Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 182242

Area Forecast Discussion
National Weather Service Salt Lake City UT
342 PM MST Sat Feb 18 2017

.SYNOPSIS...The upper level trough along the California coast
will work slowly east across the Desert Southwest through Sunday.
High pressure aloft will return by Monday before a colder system
impacts the forecast area midweek.


.SHORT TERM (Through 00Z Wednesday)...Split storm system is
hugging the Pacific coast this afternoon as it tracks eastward.
Ahead of the system, the forecast area remains under a very moist
southerly flow. PWs are currently in the 0.4 to 0.6 range for most
locations in Utah, close to the maximum observed this time of
year. Light precip is being observed in many locations, but
dynamics are limited at this time.

The stronger southern branch of the trough will track over the
Desert Southwest tonight into tomorrow, pushing a cold front west
to east across the forecast area. The front will provide a period
of heavier precip as it passes, and will also lower snow levels
just a bit (from 7,000-7,500 now to 6,000-6,500 feet by tomorrow
afternoon). Despite some better rates, think amounts will be
generally below advisory criteria and impacts will be confined to
higher elevation routes and therefore have not issued any
headlines at this time.

Precip will tend to taper off Sunday night/early Monday as the
trough and associated front exit to the east. However, EC/GFS
continuing to show some precip redeveloping in modest warm
advection Monday afternoon over northern Utah. This precip is then
progged to decrease by Tuesday as advection becomes more neutral
in increasing southwest flow ahead of the next storm system.


.LONG TERM (After 00Z Wednesday)...Medium range begins with an upper
level trough off of the west coast with broad southwest flow aloft
over the Great Basin. Both the ECMWF and the GFS depict a strong
baroclinic zone stretching from central Nevada through far northwest
Utah at this time. The ECMWF brings the front through by 1200 UTC
while the GFS is about 6 hours slower. Both models bring -10C to
-12C temperatures at 700 mb into northwest Utah Wednesday night.
Anticipate a fairly prolonged period of precipitation with this
frontal passage and in the deep northwest flow behind the mid level
trough axis. The models depict a fairly lengthy period of deep
neutral stability behind the trough axis that would support good
orographic lift, especially for those areas favored by northwest
flow. At this time it appears that this system will present the
potential for pretty decent snowfall totals for the northern
mountains. Valley precipitation may start out briefly as rain but
the deeper cold air appears to arrive quickly behind the front
changing valley precipitation across northern Utah to all snow by
Wednesday evening. The frontal band, and its associated
precipitation, push into southern Utah later Wednesday night and
Thursday with primarily orographically induced snowfall continuing
in the north.

The GFS is much more progressive bringing the upper level trough
through the west and by 0000 UTC Friday the trough is entering the
central plains with shortwave ridging building along the west coast.
At this same time the ECMWF is dropping a secondary shortwave
through the Pacific Northwest down the west side of the broader
trough. Thursday night the ECMWF continues cold advection on
northwest flow with 700 mb temps dropping to -17C to -18C across
northern Utah and down to -13C in far southern Utah by 1200 UTC
Friday. The GFS has backed the flow to southwest by this time with
broad warm advection underway and temps warming to -8C at 700 mb in
far southern Utah. The evolution toward the end of the week and into
the weekend is a little lower confidence given these differences.
However, the GFS ensemble mean is an outlier in this regard as the
ECMWF ensemble, and Canadian ensemble strongly support the less
progressive pattern reinforcing the trough and keeping colder air
place into the weekend. Feel more confident in the ECMWF solution,
at this time, and trended that way with the forecast. This will keep
a chance of snow across primarily northern Utah late in the week
into the weekend with below normal temperatures expected.


.AVIATION...There is a 40 percent chance of rain showers at the SLC
terminal through 06Z, which could briefly bring ceilings below 7000
feet. Rain across northern Utah will increase 06-12Z; while a slight
chance of MVFR conditions exists during this time, heavier/steadier
rain and MVFR conditions are more likely after 12Z.




SHORT TERM...Traphagan
LONG TERM...Graham

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