Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 190407
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
907 PM MST Thu Jan 18 2018

.SYNOPSIS...A storm system will move off the eastern Pacific and
will track across the Great Basin late Friday through the upcoming
weekend. Impacts from this storm will be felt across much of Utah
and southwest Wyoming Friday night through early Sunday.

&&

.DISCUSSION...
Water Vapor Satellite shows a deep closed low off the British
Columbia coast with ridging extending northeast from the south
Pacific through the Four Corners. 400-200mb MDCARS wind
observations place a 125-150kt cyclonic jet from off the Pacific
Northwest coast through the Pacific Northwest. GOES/SLC 00Z
RAOB/HRRR indicate that precipitable water values vary from
0.20"-0.35" mountains to 0.40-0.50" valleys. An atmospheric river
stretches from Hawaii through Central California, with abnormally
high precipitable water values extending northeast into Idaho and
Montana.

Touched up tonight and tomorrow morning, basically majority of
the pre-frontal period. 00Z KSLC RAOB indicates moistening of the
mid levels with dry low levels, and also shows warm advection with
strong winds veering with height.

Cold frontal timing has certainly slowed down during the last
several days as energy splits from the eventual northern stream
system further to our southwest. Easily can see this trend
comparing previous model runs of height falls trending further
into southern California and Arizona.

Main changes were to winds and PoPs. Took out all PoPs tonight,
lowering PoPs tomorrow morning especially further south of the
Idaho border. Even global guidance struggled to show any QPF in
the northwest Utah corner late tonight, and the high resolution
models suggest little chance of seeing measurable precipitation
through the morning as one get further south of the Idaho border
(even the traditionally early and wet NCEP WRF models).

As for winds, increased winds utilizing latest high resolution
guidance, which better depicts areas that are likely to be gusty
tonight and tomorrow morning. Already seeing winds pick up in the
usual spots such as Eureka and Vernon Hill, as well as on the
benches.

&&

.PREVIOUS DISCUSSION...
The still well defined upper low near 50N/133W late this
afternoon will gradually weaken overnight. The leftover open wave
trough will reach the west coast early Friday, then shear apart as
it enters the western Great Basin Friday night. The southern end
of this sheared trough will become better organized as it
approaches southern Utah late Saturday.

The associated surface cold front will reach far northwest Utah
early Friday morning. This surface feature will be closely tied
to the near 700mb baroclinic which should settle into far northwest
Utah by mid to late morning. Initially, precip along the baroclinic
zone will be on the light side as the colder air aloft remains
back to the northwest and lift generated by low-level thermal
advection remains quite weak.

Intensification of this baroclinic zone will occur rapidly during
the afternoon as the boundary moves south through northern Utah.
Frontogenesis will develop rapidly along this boundary leading to
strong low-level forcing as this features approaches the SLC
area. This low-level forcing supported by mid-level shortwave
energy ejecting out ahead of the advancing trough and the
favorably positioned entrance region to the jet will generate
increasingly strong and deep lift late Friday afternoon and
evening.

Looking at rain quickly changing to snow due to the strong upward
motions and much colder air pouring in from the northwest. High
intensity snowfall rates are possible along the central/southern
Wasatch Front and adjacent mountains through the evening hours.

The focus of the heaviest snow will shift into northeast, central
and southwest Utah along with extreme southwest Wyoming late
Friday night through Saturday evening. The aforementioned low-
level baroclinic zone will become nearly stationary as it tilts
more north-to south overnight and into Saturday. Low level
circulation centers forming along this boundary combined with
increasing instability and dynamic lift from the advancing upper
trough and upper divergence in the right entrance region of the
jet will bring a broad area of strong synoptic-scale lift and
widespread moderate to heavy snow to the above mentioned areas.
Significant snow accumulations are expected, with impacts expected
over a wide area near the stalled boundary. Numerous winter
weather highlights have been issued to address the potential for
heavy snow.

The upper trough axis will pass through Utah Saturday night/early
Sunday. Precip should show a decrease in areal coverage behind
the trough during the night with some exceptions. The most notable
will be along the I-15 and I-70 corridors and the adjacent
mountains. Persistent moist and unstable north-northwest flow
will maintain a convectively environment with heavy snows
Steady drying/warming aloft Sunday should bring an end to any
remaining precip during the afternoon hours.

Shortwave ridging building in from the west as the departing
trough continues to push east starts the extended period off. With
still a rather cold airmass in place (700mb temp -10C), westerly
flow begins to push in from the west as another trough digs into
the Pacific Northwest. The unsettled period of weather continues
through much of the week ahead.

A series of weak shortwaves dip down through southern Idaho into
northern Utah on Tuesday and again on Wednesday, both in more of a
warm front pattern. This should limit precipitation potential to
northern Utah, and likely to the mountains of northern Utah. By late
next week another strengthening trough system is progged to dig
south through the Great Basin. Global models are vastly out of sync
time wise, however the style and pattern looks pretty on track and
similar. This being said, kept the going forecast pretty similar to
the previous forecaster, however continued to lower snow levels late
in the week. With timing out of sync, not certain when temperatures
will really drop, but feel confident they will drop.

&&

.AVIATION...
Winds at the SLC terminal are slow to settle in from the south,
but should persist out of the south between 03-04z. Winds are
expected to strengthen by 05-06z this evening and may be gusty at
times leading up to a cold front expected at the airport after
19z.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Storm Watch from Friday evening through Sunday morning
     for UTZ008>010-517-518.

     Winter Weather Advisory from noon Friday to 9 AM MST Saturday
     for UTZ007.

     Winter Storm Watch from Friday afternoon through Saturday
     evening for UTZ006.

     Winter Storm Watch from Friday afternoon through Saturday
     morning for UTZ003-004.

     Winter Storm Watch from Friday evening through Sunday morning
     for UTZ014>016.

WY...Winter Storm Watch from Friday evening through Saturday evening
     for WYZ021.

&&

$$

PUBLIC...10/Conger/Dewey
AVIATION...Dewey

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http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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