Area Forecast Discussion
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FXUS65 KSLC 132137

Area Forecast Discussion
National Weather Service Salt Lake City UT
337 PM MDT Fri Oct 13 2017

.SYNOPSIS...A strong but largely dry cold front will cross Utah
tonight. A colder northerly flow will follow the front on
Saturday. Dry high pressure will build into the region for the
first part of next week bringing a warming trend.


.SHORT TERM (Through 00z Tuesday)...Afternoon water vapor and H5
analysis show continued amplification of the upstream trough now
sharpening over far western Idaho. Mid level baroclinicity has
further tightened downstream of this feature, with a strengthening
near H7 thermal gradient taking form over southern Idaho. In
advance of this backing flow aloft has allowed for modest warming
aloft today.

The aforementioned trough will continue to sharpen then swing
east tonight, with the attendant cold front surging south across
much of the area by early morning Saturday. A well defined cold
pool coincident with trough axis passage continues to look
favorable for light precip generation across namely far northern
Utah early Saturday morning, with the key word being light.
Breadth of cold advection could support a few flakes making it to
bench/valley levels along the norther Wasatch Front, but no accums
are expected outside of a dusting to around an inch in the higher
mountains of northern Utah. Any precip should end by mid morning.
Gusty winds, locally strong, coupled with much cooler temps will
be the most notable sensible weather changes over the next 12-24

Regarding the winds, have opted to issue a wind advisory for
Castle Country for strong and gusty west to northwest post frontal
winds late tonight through mid morning Saturday. Potential temp
cross sections in the lee of the Wasatch Plateau indicate
a modest potential for breaking wave conditions in wake of the
front, with 40-50kt H7 flow likely mixing down at times in
vicinity of Castle Dale/Ferron, and gaps near the Fremont Junction
area on I-70. Expecting some gusts in 55mph range to develop,
likely strongest just before and around dawn Sat morning.

Elsewhere, gusty winds in the prone downslope areas should remain
more modest, with H7 support/CAA not quite favorable enough for
strong Washington County gap formation Saturday. Will continue to
monitor conditions as they come to form however.

The progressive nature of the trough will allow heights to build
in rapidly beginning Saturday afternoon, continuing thereafter
into next week. Increasingly stable and tranquil conditions will
be the norm Sunday on, with warming trend temps aloft. This said,
valleys (especially northern valleys) will be slow to modify due
to the remnant airmass and minimal mixing potential.

.LONG TERM (After 00Z Tuesday)...Strong ridging remains over the
southwestern CONUS heading into Monday night, with the ridge axis
shifting east across the Rockies through the day Tuesday. Behind
this ridge axis, a weak shortwave trough slides through the Great
Basin on Tuesday, increasing southwesterly flow over Utah. Combined
with somewhat better mixing, this should allow Tuesday to be several
degrees warmer than Monday in some valley locations.

Wednesday and Thursday are expected to remain warm and dry, with a
mostly zonal flow Wednesday giving way to increased southwesterlies
again Thursday ahead of a stronger trough pushing across the Pacific

Thursday night and beyond is where the forecast starts to get more
interesting and, subsequently, a lot more uncertain. The global
models and their ensembles have a variety of solutions on how to
handle the incoming Pacific trough, with some (including the
operational ECMWF and Canadian) keeping the best moisture and
instability north of Utah on Friday, while more aggressive models
(including the operational GFS) have a much deeper trough that
brings widespread precipitation to the state Friday and Saturday.
For now, have kept the forecast as a compromise between these two
solutions, with modest cooling and 20-40 percent POPs across the
area. Hopefully models will come into better agreement about this
trough sooner rather than later.


.AVIATION...Winds at the SLC terminal could become light and
variable or light southeast at times between 02Z and 06Z, before
northerly winds increase again with a cold front around 05-08Z.
Showers will also increase across northern Utah overnight, with a 50
percent chance of ceilings dropping below 7000ft after 06Z.


UT...Wind Advisory from midnight tonight to 9 AM MDT Saturday for




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