Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 141044
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
344 AM MST Thu Dec 14 2017

.SYNOPSIS...Northerly flow will prevail across the region today
in the wake of a departing storm system. A stronger system will
cross the area Saturday, sweeping out valley inversions and
bringing a chance of snow to to northern and western Utah.

&&

.SHORT TERM (Through 00Z Monday)...Early morning satellite imagery
and objective analysis depict an upper ridge centered off the
Pacific Coast near 130W, with northerly flow extending downstream
across the Great Basin. A shortwave trough which passed through
the forecast area Wednesday evening into the overnight hours is
currently located near the Four Corners region and will continue
moving away from the area today. This feature brought a bit of
light snow, but more importantly has cooled temperatures aloft and
significantly weakened valley inversions across northern Utah.
700mb temps are analyzed by the RAP at near -5C this morning, and
this coupled with 700mb flow near 30 kts and a bit of a northerly
surface gradient has both eroded the fog/low stratus bank which
has plagued northern Utah over the past few days, as well as
improved air quality based on DEQ sensors around the Wasatch
Front. As such have removed fog/stratus from the forecast for
today as well as bumped valley temperatures a bit, although have
retained haze as the airmass has not fully mixed out overnight,
and may see the inversions strengthen a bit today as temperatures
warm aloft.

Further south, a northerly gradient coupled with strong 700mb cold
advection is resulting in strong gusty winds along the I-15
corridor between Cedar City and St George, as well as other wind
prone areas of Washington County. Will maintain the going High
Wind Warning through the morning hours where gusts to 60 mph will
remain possible.

Shortwave ridging will temporarily build across the region tonight
through Friday bringing warming temperatures aloft, and likely
allow valley inversions to strengthen a bit. Have included a
mention of patchy fog for late tonight through Friday across these
areas.

The models remain in good agreement digging a splitting shortwave
through the Great Basin late Friday night into Saturday. Although
this wave will be splitting, the cold front coupled with 700mb
temps falling below -10C and a northerly gradient should sweep out
lingering inversions from northern Utah Saturday morning.

The baroclinic zone coupled with the passage of the trailing mid
level trough should be sufficient to bring a few inches of snow to
the higher terrain although this system does not look like a
major snow producer. The frontal band will shift south into
southwest Utah by Saturday afternoon bringing a chance of snow as
far south as Cedar City. For now have increased PoPs into the
70-80 percent range across the northern mountains while leaving
the valleys of northern and western Utah in the chance category
for now. This trough will gradually exit the region Saturday night
which should leave a drying trend for Sunday as shortwave ridging
builds into the region.

.Long Range (After 00z Monday)...Heading into early next week,
shortwave ridging will build in but extended models all point to a
short duration ridging with very weak (if any) warming aloft. The
big picture weather pattern does continue to point to an
unsettled disturbance sliding through the Western CONUS and the
Great Basin by mid-week. A nice change in the pattern as the ridge
of high pressure seems to retrograde and weaken a bit.

Long range models are pointing to a cold weather disturbance by
mid- late week, and models are pretty close in timing so that
helps confidence a bit. However, with the history of shortwave
energy weakening at the last minute, have low confidence in the
finer details of this system. Am feeling more comfortable about
the fact that inversions will struggle to redevelop after the
storm system the previous weekend, and with temperatures at 700mb
of -13C to -17C being advertised, have higher confidence this will
continue to keep valleys mixed out.

The shortwave trough models are advertising for Thursday are in
decent agreement at this time and even advertising a decent amount
(compared to recent events) of moisture. Won`t get too excited at
this time in the prospect of moisture/precipitation chances, but
with the flow pattern and nature of the trough, we may see some
winter precipitation before the end of 2017 after all.

&&

.AVIATION...Low stratus and haze will continue to be the primary
operational weather concern at KSLC today. MVFR/IFR visibilities
and ceilings will persist through the day. A 20 percent chance
that conditions will drop into LIFR in fog. Light winds will
prevail with a northwest direction until 03z or so this evening
when a switch back to the southeast takes place. A 30 percent
chance that low IFR conditions set up after 00z this evening in
fog and haze.


&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...High Wind Warning until 9 AM MST this morning for UTZ019.

WY...None.
&&

$$

Seaman/Dewey

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