Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 212312

Area Forecast Discussion
National Weather Service Salt Lake City UT
412 PM MST Wed Feb 21 2018

.SYNOPSIS...An active weather pattern will prevail through the
next 7 days as a series of weather systems move through the


.SHORT TERM (Through 00z Sunday)...Afternoon water vapor and H5
analysis continue to place the eastern Pacific ridge along 150W,
with a long wave trough downstream encompassing the western CONUS
attm. Several short waves are either currently embedded within the
western flank of this trough, or lined up to crest the ridge and
become so moving forward through the forecast period. Due to the
amplitude and orientation of the western CONUS trough, an
unseasonably cold airmass will remain in place coincident, and
unsettled conditions with periodic snow will be the norm more
than not over the next 7 days.

Not much to note regarding the weak short wave currently dropping
southeast into southern Nevada. Meager forcing/moisture and track
well west/south of the area will only promote clouds with
isolated snow showers over the central/southern mtn spines.

Of more note is the next short wave currently showing signs of
sharpening along the southern Britcol coast. Good agreement exists
in guidance that this wave will continue to sharpen as it
translates into the Great Basin tomorrow and then over the area
tomorrow night through Friday. Although this next short wave will
still largely have a continental track and sub par moisture/weak
flow, enough forcing will be present across the area to generate
snowfall. Upper jet support and modest PVA (south), a mid level
circulation/shear axis (northwest), and mid level axis passage
(areawide Friday) look to be the primary mechanisms to drive this
light to moderate snowfall at times tomorrow night through Friday.
Additional, the cold airmass in place and weak CAA with the wave
will allow even light amounts of new snow to stick to area
roadways periodically through that time. Focused concern areas
include I-15 largely south of SLC, the southern 89 corridor down
to the UT/AZ line, and portions of I-70, especially Thursday
night/Friday morning.

Opted to not issue headlines at this time but have began messaging
for the likelihood of travel impacts (even light amounts of new
snow), as models are still lining up details regarding evolution
of some of these forcing mechanisms within this trough. Do have
highest confidence in forecast snowfall amounts across
central/southern portions of Utah where large scale ascent looks
more prolific, but some areas in the north remain questionable at
this time. Many areas will most likely see borderline/sub normal
advisory criteria, but the travel impacts look to warrant
headlines in future shifts.

Downstream translation of the trough Friday night into Saturday
will allow for a brief and relative break from snowfall, but the
next sharpening wave will be right on the doorstep by late day as
the next progressive short wave moves in from the northwest.

.LONG TERM (After 00Z Sunday)...The long term period remains
dominated by a mean longwave trough over the western CONUS. As a
result, an active pattern featuring a series of shortwave troughs
crossing the area with periods of precipitation and unseasonably
cold temperatures will continue.

A fast-moving trough is still on track to cross the area Saturday
night into Sunday. The 12Z and now the 18Z GFS runs have trended
deeper with this storm. As a result, have spread higher PoPs farther
south. The next storm, a deeper and slower-moving system, is still
forecast to affect the area late Monday through the middle of next
week. However, this storm system is starting to trend towards a more
splitting pattern, especially now in the 18z GFS. While there is
lower confidence in the evolution of this particular system for now,
the global longwave pattern is not exhibiting any signs of a
significant change.


.AVIATION...Northwest winds at the SLC terminal are expected to
switch to the southeast by 03z this evening. Scattered high clouds
will persist overnight.





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