Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 132138

Area Forecast Discussion
National Weather Service Salt Lake City UT
338 PM MDT Wed Sep 13 2017

.SYNOPSIS...A series of storm systems will bring cooler and
wetter conditions to Utah and southwest Wyoming Thursday and
Friday. A milder southwesterly flow will return for the weekend.


.SHORT TERM (Through 00Z Saturday)...Ridge axis has shifted
eastward into Colorado today, and the resultant southerly flow
is drawing moisture northward into the forecast area. Associated
showers and thunderstorms are primarily focused over the higher
terrain this afternoon, and will continue into the evening hours.
Increase in moisture and cloud cover is resulting in maxes a few
degrees cooler than yesterday, but still in excess of 5F above
seasonal normals.

The storm system just off the California coast is progged to move
onshore this evening and track east overnight, reaching southwest
Utah tomorrow morning. This system will then continue over
southern Utah through tomorrow night, producing good coverage of
showers and thunderstorms for much of the CWA, with heavy rain
possible over southern and eastern Utah. Have added mention of
this in the grids.

A second system dropping down from the Pacific Northwest is
expected to follow quickly on the heels of the first system,
entering northwest Utah by Friday morning. This system should
bring a fairly well defined cold front as far south as southern
Utah by Friday afternoon, with guidance indicating 700 mb temps as
low as 0C to -1C to northwest Utah. Best precip will be confined
to northern and central portions of the forecast area with this
system, with snow possible at higher elevations.

.LONG TERM (After 00Z Saturday)...The axis of the cold northern
branch trof is forecast to be moving into wrn UT at the start of
the long term period with the frontal zone precip shifting over
the east and south, and tapering off.

Expect most of the CWA to clear out overnight as the trof axis
shifts east across the area and the lift moves away. Some
instability showers could linger across the north into Sat morning
with snow levels dipping below 8000 ft but don`t expect much
impact or accumulation from these.

The GFS is faster moving the trof out on Sat and flipping the
flow back to swly by the end of the day. It also generates a
little convection over the srn mtns late that lifts nwd overnight
with a weak short wave ahead of the next deep trof. The wave
crosses the nrn CWA on Sun with a continued slight chance of

The next cold trof deepens to our west on Mon with increasing
south winds across the CWA. The GFS and EC part ways at this
point with the GFS much more aggressive moving the trof inland
with the associated cold front into the nwrn corner of the CWA by
Mon eve. It then swings a strong front thru the CWA along with
widespread precip Mon night thru Tue with snow levels possibly
down to the benches by Tue aftn.

The EC is much slower moving the upper trof into the Great Basin,
and digs the trof down the coast Tue forming a closed low over
the wrn Great Basin by Wed aftn. This keeps the CWA in a strong
mild southerly flow thru Wed with only a slight chance of precip
over the far nw.

I continue to think the EC has a better handle on the evolution of
this trof, and that GFS is too fast and cold. So have based the
long term forecast on the EC solution keeping temps much milder
thru Wed with little if any chance of precip.


.AVIATION...Light and variable winds at the SLC terminal as of 21Z
could continue through the afternoon, though the most likely
scenario is still increasing northwesterly winds between 22Z and
00Z, before switching back to the southeast around 02-04Z.
Scattered showers will continue across northern Utah through the
evening hours, which could also cause erratic winds at times.


.FIRE WEATHER...Increasing southerly flow has allowed moisture to
spread into the district today ahead of the first in a series of
storm systems. This has allowed for convection to develop over the
higher terrain of Utah that will continue into the evening. The
first system is progged to southwest Utah tomorrow morning,
tracking across the rest of southern Utah through Thursday night.
Wetting rains are likely across southern Utah as this system moves
through, with locally heavy rainfall possible.

Behind this first system, a second system will dive in from the
Pacific Northwest Friday morning, bringing a good cold front into
southern Utah by Friday afternoon/evening. Precipitation with this
system will be focused over northern and central Utah, with some
snow possible at higher elevations into Saturday morning. High
pressure will briefly return behind this system, but another
trough is expected to impact the area early next week.




LONG TERM...Wilensky

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