Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 190448

Area Forecast Discussion
National Weather Service Salt Lake City UT
948 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.


Water Vapor Satellite shows a split between a northern trough
moving across the interior west, and a cutoff low over southwest
Arizona. 400-200mb MDCARS wind observations place a 150-185kt
anticyclonic jet over the eastern Pacific. GOES/SLC 00Z RAOB/HRRR
indicate that precipitable water values vary from 0.15"-0.40"
mountains to 0.50"-0.75" valleys.

Deformation zone north of the closed low will traverse southern
Utah tonight and tomorrow, bringing another round of fairly
widespread precipitation. Southeast facing slopes will see locally
higher totals due to orographic effects.

Northern trough approaches tonight and passes through tomorrow.
850-700mb flow generally remains southwesterly, turning westerly
with the frontal passage. Based on the trough positioning and
areas favored in southwest flow, many areas of the northern
Wasatch will do well, especially above 8000 feet. Put out a Winter
Weather Advisory for this area. Pockets of the mountains across
north central and central Utah will do well but impacts should
remain intermittent based on precipitation intensities due to a
snow level near 7000 feet and a relatively higher sun angle (mid-
late Feb) during the day tomorrow.


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.

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.


UT...Winter Weather Advisory until 4 PM MST Sunday for UTZ007.




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