Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 230042

Area Forecast Discussion
National Weather Service Cheyenne WY
642 PM MDT Tue Aug 22 2017

.SHORT TERM...(This evening through Thursday night)
Issued at 1249 PM MDT Tue Aug 22 2017

Upper ridging in the northern stream will remain in place over
the northern Rockies through Thursday before migrating out into
the plains on Thursday night. A southern stream trough off the
California coast will dampen as it undercuts the ridge through
Thursday. This trough will serve to start advecting more monsoonal
moisture into the area starting Wednesday afternoon and
continuing through Thursday. With this increase in moisture we
should see an uptick in convection Wednesday especially over the
mountains and areas west of I 25 in southeast Wyoming. Shear will
be weak as the strongest mid level flow remains well to our north
so do not expect much in the way of severe threat. There is some
threat for gusty downdraft production tomorrow especially over
Carbon County as model sounding show inverted-v low level profiles
combined with PW values around 0.8 inches.

A mid level shortwave emanating from the trough over the SW US will
track eastward over the CWA on Thursday.  Increased POP over the
plains on Thursday as the favorable timing of the wave will combine
with a surface trough along the Laramie Range to produce at least
scattered convection. MLCAPE values will range from 500 to 1000 J/kg
over southeast Wyoming to as high as 2000 J/kg over the Panhandle.
Low level SE flow will be topped by modest 20kt NW flow leading to
bulk shear values around 30kt.  Thus may see some stronger multi-
cell storms Thursday afternoon with strong/gusty outflow winds and
small hail being the main threat.  This convection may last into the
late evening hours before the mid level wave pushes east into
central Nebraska.

.LONG TERM...Friday through Tuesday)
Issued at 1249 PM MDT Tue Aug 22 2017

Models in fairly good agreement in showing the upper level ridge
breaking down a bit on Friday as a weak mid level shortwave
crosses northern Wyoming/southern Montana into the northern
Plains. A few showers/thunderstorms are expected to accompany
this shortwave, but due to lack of moisture it will most likely
not be widespread. This feature will send a frontal boundary
through areas mainly east of the Laramie Range on Friday which may
yield to some cloud cover ahead of it. Behind this shortwave,
things should dry out fairly quickly which may lead to more
critical fire weather conditions especially in areas west of the
Laramie Range.

Beyond this timeframe, there is some uncertainty on how fast the
upper level ridge builds across the western U.S. The ECMWF/GFS are
drying things a little bit more than the GEM, due to the upper
level ridge building over the area a bit more quickly. Either way,
it appears like only small chances of showers and thunderstorms
with those chances focused east of the Laramie Range. Areas on
the west side of the Laramie Range will be looking at mostly dry
weather along with potential for critical fire weather conditions
through early next week.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 640 PM MDT Tue Aug 22 2017

VFR conditions will prevail through the period, with just some
periodic high-level clouds passing thru. Routes over the southeast
Wyoming mountains will be impacted with afternoon convection after
19Z Wednesday, but confidence is too low to add to the terminals
at this time.


Issued at 1249 PM MDT Tue Aug 22 2017

Deeply mixed and dry boundary layer has developed
this afternoon over lower elevations of Carbon County.  Winds have
been below 20 mph for the most part and this has precluded the need
for any Red Flag Warnings at this time.  Some elevated conditions
are likely through early evening over western and northern parts of
Carbon County.  With increasing moisture and cooler temps Wed and
Thurs, expect little in the way of fire weather concerns.  Will need
to watch for elevated conditions late in the week and the weekend as
drier and breezier conditions return especially west of the Laramie




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