Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 061128

Area Forecast Discussion
National Weather Service Salt Lake City UT
428 AM MST Tue Dec 6 2016

.SYNOPSIS...A cold storm system will bring light snow and the
coldest temperatures of the season to most of Utah and southwest
wyoming today. 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 00Z Sunday)...A broad cyclonic flow
encompasses much of the northern and central U.S. early this
morning, anchored around an upper low which continues to spin over
northern Saskatchewan. Within this flow a shortwave trough is
noted rotating across the Pacific Northwest. This feature is
forecast to dig southeast today, crossing the forecast area late
this afternoon through tonight.

Low level warm advection downstream from this wave is forecast to
develop this morning across portions of northern Utah north of
I-80, which will bring a chance of snow primarily to the higher
terrain. Models have continued to slow the onset of this
precipitation, and as such have lowered POPs across locations
south of I-80 for the morning hours. The upstream wave is forecast
to push a cold front through northern Utah this afternoon, before
the parent mid level trough crosses the area this evening. Periods
of snow will likely develop in the wake of the surface front
within a region of mid level ascent ahead of the mid level trough
axis, however moisture looks to be somewhat limited and forcing
not particularly strong, and as such expecting snow totals to be
fairly limited across northern and central Utah through this
evening, with most valley locations only seeing an inch or so of
accumulation. Much colder temperatures will accompany this wave,
with 700mb temps near -18C forecast for Wednesday morning across
northern and central Utah. With this cold airmass there is some
potential for lake effect snowfall downwind of the GSL across Salt
Lake and Tooele Valleys late this evening through the morning
hours Wednesday. This is supported by hi-res models as well as
local lake-effect guidance, and as such have bumped overnight POPs
south of the GSL by 10-20 percent.

This cold snap is forecast to be short lived, as a mild and
increasingly moist westerly flow develops across the eastern
Pacific into the interior West Thursday into the upcoming weekend.
The initial surge of warm advection and associated moisture is
forecast to spread across northern and central Utah during the day
Thursday. Given the cold airmass in place precipitation is
expected to begin as snow, however the models suggest this cold
dome will gradually erode/modify allowing for a transition to rain
across most locations below roughly 6000 feet sometime Thursday
night into Friday morning. A few sheltered locations such as the
Cache Valley and portions of the Wasatch Back may locally keep the
cold dome in place into the morning hours Friday. This moist
westerly flow maintains into the weekend, and although the EC
presents a drier forecast in terms of model QPF output, both the
operational EC and the GFS depict a fairly continuous moisture
axis/modest atmospheric river structure extending inland into
northern Utah Friday through Saturday. As such have increased POPs
considerably across the northern half of the forecast area, with
snow levels gradually rising above the valley floors Thursday
night into Friday morning. A wave embedded within this westerly
flow may push colder air back into northern Utah during the day
Saturday lowering snow levels back to the valley floors.

.LONG TERM (After 00Z Sunday)...A relative lull in precipitation
is expected Sunday into early Monday with subsidence temporarily
moving in behind the exiting wave. GFS is a bit faster tapering
off the precip compared to the EC as it moves the wave out faster,
but by Sunday night both show limited precip. Have indicated this
trend in the going POP grids.

By Monday afternoon, EC/GFS both indicate warm advection returning
in westerly flow aloft, bringing a rather moist airmass back over
northern and central portions of the forecast area, and remaining in
place through day seven. Have nudged POPs upward slightly as models
have shown reasonable run-to-run consistency in this occurring. The
warm airmass is expected to keep precip as rain in the valleys, but
the pattern could produce significant snow in the mountains if it


.AVIATION...The SLC terminal will see VFR conditions through the
morning with cigs generally around 6000 feet AGL. Conditions are
expected to deteriorate to low-end MVFR to high-end IFR by mid-
afternoon as snow moves into the terminal. Southeast winds are
expected to shift to the northwest with the passage of a cold front
between 19Z and 21Z.





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