Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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730
FXUS65 KSLC 061542
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
842 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.

&&

.DISCUSSION...
Water Vapor Satellite shows the next cold storm system approaching
from the Pacific Northwest and Idaho. Meanwhile another storm
system is crossing the Eastern Pacific. 400-200mb MDCARS wind
observations place a 120-160kt northwesterly jet from well off the
Pacific Northwest coast into northern California and western
Nevada. GOES/KSLC 12Z RAOB/HRRR indicate that precipitable water
values vary from 0.05"-0.10" mountains to 0.15"-0.25" most
valleys. Blended precipitable water product shows an atmospheric river
from near Hawaii into the aforementioned Eastern Pacific storm
system.

Currently have a cold and dry airmass in place. Models have been
trending with a later onset time for snow, and with the dry column
that will only aid in the delay. Cut PoPs back to slight chance to
the north and east of SLC this morning, leaving chance PoPs closer
to the Raft River Mountains of far northwest Utah. Very little
change to the afternoon PoPs with good agreement in various
guidance.

Valley accumulations through the afternoon look to be less than an
inch most valley locations, with several inches in the adjacent
mountains. The timing of course is not good for the afternoon
commute. UDOT will be pre-treating which will help with road
conditions despite the cold temperatures. Still expect some slick
spots on ramps and overpasses, and additional issues to arise
after sunset.

For the afternoon package will be taking a closer look at the Lake
Effect potential tonight along with the slew of storm systems
Thursday into next week.

&&

.PREVIOUS DISCUSSION...
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.

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
materializes.

&&

.AVIATION...
Operational weather concerns will become more notable at the KSLC
terminal in the 20-02z window as light snow and lower cigs develop.
Conditions are expected to deteriorate to low-end MVFR to high-end
IFR by mid-afternoon though this light snowfall may be periodic in
nature at times into the evening. Patchy light dustings of slush
are expected after 22z on untreated surfaces with a threat of
light icing becoming increasingly likely after 01z as temps drop.
Southeast winds are expected to shift to the northwest with the
passage of a cold front between 19Z and 21Z.

&&

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

$$

PUBLIC...Rogowski/Seaman/Traphagan
AVIATION...Merrill

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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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