Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 201017
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
417 AM MDT Wed Sep 20 2017

.SHORT TERM...(Today through Friday)
Issued at 255 AM MDT Wed Sep 20 2017

High winds have subsided across the CWA this morning as a result of
weakening low-level gradients. Precipitation chances will end today
with significant mid-level drying beneath strong W-SW flow aloft on
the southern side of a deep low over the Pacific NW. It will likely
be another windy day with excellent mixing after 15z and 25-35 knot
flow at H7-H8 over much of the area. However, overall gradients are
not expected to support significant wind gusts that would warrant a
High Wind Warning. Some fire weather concerns today, mainly for the
east central WY and northern NE Panhandle zones (FWZ 301, 311) with
the expected strong winds. Early mixing may force RH values near or
below 15 percent, yielding a brief period of Red Flag conditions by
early afternoon. However, the models suggest increasing moisture in
the mid-levels after 18z which would help moderate dew points, plus
we would expect to see some increase in low-level moisture with sfc
winds turning to the south and southeast, especially across the Neb
Panhandle. That said, our confidence in a sustained period (3 hour)
period of Red Flag conditions is too low to warrant the issuance of
a headline at this time.

No weather impacts for this CWA on Thursday as an upper-trough digs
into the Great Basin. Southwest flow aloft will increase during the
day, with comfortable temperatures for many areas as H7 temps climb
to around +10 to +12 deg C. Thursday will be one day this period to
enjoy, because Friday marks the beginning of a dramatic change to a
much colder and wetter pattern. Upper-level troughing advances from
west to east with strongly difluent flow overhead. Scattered shower
and isolated thunderstorm activity will return. Highs will be quite
tricky, with a range from the low/mid 50s northwest to around 80 in
the far southeast portion of the CWA.

.LONG TERM...(Friday night through Tuesday)
Issued at 340 AM MDT Wed Sep 20 2017

All models continue to show a cool and unsettled weather pattern
for the weekend and well into next week. Medium to long range
models are in good agreement with the position of the strong
Pacific upper level trough as it moves eastward into the eastern
Great Basin region and stalls as it moves into the Rocky Mountain
area. A potent 110 to 130 knot jet will set up and extent
northward out of central Colorado. Models indicate numerous jet
streaks and vorticity maximum moving northward ahead of the
trough axis. With upslope flow and periods of diffluent flow
aloft, expect light to moderate rainfall with periods of heavy
rainfall, especially Saturday afternoon and Saturday night as the
second vigorous jet max moves across the high plains. Continued to
increase POP due to good model agreement even though they don`t
agree on the exact position of the heavier rainfall. These
conditions should continue into late Sunday as another jet max
lifts northward after a brief lull in the precip early Sunday.
High temperatures will struggle to reach 50 degrees across most of
southeast Wyoming, with mid to upper 50`s across western Nebraska.
Periods of fog and drizzle are also possible during this period
due to moist llvls and upslope flow near the surface. Snowlevels
will gradually lower to between 7500 to 9000 feet from north to
south. Some snow is possible in the higher valleys of Carbon and
Albany county, but timing and location are uncertain at this time.
Will likely need Winter headlines for the Sierra Madre and Snowy
Ranges since this will likely be their first heavy snowfall of the
season.

For early next week, The storm system will move very little with a
position somewhere near the four corners area. This is the portion
of the forecast that is a little more uncertain as some model and
ensemble runs want to form a closed low near the four corners
region while others keep an open wave trough over the Rockys and
progress it eastward through Tuesday. One thing all models agree
on is another round of moderate rainfall on Monday as the trough
axis/upper level low moves towards the area. Based on model
soundings, this will likely be the most favorable time period for
rain to mix with snow down to the valley floors and adjacent
plains down to 6500 feet east of the Laramie Range. Monday may
actually end up being the coolest day in the extended with highs
in the 40s along and west of I-25. By Tuesday, the trough axis
should be over the region with slightly improving conditions.
However, the Canadian model as well as a few ensemble forecasts
show the trough becoming a stalled upper level low near the
Colorado and Wyoming border, which would further enhance
precipitation rates and bring even colder air into the southeast
Wyoming. Kept POP between 10 to 20 percent for now through Tuesday
night due to considerable uncertainty.

&&

.AVIATION...(For the 12Z TAFS through Wednesday night)
Issued at 255 AM MDT Wed Sep 20 2017

VFR conditions are expected today with skies becoming mostly
clear. Winds have remained gusty overnight, but have subsided
noticeably after midnight. Winds should not be as strong today,
but south to southwest winds between 20 to 25 knots with gusts
over 35 knots are possible mainly near KRWL and KLAR. Gusts around
25 to 30 knots are expected further east.

&&

.FIRE WEATHER...
Issued at 255 AM MDT Wed Sep 20 2017

Elevated to near-critical fire weather conditions are likely across
east central WY and the northern NE Panhandle today, with RH values
falling to near 15 percent and wind gusts over 25 MPH. FWZs 301 and
311 are the most likely areas to experience this in the hours right
around noon MDT today. Conditions should quickly improve by mid, to
late afternoon. A transition to a much colder weather pattern along
with good chances for precipitation by this weekend suggests little
or no fire weather concerns after today.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

SHORT TERM...CLH
LONG TERM...TJT
AVIATION...TJT
FIRE WEATHER...CLH


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