Scientific Forecaster Discussion
fxus65 kslc 142305
Area forecast discussion
National Weather Service Salt Lake City Utah
405 PM MST Sat Dec 14 2019
Synopsis...a drier northwesterly flow aloft will continue to
spread into northern areas tonight ending the long duration storm
for the north. A weather disturbance will track across southern
Utah tonight through Sunday, and will bring largely snow to far
southern Utah/Arizona border region through that time. High pressure
returns for next week.
Short term (through 00z wednesday)...afternoon water vapor and 500 mb
analysis depict a short wave tracking downstream and into central
Wyoming, and a stronger negatively tilted trough translating southeast
into the western Great Basin attm. A near zonal jet in advance
stretches from the eastern Pacific inland while centering over
southern Utah as well. Msas mslp indicates strong pressure rises
over northern Utah, with a near stationary boundary draped across
the Utah/Arizona border region. The primary mid level baroclinic zone
stretches north into central Utah from this. This baroclinic zone,
the jet aloft, and track of the trough across southern portions
of the state tonight will create a developmental environment with
increasing deep layer lift and strong frontogenic forcing as the
key variables above interact with each other through Sunday
Models have been steadfast in this development over southern Utah
for many runs now, with primary uncertainty focused around
details regarding exact placement of the boundary. Recent trends
in guidance have shifted colder however, with stronger cold
advection and convergence into this boundary beginning around
midnight tonight. Lower snowlevels the outcome in thinking, and
expect snow to fall down to the lowest elevations of the Grand
Staircase/Escalante regions beginning later tonight...even if
these lowest elevation areas initially begin as rain. Breadth of
forcing/moisture in place support moderate to heavy snowfall
focused over the southern mountains south to the Arizona line. Have
issued winter weather advisories for these areas as accums will be
modest and area roadways impacted.
Further north, continued downstream translation of the short wave
into Wyoming is allowing a much drier northwesterly flow to spread
across the region this afternoon...trends in this to continue
moving forward tonight. With this, continue to expect a rapid
tapering trend to lingering snow showers/squalls through sunset.
Anticipating going headlines in the north to expire on time as
The mean trough position will continue to amplify south into the
4-corners region Sunday night with the western periphery remaining
just upstream over Nevada...much colder temps across the area with
this. Guidance continues to indicate an embedded vort Max will
track northwest- southeast across northern Utah during this time allowing for
the potential of another round of light northern mountain
snowfall, and potential for at least lake enhancement southeast of the
Great Salt Lake overnight into Monday morning. Could be sneaky and
something to watch moving forward.
Thereafter heights will rapidly build from the west as a strong
ridge of high pressure amplifies inland into the Great Basin
region into the midweek period.
Long term (after 00z wednesday)...a mean longwave ridge will
encompass the western U.S. Through the long term period. Ensembles
agree on a weakening shortwave pushing across Utah on Wednesday or
Wednesday night. Lack of moisture and significant dynamics with this
wave should preclude meaningful precipitation with its passage.
Other than some low pops on Thursday hugging the Idaho border, the
forecast for the entire long term period is dry.
Rising heights with the mean ridge will strengthen valley
inversions, especially in the wake of wednesday's weak
disturbance. Deteriorating air quality is expected starting
Thursday, lasting into the weekend.
Aviation...VFR conditions are expected across the slc terminal
through this taf period. Breezy, west-northwest winds will
continue for another hour before dropping below 10 knots after
00z. Cigs will continue to lift and should rise above 7000ft above ground level
between 01z and 02z. A steady northerly flow will shift to a light
southerly flow around 08z.
Utah...Winter Weather Advisory from 10 PM this evening to 11 am MST
Sunday for utz020.
Winter Storm Warning until 5 PM MST this afternoon for
Winter Weather Advisory until 5 PM MST this afternoon for utz006.
Winter Weather Advisory until 10 PM MST this evening for utz517.
Winter Weather Advisory until 11 am MST Sunday for utz518.
Wyoming...Winter Weather Advisory until 5 PM MST this afternoon for wyz021.
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