Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 041157
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
457 AM MST Sun Dec 4 2016

.SYNOPSIS...A dry northwest flow over Utah today will give way to
a very cold air mass for the first half of the upcoming week. The
coldest temperatures of the season are anticipated with snow for
mainly the northern half of the state.

&&

.SHORT TERM (Through 00Z Thursday)...A series of shortwave troughs
rotating through the Gulf of Alaska will carve out a longwave
trough across much of the western and central CONUS during the
early half of the upcoming week. The first of these features,
currently approaching the Pacific Northwest coast near 130W, will
move inland today before crossing the northern Rockies tonight.
Warm advection ahead of this wave is forecast to develop across
northern Utah and points northward this morning, resulting in a
considerable increase in mid and high level cloud cover, while any
substantial precipitation threat remains north of the Idaho
border. As this wave passes north of the area tonight, a shallow
baroclinic zone will push south through northern Utah overnight
through Monday morning, before stalling across central Utah during
the afternoon. Snow is expected to develop ahead of this boundary
along the terrain, while any chance for valley snowfall appears to
be tied to the boundary as it sags through the northern Wasatch
Front/Cache Valley overnight, then through the central/southern
Wasatch during the pre-dawn hours. Given that the jet axis and mid
level cold air remains north of the area, and any forcing remains
in the low levels confined to the boundary, little if any
accumulation is expected in the valleys through Monday morning
with perhaps a few inches in the Wasatch range.

As this lead shortwave moves east Monday, the associated surface
front is expected to stall across central Utah, while an upstream
wave moves onshore Monday afternoon across the Pacific Northwest.
Given the shallow nature of the baroclinic zone anticipate a
dry window from late Monday afternoon until the upstream wave
begins to interact with the stalled boundary sometime late Monday
night into the day Tuesday. Unfortunately the models continue to
struggle with the timing and amplitude of this wave, as well as
the placement of the stalled boundary once it comes under the
influence of the upstream wave, with the EC offering a slower and
somewhat more amplified solution. Leaned a bit toward the EC in
terms of timing, and as a result trimmed POPs for Monday evening
while adjusting them upward 10-20 percent on Tuesday across
northern Utah, then Tuesday evening across the south. This wave is
expected to move east by Wednesday, however a very cold airmass in
its wake will usher in the coldest airmass of the season thus far,
with 700mb temps Wednesday in the -18C to -20C range across
northern Utah.

.LONG TERM (After 00Z Thursday)...The trough continues to exit the
region Wednesday night, and clearing skies will likely lead to the
coldest morning of the week on Thursday morning. Warm advection
aloft through the day Thursday will probably create inversion
conditions, with a fair amount of warming in the mountains and
little to no warming in the valleys compared to Wednesday. Have
continued to trend the Thursday max temperature forecast toward this
idea.

The GFS and ECMWF have some timing differences heading into Thursday
night and Friday, but both models promote the idea of a strong and
moist zonal flow moving across the northern half of the CWA, leading
to increased cloud cover and likely precipitation. With a good
moisture tap and increased instability from a shortwave moving
across the area Friday night or Saturday, this pattern could be a
pretty good precipitation producer across northern Utah. If mixing
increases enough to warm the valleys, this precipitation could start
out as rain in the lower elevations Friday, changing to snow with
the cold front Friday night/Saturday.

Have generally increased POPs through the Thursday night to Saturday
period to account for this moist set-up. That said, the model
agreement in this solution has not been around for too many runs, so
would not be surprised to see details change over the next couple
days.

&&

.AVIATION...Ceilings at the SLC terminal are expected to drop below
7000 feet around 16-19Z, though there is a slight chance that
ceilings remain in the 7000-9000 foot range most of the afternoon.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

Seaman/Schoening

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http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php


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