Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 171032
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
432 AM MDT Tue Apr 17 2018

.SYNOPSIS...A cold Pacific trough currently overhead will shift
downstream over the next 24 hours. High pressure will return
Tuesday night through midweek, followed by the next storm system
late in the week.

&&

.SHORT TERM (Through 00z Saturday)...Early morning water vapor and
H5 analysis depict a positively tilting trough axis exhibiting
trends of a rapid lift to the northeast. MSAS MSLP analysis places
the attendant surface cold front along a rough line from northern
portions of Castle Country SW across the Grand Staircase, with
upstream baroclinicity along the mid level front from SW Wyoming
into west central Utah. As of 4am areal extent of snowfall is
greatest beneath this mid level front, though do note a rapid
thinning from the northwest across northern Utah as drier
boundary/mid level air has begun to advect in coincident with
downstream frontal progression.

Currently noting slush impacts along the higher passes of I-15/70
in central Utah where a combination of forcing along the
baroclinic zone and orographics has allowed for pockets of heavy
snow (especially noted in vicinity of Cove Fort). Select passes
across the central mtns and the Wasatch have and continue to see
impacts as well. Continue to feel this is a near term issue
largely through dawn due to the aforementioned evolution of the
upper trough and continued downstream progression of the front.
Surface visibilities have been trending up across those areas, and
radar signatures have begun to wane supporting this thinking.
Have maintained winter headlines through 10am in the mtns, but may
readdress over the next few hours if trends of improvement
continue and impacts trend more negligible.

Pre-frontal winds remain elevated across Castle Country/SE with
gusts generally borderline/low end advisory. Expect speeds will
generally decrease a bit with frontal passage mid morning (outside
of the SR-10 corridor where they should briefly increase). Have
maintained the wind advisory through mid morning there for now.

Low/mid level instability will promote convective snow showers
late morning and afternoon, but areal coverage should become
primarily focused over the north where the H5 cold pool and
steepest lapse rates will reside. Do not foresee much/if any
impact from these, with coverage primarily isolated in the valleys
and scattered in the mtns. Any convection will wane rapidly late
day however as mid level ridging builds in, orienting over the
area through tomorrow.

Upper ridging will shift downstream Wednesday night as the next
upper trough closes and digs SE into the lower Colorado River
basin. Globals continue to indicate a tightly wound upper low
developing along the UT/AZ border Thu night through Friday, with
precip initially tied closest to the northern flank of the mid
level circulation. Have decreased PoPs across the north as
orientation of heightened mid level isentropic lift will focus
across the southwest, but increased PoPs There. Return flow and
modest upglide/orographics will however promote a better chance of
precip across the NE and (Uintas especially) as the trough begins
to shift downstream mid/late Friday.

.LONG TERM (After 00Z Saturday)...The early part of the long term
period will feature a closed upper low over the southern portion
of the Great Basin. Global modes all support low pulling away from
region, though GFS offers a slower solution than ECMWF and
Canadian. Even more progressive models solutions suggest a
continued chance for some light precipitation across eastern
portions of forecast area Friday night. Snow levels looks to be
generally in the 7,000-8,000 foot range.

In the wake of the system exiting the Great Basin, a ridge of high
pressure will gradually build over the region on Saturday.

The ridge of high pressure will slide east of the forecast area on
sunday, as a decaying low amplitude disturbance crosses the northern
Great Basin, far ahead of an upper level trough of low pressure off
the Pacific coast.  Any impact of this system will be limited to
some cloud cover, and perhaps few light snow showers over the higher
terrain of northern Utah and extreme southwest Wyoming.

GFS and ECMWF are in general agreement with holding upper trough off
of Pacific Coast Monday and Tuesday, with weak systems ejecting out
of trough scraping across the northern portion of the Great Basin.
With only weak forcing and little moisture to work with, precipitable
water values still less than a half an inch across much of forecast
area, will only opt for some slight chance wording for precipitation
across primarily higher terrain of northern Utah and extreme
southwest Wyoming.  With warming temperatures aloft and an
increasing southwest flow, high temperatures will be 5 to 10 degrees
above normal.

&&

.AVIATION...Generally VFR conditions will prevail at KSLC
through the valid TAF period, though ceilings could remain below
7,000 feet through around 16Z.  Additionally through 16Z, there will
be a chance for lake effect snow showers with MVFR ceilings and
visibilities.  Northwest flow should increase as the morning
progresses.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory until 9 AM MDT this morning for UTZ013-021.

     Winter Weather Advisory until 10 AM MDT this morning for UTZ007-
     008-517.

WY...None.
&&

$$

Merrill/Barjenbruch

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