Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 131214
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
514 AM MST Tue Feb 13 2018

.SYNOPSIS...The upper level low pressure system over the California
coast will drift offshore today. The next trough will move in from
the Pacific Northwest Wednesday night through Thursday.

&&

.SHORT TERM (THROUGH 12Z FRIDAY)...The upper low that is currently
affecting the weather over the area is centered over the
central/southern California coast this morning. The 700mb boundary
remains across southwest through east-central Utah, but will start
to lift north as warm advection increases this morning. Currently, a
band of precipitation remains in place near the baroclinic zone,
with snow having been reported down to below 5000ft across south-
central Utah. Rapid road-surface accumulations earlier prompted the
expansion of Winter Weather Advisories into south-central Utah and
higher elevations across the San Rafael Swell where favorable
upslope flow will continue into this morning. No additional changes
were made to winter weather highlights with the morning package.

With the upper low continuing to head offshore, the forcing
resulting from the combination of upper diffluence and low-level
convergence will gradually weaken through the morning hours.
Precipitation should diminish significantly by early afternoon.

Otherwise, lingering low-level moisture has resulted in some patchy
stratus last night and into this morning in some northern valleys
and even a small area of fog near Evanston and the Bear River
Valley. These should also diminish by afternoon.

Southwesterly flow will increase late this afternoon and into
tomorrow as the next trough comes ashore over the Pacific Northwest.
This trough will cause to closed low to weaken and eject across
southeast Utah tomorrow night. This weakening low will pull in some
moisture ahead of it, so expect precipitation to begin filling back
into the area again beginning tomorrow afternoon. However, with the
strengthening southerly flow, the airmass should be fairly well-
mixed, leading to mild temperatures and keeping snow levels above
many valleys initially.

As the trough drops in from the northwest late tomorrow night into
Thursday, the accompanying cold front will cause snow levels to fall
to the valley floors. However, this storm in general is not expected
to be especially moist, and valley accumulations should remain
minor. There looks to be a brief period of good orographic forcing
on Thursday which would help increase amounts along northwest-facing
slopes Thursday morning to early afternoon, but drier air aloft will
then quickly work its way into the area, bringing an end to
precipitation by the evening hours.

.LONG TERM (After 12Z Friday)...As the shortwave trough slides to
the southeast by early Friday, warm advection northwest flow filters
in across northern Utah. Both extended models hint at a weak brush-
by shortwave late Friday that will just clip northeastern Utah and
move into southern Wyoming and northern Colorado. This should
increase cloud cover a bit, but keep most precipitation well to the
north of our forecast area.

Going into the weekend the models begin to really diverge from one
another, surprise surprise. However, both the GFS and the ECMWF
remain consistent run to run with the respective model runs, so
there`s that going for them. In past experience over the last week
or so, the EC seems to have been more consistently unchanged while
the GFS tends to flip flop more, so I leaned more towards the EC.

A digging trough slides south through the Great Basin Saturday into
Sunday from the EC, while the GFS opens a broad longwave trough
draped across the northern Rockies. The EC has a better advertised
baroclinic zone with more frontogenetic forcing Sunday, with 700mb
temps dropping to -14C with better available moisture. While the GFS
finally digs a trough into the Great Basin by Tuesday, the EC
further enhances the trough with a cut off low forming overhead of
Utah by late Monday. Differences aside, this pattern looks very
active and through the weekend a storm system looks promising to
bring some much anticipated winter precipitation to at least a good
portion of Utah.

&&

.AVIATION...North winds are expected to persist at the KSLC terminal
throughout the day, with a shift to the southeast by 03z this
evening. Patchy areas of cigs near 2500 feet are noted across
northern Utah, and there is a 30 percent chance that they will
periodically spread over the terminal through 19z.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory until noon MST today for UTZ013-020.

     Winter Weather Advisory until 4 PM MST this afternoon for UTZ517-
     518.

WY...None.
&&

$$

Cheng/Dewey

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visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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