Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 071153

Area Forecast Discussion
National Weather Service Salt Lake City UT
453 AM MST Wed Dec 7 2016

.SYNOPSIS...A cold airmass will reside across the region through
tonight. This cold snap will be short lived, as a relatively mild
but active westerly flow develops late in the week through the
upcoming weekend.


.SHORT TERM (Through 12z Sunday)...A longwave trough evident in
water vapor imagery encompasses much of the U.S. this morning. A
shortwave trough currently rotating into the base of this larger
scale trough is crossing northern Utah at this time, and although
moisture is quite limited with this feature, the coldest airmass
to advect into the forecast area this season accompanies this
wave, with 700mb temps of -18C observed this morning at KSLC. This
cold air coupled with weak ascent from the passing trough has
helped generate light snow showers off the Great Salt Lake into
western Salt Lake County. Web cams show some minor accumulation
along portions of Mountain View and Bangerter Highways thus far.
Hi-res models suggest this activity shifts west into the Tooele
Valley this morning, and have increased POPs into the likely
category in this small area, where snow accumulations around an
inch or so are possible.

The upper trough is forecast to slowly shift east this afternoon,
while the cold dome at the surface remains in place. This will
likely result in another very cold night tonight, although mid and
high level moisture is expected to begin spreading across the area
from the west during the overnight hours.

The recent cold snap will be short lived, as a mild and
increasingly moist westerly flow develops and spreads inland,
evolving into a modest inland-penetrating atmospheric river across
the northern Great Basin beginning Thursday, and continuing
through Friday and into the day Saturday. The initial surge of
warm advection will over-run the pre-existing cold dome currently
in place across the region, which will likely allow for snow
across most valley locations of northern Utah during the onset
Thursday afternoon into Thursday evening, however strong warm
advection is expected to erode this cold dome by early Friday
across most locations allowing for a transition to rain below
roughly 6000 feet. It`s possible a few sheltered locations such as
the Cache and Ogden valleys could hold onto the low level cold air
longer, resulting in an extended period of snow. Keep in mind the
models often struggle with handling low level inversions and the
erosion of cold pools, particularly within complex terrain, and
thus confidence in the timing of the transition in precipitation
type across the valleys is somewhat low confidence.

Eventually a more amplified shortwave trough is expected to push
colder air back into northern Utah during the day Saturday
allowing for a changeover back to snow, as well as temporarily
disrupting the atmospheric river structure.

Given the prolonged period of theta-e advection across northern
Utah will likely see SWE significant (snow-water equivalent)
accumulation across the northern mountains with relatively high-
density snowfall. This coupled with strong 700mb winds during
periods of stronger advection has prompted the issuance of a
Winter Storm Watch for the mountains of northern Utah. Held off on
any headlines for the valleys north of I-80 given uncertainty in
precipitation type.

.LONG TERM (After 12z Sunday)...Global models continue to indicate a
relative lull in precipitation Sunday morning with a more stable
airmass developing behind the exiting wave. However, this breaks
seems to be short lived, albeit a bit longer in the EC.

All models show a return to the warm advection/atmospheric river
regime, but this occurs late Sunday in the 06Z GFS as opposed to
Monday night/early Tuesday in the 00Z EC. 00Z GFS was still faster
than the current EC, but not as fast as the 06Z run. Have kept POPs
predominately in the slight chance range for Sunday and Monday over
northern portions of the forecast area before ramping them up a bit
for Tuesday. This still looks like an event that could produce
significant snow for the northern mountains, with rain prevailing in
the valleys.

EC indicates precipitation continuing over northern Utah and
southwest Wyoming through day seven, while the 00Z GFS indicated
moisture would retreat northward into Idaho by that time. 06Z GFS
looks more active than either of the 00Z runs, particularly after
day seven. For now, have moved POPs back into the slight
chance/chance range for day seven. The continued warm advection
pattern would keep temperatures near or slightly above seasonal
norms throughout the extended period.


.AVIATION...VFR conditions are expected at the SLC terminal through
the morning. Cigs at or below 6000 feet are expected to dissipate
between 14Z and 16Z but there is a 30 percent chance that they will
hang around longer than expected. The terminal will see a bit of
light snow early this morning but accumulations should be minimal.
Northwest winds are expected to continue during the daylight hours.

UT...Winter Storm Watch from Thursday afternoon through Saturday
     afternoon for UTZ007>009.




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