Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 220511

Area Forecast Discussion
National Weather Service Cheyenne WY
1011 PM MST Tue Feb 21 2017

Issued at 758 PM MST Tue Feb 21 2017

We are within 12 hours of the start of snow in the Snowy and
Sierra Madre Ranges. So went ahead and upgraded to a warning.
Latest mosaic radar showing a band of fairly strong showers just
approaching eastern Sweetwater County now and should be into the
mountains of Carbon County in the next several hours. Updates have
been sent.


.SHORT TERM...(This evening through Wednesday night)
Issued at 343 PM MST Tue Feb 21 2017

Winds easing over the area this afternoon as the gradient likewise
is slackening. Expect a fairly quiet night across the CWA with
some increase in cloudiness mainly out west. Snow showers will
persist over the higher mtns as moisture rides up the west-facing

One more mild day across the CWA Weds ahead of an upper trough
that will be over the western CONUS. Sctd rain and snow showers
will continue over far western parts of the CWA during the day as
the moisture feed continues over that area. Should see a slow
increase in clouds elsewhere. Meanwhile a lee cyclone will begin
developing over eastern Colorado by late in the day. More
significant changes should ensue Weds night as the lee cyclone
continues to deepen over eastern Colorado and colder air sinks
south across the CWA. Some diffluent upper flow and jet dynamics
should allow for an increasing area of snow with some banding
expected. Main problem will be the location of the heavier bands.
Models continue trend of shifting heavier activity a little
northward. Given trends in models have advanced the Winter Storm
Watch over the higher mtns to early Weds morning then started a WS
Watch over most of the rest of the CWA Weds night.

.LONG TERM...(Thursday through Tuesday)
Issued at 343 PM MST Tue Feb 21 2017

Significant winter storm is still on track for late Wednesday night
through Thursday night, taking aim at the CWA.  The biggest change
in the 12Z guidance compared to previous cycles was a northward
shift in the axis of heaviest QPF, away from areas along the
Colorado Border, and toward the northern half of the CWA.  Large
scale ascent (QG convergence) really gets going between 03-06Z
Thurs.  There will likely be some banding also as the GFS indicates
good midlvl frontogenesis and elevated instability.  Went ahead and
issued a Winter Storm Watch for almost the entire CWA with
widespread 6-12 inches over lower elevations.  Was least confident
about amounts in areas along Interstate 80 due to the aforementioned
northward shift in the best forcing.  Patchy blowing snow will be
possible through the event, especially by Thursday aftn/evening as
the pressure gradient strengthens and north winds gust to 30-35 mph.

Snow will begin to decrease in intensity by late Thursday evening
and into the night. The remainder of the extended will feature
below normal temps in northwesterly flow aloft, however it will be
mostly dry after Friday morning.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 950 PM MST Tue Feb 21 2017

Area of snow west of Laramie Range will impact both KRWL and KLAR
during the overnight hours. Expect IFR/LIFR conditions to remain
ongoing at KRWL through sunrise with timing at KLAR around 09Z.
HRRR is about 2 hours too slow on the onset, so adjusted
accordingly based off trends. A widespread snow event looking
likely across southeast Wyoming and nebraska Panhandle Wednesday
night through Friday morning that will have major impacts on all
airports across the area.


Issued at 343 PM MST Tue Feb 21 2017

Fire weather concens will be quite low with a return to colder and
wetter conditions over the next few days.


WY...Winter Storm Watch from Wednesday evening through Friday morning
     for WYZ101>110-113-115>119.

     Winter Storm Warning from 5 AM Wednesday to 11 AM MST Friday for

NE...Winter Storm Watch from Wednesday evening through Friday morning
     for NEZ002-003-019>021-095-096.

     Winter Storm Watch from Thursday morning through Friday morning
     for NEZ054-055.



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