Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 161730
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
1030 AM MST Thu Nov 16 2017

.SYNOPSIS...Southwest flow will increase over the area today. A
cold storm system will cross the region tonight through Friday
with a return to mild weather into the weekend.

&&

.DISCUSSION...Gusty southerly winds continuing to increase this
morning across the western half of Utah, as well as eastern Nevada
and southern Idaho. Water vapor imagery has the Pacific NW cold
core trough continuing to dig and deepen as it carves into
northern California this morning. A nice stream of cold upper
level clouds are stretching from the southern Pacific into the
Sierras and through the Great Basin. With the amplifying trough
moving through the Great Basin today and building high pressure
off the Baja Peninsula, the surface pressure gradient is expected
to continue to tighten and winds will continue to howl across
western Utah. With increasing winds in the Tooele and Salt Lake
valleys expected, have opted to include these areas in the going
wind advisory through this evening.

With the winds picking up late last evening, the surface
temperatures have also warmed and kept mostly steady overnight,
with Salt Lake City remaining 61-62F since about 11 pm last night.
With the cloud cover today, not thinking temperatures will vary
much during the daytime.

A decent upper level moisture stream from central California into
northern Utah and southern Idaho is being somewhat shadowed by the
Sierras, with radar highlighting showers across far northern Utah
and southern Idaho this morning. As the upper level trough swings
farther east today, expecting the extent of showers to continue
dropping southward and moving south down the Wasatch Front this
evening. Made updates to the forecast to account for gusty winds
today, regarding the wind advisory in place. Previous discussion
describes the expected impacts and event for Friday very well.

.Previous Discussion (541 MST Thursday)... Things begin to get
more interesting overnight tonight and especially Friday. The
trough remains poised to translate ESE inland at that time,
coupled with a more favorable moisture advection trajectory as the
atmos river sags south towards the southern Sierra region. Less
lee-side blocking and deeper inland moisture penetration is
expected to eventually ensue late tonight/Friday (still not ideal
though).

Models largely agree now regarding timing of cold frontal passage
in the north Friday morning, with the front becoming increasingly
frontogenic midday Friday into the afternoon as the upper support
catches up the surface boundary in place across central/southwest
Utah. Expecting a period of widespread and locally significant
precip to develop along this boundary from the central Wasatch
Front south during that time (though areas across the east
downstream of the terrain should remain largely shadowed), with
embedded convection becoming likely due to rapid destabilization
below H5. Have upped water totals across many areas for these
areas this package due to the better forcing potential coincident
with the deep layer moisture in place. Snow levels will remain
quite high initially, then drop rapidly as the front arrives
(8kft+ on average through tonight, then to valley levels). This
said, not expecting much in the way of valley impacts as much of
the deep layer moisture will continue to reside on the warm side
of the trough.

Mtn advisories still look good (maybe a little overdone in the
far northern mtns), and expect focused accums to be in the
Central/Southern Wasatch and portions of the western Uintas.
Central/southern Wasatch valleys should see graupel showers in the
increasingly convective environment Friday afternoon, with the
central I-15 corridor likely seeing a brief but intense snap over
to snow during the afternoon. Will continue to have to monitor
potential of expanding winter advisories into the Wasatch Mtn
Valleys and the central I-15 corridor for Friday, but will leave
that to day shift after coordinating with partners as accums
should remain quite low due to a limited window of potential.
Intensities would be the primary factor.

A rapid building of heights remains forecast by Friday evening as
the trough axis slides east of the area. Expecting any and all
showery post frontal precip to end by midnight, with the last
holdovers tapering across the central/southern mtns. Much colder
temps are expected areawide for the weekend under clearing skies,
but the upper elevations will gradually warm Sunday within a
backing flow environment.

&&

.AVIATION...Breezy south winds with gusts to 35 mph will prevail at
the SLC terminal through today and into tonight. There is a 20
percent chance that winds could shift to NW this evening if a band
of rain sets up to the north of the terminal. Cigs will lower this
afternoon and evening near 6000 feet AGL with the onset of rain
showers this evening.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory until 10 PM MST this evening for UTZ003-015-016.

     Winter Weather Advisory from 11 PM this evening to 5 PM MST
     Friday for UTZ007>009.

WY...None.
&&

$$

Dewey/Traphagan

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