Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 210116 AAA

Area Forecast Discussion...UPDATED
National Weather Service Salt Lake City UT
716 PM MDT Tue Mar 20 2018




.SYNOPSIS...A developing storm system along the west coast will
bring rain, high elevation snow and gusty south winds to much of
the state for the latter half of the week. This storm will move
east through the area this weekend, leading to cooler temperatures
and lowering snow levels through Sunday.


.SHORT TERM (Through 00z Saturday)...Afternoon water vapor and H5
analysis place a near stationary closed low off the Cali coast
along 140W, and a deeper northern branch trough translating
southeast along the BritCol coastline. Orientation of the former
has allowed for a substantial sub-trop moisture tap with an
ensuing atmos river (AR) continuing to take form from the central
Cali coast and points upstream to the central Pacific. The latter
will provide the kick to allow downstream translation of the
remnant energy from the Cali low and inland penetration of the
deep moisture. This will occur Thursday.

Previous to this, low amplitude ridge will continue to build over
the area, with remnant mid/high moisture downwind of the Sierra
combined with mid level warm advection should induce a few light
showers Wednesday, especially across the far north.

Orientation of the AR will begin to take better form Wednesday
night as the southern fringe looks to begin to shift south of the
high Sierra allowing for increasing PWAT and resultant orographic
precip for the SW flow favored areas across the western half of
the area. Snow levels will be on the rise however, with the
rain/snow line likely pushing 9kft by dawn Thu. Orientation of the
AR will become most favorable locally Thursday afternoon into the
overnight hours as trajectories will allow for better inland
penetration as SW trajectories will then be south of the Sierra
minimizing upstream blockage on the large scale. This should be
the period of heaviest precip focused across the the southwesterly
flow favored areas, and highest snow levels (pushing 10kft).

Outside of onset of precip, have to note H7 flow increasing
rapidly across the west Thursday, with a core of 60+kt flow
likely developing over west central and southwest Utah. This
coupled with a modest 5mb s-n surface gradient across the southern
half of the area should be enough for strong southerly wind
development there. Have some concern that cloud cover and
increasing potential of precip could inhibit full mixing, but do
expect some shadowing/downsloping from the Pine Valleys upstream.
Opted to issue a high wind watch for now, but will monitor.
Advisory levels look like a slam dunk at the least.

Interesting note in guidance to point out over the last several
runs. NAEFS IVT/PWAT anomalies continue to look even more
impressive, but model QPF has begun to decline in both
significance and areal coverage in many areas. A look vertically
at the BUFR data adds a little light to this as much of the deep
layer moisture is progged to be above H7, with dry boundary layers
being maintained across the valley areas. Although the remnant
wave passage will still occur and provide a kick as it ejects
across the area later Thu into the overnight hours still allowing
for widespread precip, expecting a wide distribution of rain and
high elevation snow due to the orographics providing the main
forcing mechanism.

More orographic or not, still looking at the potential for quite a
few impacts from this event. Expecting notable river rises across
the Virgin River basin, portions of the Bear, and Blacksmith Fork
(with the latter two forecast to bankful) at the least, though
lack of low/mid elevation snowpack will keep much of the hydro
concerns in check. Dense/heavy snow and blowing snow for
elevations in excess of 9kft (slightly higher at times), and
strong winds.

Influence of the northern branch trough will take form Friday on,
with a net cooling in wake of a mid level baroclinic passage. Will
likely see a brief period of snow down to 6kft with passage.

.LONG TERM (After 00Z Saturday)...The parent upper level low remains
just off the Pacific Northwest coast Friday evening. Global forecast
models begin to move the trough inland through the weekend with some
different solutions among models. Per the previous forecast shift,
the GFS continues to be more organized bringing a cold front through
Utah and the Great Basin area Saturday through Sunday. The GFS also
advertises a more frontogenetic front with colder 700mb temperatures.
The ECMWF has a slightly different approach; is slower and warmer
and much less organized as it splits a wave farther north through
Idaho and Wyoming while the deeper portion of the trough swings
farther south. Both models do however swing the front and associated
trough through Utah by late Sunday with similar airmass features
behind the trough.

Heading into next week, Monday and onward, models continue to
change, both with respect to one another and run to run. The big
picture paints the weather pattern to remain unsettled but the
tricky part will be to determine just what part of Utah and southern
Wyoming see more activity. The EC drags the system farther south and
west and as of the latest 12z run creates a cutoff Low over the
Desert Southwest heading into mid-week, which could translate to
easterly winds across northern Utah. The latest GFS also sags the
trough southward while several small perturbations rotate around the
larger trough through the Desert Southwest. As confidence decreases
greatly in the extended period, and with continued model
disagreement, kept forecast with a broad brush blend. That said, the
far extended forecast may flip flop a bit until we can get a more
clear idea of what will play out.


.AVIATION...Light northwest winds at the KSLC terminal will shift to
the southeast around 04z this evening. Increasing clouds filtering
in from the west will gradually lower ceilings overnight but
remain above 7000 feet.


UT...High Wind Watch from Thursday morning through Thursday evening
     for UTZ015-016.




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