Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 231135
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
435 AM MST Fri Feb 23 2018

.SYNOPSIS...An active cold weather pattern will prevail through
at least the middle of next week as a series of weather systems
move through the region.


&&

.SHORT TERM (Through 00Z Tuesday)...An expansive longwave trough
continues to encompass the western CONUS early this morning, and
will continue to do so throughout the short term period. A
shortwave currently rotating through the base of the mean trough
is currently crossing the Great Basin and will continue to
translate eastward today, while further upstream another shortwave
is noted digging along the British Columbia coast. This secondary
feature will impact the forecast area late Saturday through
Saturday night.

The best large scale forcing associated with the Great Basin
shortwave continues to remain focused across southern Utah, owing
to a combination of thermal advection, upper level diffluence, and
increasing vorticity advection with the approach of a vorticity
maxima within the base of the trough. This activity should
continue to spread east/northeastward over the next few hours
across south central Utah, while drier air works into portions of
southwest Utah including the St George area. Going Winter Wx
Advisories through late morning handle this well, although one
area which has struggled thus far is Castle Country, where a
northeasterly surface wind is negating any potential upslope
component.

Further north, a weak 700mb circulation analyzed in RAP data and
evident in satellite/radar imagery is reforming across northwest
Utah early this morning, which is aiding an area of snowfall from
near Dugway northeastward into the northern Wasatch Front and
Cache Valley. This feature is expected to weaken slowly as it
continues eastward this morning, and areas south of I-80 along the
Wasatch Front as well as points east may not see the onset of
appreciable snowfall until mid morning or so, as this feature
finally passes and the flow aloft switches to the northwest. As
such the greatest impacts during the morning commute will likely
come across Weber and Davis counties. Eventually snowfall should
fill in across the central and southern Wasatch Front and points
east by mid morning, and continue into the early afternoon before
slowly tapering off.

A brief break in snowfall will ensue tonight into Saturday
morning as a low amplitude shortwave ridge quickly translates
across the area, in advance of the upstream British Columbia
shortwave. This break will be short lived, as thermal/moisture
advection strengthens across northern Utah as early as late
Saturday morning. This will strengthen Saturday afternoon bringing
snow to the higher terrain of northern Utah as well as valleys
near the Idaho border. The fast moving trough is expected to sweep
through the area bringing another round of snow to the I-15
corridor from Brigham City south to Cedar City Saturday evening,
with snow quickly ending overnight as the trough axis races east.

Another brief dry period will follow Sunday, and the flow backs
and warmer air spreads back into the region, valley temperatures
may reach or exceed the 40 degree threshold for the first time in
a bit along the Wasatch Front Sunday and again Monday. Yet another
shortwave is forecast to push a cold front into northern Utah
later in the day Monday potentially bringing another round of snow
to northern Utah.

.LONG TERM (After 00Z Tuesday)...EC/GFS have come into much
better agreement compared to yesterday at this time with respect
to the trough progged to be along the northern California coast
Monday evening. They both show the center of the system digging
across Arizona and New Mexico Tuesday into Wednesday, sliding a
front through the forecast area Tuesday. This front is noticeably
stronger in the GFS, which shows a good band of precip
accompanying it compared to the more spotty precipitation in the
EC. Have leaned toward the GFS here as it has been more
consistent, but left POPs primarily in the scattered range to
account for the EC solution.

After this system exits, all models continue to show a continued
unsettled pattern through day seven with temperatures generally
remaining near or slightly below seasonal normals but continue to
differ with respect to the details. GFS indicates another system
follows quickly on the heels of the Tuesday trough for Wednesday,
bringing a reinforcing shot of cold air and another round of
precip for primarily northern and central Utah. It shows a more
zonal flow developing behind that system with moisture lingering
over northern Utah and temperatures gradually warming. The EC
shows a much weaker trough behind the Tuesday one and keeps it
primarily away from the forecast area, allowing moist warm
advection to develop for Thursday before showing a broad storm
system impacting the area for day seven. With this uncertainty
remaining, have broadbrushed POPs and kept them generally isolated
across northern and central Utah with the exception of the higher
terrain.

&&

.AVIATION...At the SLC terminal...VFR conditions are expected to
deteriorate to prevailing IFR in snow by 14Z with the passage of a
cold front. There is a 30 percent chance IFR conditions will
begin later than expected. Light winds will become prevailing
northwesterly as the front moves through. Snow accumulation on
untreated surfaces is expected to be 1 to 3 inches through
midafternoon Friday.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory until 11 AM MST this morning for UTZ020-
     021.

     Winter Weather Advisory until 5 PM MST this afternoon for
     UTZ002>006-008>010-012-015-016-517-518.

WY...None.
&&

$$

Seaman/Traphagan

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