Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 141702

Area Forecast Discussion
National Weather Service Salt Lake City UT
1002 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.


.DISCUSSION...Heights are beginning to build in from the west this
morning as the departing short wave continues to dig well south into
the desert southwest. The influence of the prevailing upstream
ridge will once again begin to dominate local weather over the
next 36 hours, increasing stability whilst warming the airmass
aloft (H7 temps will trend from -6 C to +2 C through that time),
owing to another period of strengthening inversions for the
valleys/basins of primarily northern/central Utah.

In the near term however, a combination of cooling aloft and
modest low/mid level flow allowed for a fair bit of ventilation
vs. 24 hours ago. Though haze will remain for the inverted areas,
much of the stratus/fog has waned for the time being. Made some
subtle changes to Sky grids, and removed mentioned of fog in the
Cache Valley as the only updates this morning.

Allowed the High Wind Warning for Washington County to expire last
hour as scheduled. Beginning to see the MSLP gradient/H7
winds/CAA relax vs. earlier this morning, and as such the gust
speeds through gaps/canyons are as well. Will likely continue to
see some gusts in the low 40 mph range for the next few/several
hours in the Leeds Canyon area, but impacts will be on the
decline. No other updates made or planned. Previous discussion

PREVIOUS DISCUSSION...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.

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...Light and variable winds will likely continue at the SLC
terminal through the day, with winds unlikely to exceed 6 knots from
any direction. Visibilities will likely fluctuate in the 4-7SM
range. There is a 20 percent chance that MVFR/IFR ceilings develop
before 00Z, with that probability increasing after 00Z.





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