Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 052244

Area Forecast Discussion
National Weather Service Salt Lake City UT
344 PM MST Mon Dec 5 2016

.SYNOPSIS...A  colder storm system will bring light snow and the
coldest temperatures of the season to most of Utah and southwest
wyoming for tomorrow into midweek. An active and potentially
wetter pattern will again set up late week through the weekend.


.SHORT TERM (Through 00z Friday)...Primary feature to note in
afternoon water vapor imagery and H5 analysis is an upstream short
wave diving southeast into the PacNW, on the back edge of a mean
long wave trough encompassing much of the northern CONUS. This
wave will cross the forecast area later tomorrow into Wednesday
bringing light snow and coldest temps of the cold season thus far
into midweek.

A largely continental orientation of this upper low has and will
continue to limit moisture potential with this wave as it crosses
the area. Although instability will be more than sufficient for
vertical motion, and the upper jet will be aiding lift, not seeing
strong coupling of deep moisture in any of the BUFR profiles, nor
in operational runs. Limited surface reflection exists. QPF thus,
is quite limited in guidance and this makes sense looking forward
regarding sensible weather and amounts.

What is high confidence is that light snow and snow showers will
expand areally tomorrow afternoon across northern Utah within
modest PVA and CAA as the cold core of the trough approaches from
the northwest. Do expect around an inch of snow in the northern
valleys, and 2 to 6 inches in the northern/Central mtns by
Wednesday morning. Breadth of CAA will allow for accumulation on
surfaces, though the bulk of these moderate impacts are
anticipated to occur over the mtn routes/canyons the sun angle
lowers and eventually sets tomorrow evening.

NAM (per norm) seems to overdoing low level moisture potential
Wednesday morning, thus favors the potential for lake effect
downwind of the GSL. Not buying off on this, as pattern does not
look suitable for much low level moisture advection across the
lake. Other variables are there including wind orientation and
lake induced instability...lack of low level RH looks to be the
limiting factor. Will continue to assess.

The main take away heading into Wednesday/Thursday will be the
cold temps...the coldest yet this cool season. Good agreement
exists regarding H7 temps of -18 to -19C across the north, with H5
temps as cold as -38C come Wednesday. Prep to bundle up through
Thursday beneath low amplitude ridging as temps will be running
some 15 or more degrees below normal.

The pattern begins to reload across the eastern Pacific Thursday with
the next strong trough moving into the PacNW region. A near zonal
flow oriented with the jet coincident with this looks poised to
drive at least a modest river event into the intermountain region
late week through the weekend. We look to be on the southern
periphery of this. Details below.

.LONG TERM (AFTER 00Z FRIDAY)...The extended period forecast
starts with a weak ridge pattern and westerly flow aloft as the work
week comes to an end. Warm advection in the mid levels is evident on
the 700mb level as temperatures begin to warm near -2C. Differences
among the global forecast models as to how much warming occurs, the
forecast challenge will be in the precipitation type.

An atmospheric river set up looks evident by Thursday, with mixing
ratio values along the eastern Pacific surpassing 9 g/kg under a
110kt jet streak with mostly zonal flow into north-central
California. With warm advection and mostly zonal flow, not much
stopping the moisture advection this far inland. Upslope conditions
along the Wasatch front with west- northwest flow Friday into
Saturday, have slightly bumped up PoPs to hint at a longer duration
precip event.

With the better baroclinic front dropping through northern Utah late
Friday, and the cold air pool dropping down just behind the front,
precip will likely change from rain to snow sometime late Friday.
Models are still a bit inconsistent with this timing, so will have
to reevaluate with the next several model runs.


.AVIATION...Conditions at the SLC terminal are expected to
remain at VFR levels through the TAF period. Northwest winds are
expected to continue before switching after 03z to a southeast





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