Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 151005

Area Forecast Discussion
National Weather Service Cheyenne WY
305 AM MST Wed Nov 15 2017

.SHORT TERM...(Today through Thursday night)
Issued at 305 AM MST Wed Nov 15 2017

Main concerns for this period will be the potential for high winds
over western portions of SE Wyoming on Thursday as well as a
period of accumulating snow for the mountains.

One more fairly nice day in store for today as modest ridging
builds over the Rockies. It will be seasonably cool with highs in
the low to mid 50s over most lower elevation sites. Breezy
southwest winds will develop during peak mixing over western
Carbon County.

Strong upper trough/closed low off the coast of Washington and BC
early this morning will begin to shift E`ward tonight and Friday.
This will serve to beat down the ridge over the Rockies, however
ridging will persist in the subtropical stream over the desert SW.
Intensifying upper jet will nose into SE Wyoming by Thursday
morning as a lead shortwave trough ejects NE`ward across northern
MT and southern Alberta. As this happens deep layer unidirectional
SW flow will increase rapidly. Best surface pressure falls
associated with the ejecting shortwave will initially be over
southern Alberta but both the NAM and GFS indicate a second area
of significant pressure falls developing over NE Wyoming during
Thursday afternoon. The combination of the developing surface low
to the NE, and 50kt winds as low as 2000 feet on BUFKIT soundings
from the NAM and GFS tend to increase confidence in the potential
for high winds especially in the Arlington area and the immediate
lee of the North Laramie Range. Decided to go ahead with a high
wind watch for lower elevations of Converse and Carbon Counties as
well as the wind prone locations near Arlington. These areas were
where the best overlap of strong low level SW flow, deeper mixing,
and subsidence will reside. Unfavorable location of the strongest
pressure falls as well as weaker subsidence led to leaving the
Bordeaux wind corridor out of the watch for now.

As the lead shortwave moves east and mixing ends on Thursday
evening, think we will see a brief diminishing trend in the winds
for Thursday night. The trailing portion of the main upper trough
will then begin to approach late Thursday night. Increasingly
moist and strong WSW flow will lead to orographically enhanced
snow developing over the higher elevations of the Sierra Madre and
then the Snowy Ranges by late Thursday night. Snow levels will
initially be quite high, around 9000 feet but will begin to fall
by late Thursday night.

.LONG TERM...(Friday through Tuesday)
Issued at 305 AM MST Wed Nov 15 2017

Forecast guidance in good agreement bringing a strong cold front
through Carbon and Albany Counties through the day Friday. This is
a really good setup for a heavy mountain snow event for the Snowy
and Sierra Madre Ranges as they will be on the northern side of a
150kt jet associated with the front providing strong dynamics.
Look for falling temperatures through the day. We need to be
watching surrounding foothills for additional winter headlines
during this time. Did lower snow levels over guidance by roughly
500-1k feet and still getting rain for lower elevation for a time
before it all turns over to snow later Friday morning. Later
forecasts may need to transitions rain over to snow earlier than
what we have in there now.

Precip comes to an end Friday night into Saturday and upper
ridging returns. Rest of extended looks warm and dry through


.AVIATION...(For the 06Z TAFS through Wednesday evening)

VFR conditions expected tonight and Wednesday with increasing high
clouds by late Wednesday morning. South to southwest winds will be
gusty at times Wednesday afternoon with gusts up to 25 to 30 knots.


Issued at 305 AM MST Wed Nov 15 2017

Cooler temperatures today will mitigate any fire weather concerns.
It will be dry and windy on Thursday but temperatures will remain
cool enough in areas where the winds are strongest to keep
elevated or critical conditions at bay.


WY...Winter Storm Watch from Thursday evening through Friday evening
     for WYZ112-114.

     High Wind Watch from late tonight through Thursday afternoon for

     High Wind Watch from Thursday morning through Thursday afternoon
     for WYZ101-104-105-109.



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