeast across northern Utah tomorrow morning, maximizing over southwest
Wyoming tomorrow afternoon/eve. Passage of this low will drive cold
frontal passage from northwest-southeast tomorrow morning supplying aided
forcing when deepest moisture is in place, and strengthening cold air advection
allowing snow levels to fall back to the valley floors south of
Ogden by late in the morning. Expecting greatest impacts in the
slc Metro area to occur midday/afternoon Monday with 1 to 4+
inches of rapid accumulation into the evening commute. Will let
next shift assess timing and need for a highlight focusing on the
Snowfall will gradually wane Monday evening/overnight, though flow
aloft will be veering towards a cyclonic northerly flow by
Tuesday. NAM/GFS drop a wrap around wave from north to south
across the northern half of the area during the day within this
flow. With cold air advection continuing...700 mb temps falling to around
-15 c at kslc...do expect additional light snowfall to have an
impact at times during the day. Bumped pops to high chance valleys
and likely in the northern mtns.
Long term (after 12z wednesday)...a cold unstable cyclonic nwly
flow aloft is forecast to be over the nrn Great Basin at the start
of the long term period with sufficient moisture to keep a threat
of orographic precip going along the Wasatch front and in the
Heights start to rise during the day Wed and the airmass starts to
stabilize as the parent trof moves off to the east. Any lingering
snow showers will taper off through the aftn. Low levels stay
cold however and expect valley temps to remain below freezing.
A weak trof breaking under the upstream ridge phases with one last
lobe swinging south around the western end of the plains trof and
brings a period of warm advection to the nrn and central County Warning Area Thu
thru Thu night. Models are fairly generous with the 700 mb moisture in
this pattern but any lift appears weak so the threat of precip is
The skies clear Fri morning as the upstream ridge noses into the
western states and this stays centered to our west thru the
remainder of the long term period keeping a dry stable airmass in
place. Temps aloft warm but valley inversions will likely become
strong with cold air trapped well into next week.
Aviation...expect VFR to prevail at the slc terminal overnight
into Mon morning with occasional MVFR in light snow at times.
Gusty south winds will continue through the night as well. A cold
front will cross the airfield around 15-16z Mon with IFR in snow
developing behind the front.
Utah...Winter Storm Warning until 10 PM MST Monday for utz006-012-020.
Winter Storm Warning until 4 am MST Tuesday for utz007>010-517-
Winter Weather Advisory until 10 PM MST Monday for utz001-002-
Wyoming...Winter Weather Advisory until 10 PM MST Monday for wyz021.
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