Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 172345
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
445 PM MST Sat Feb 17 2018

.SHORT TERM...(Tonight - Monday Night)
Issued at 252 PM MST Sat Feb 17 2018

An extremely active period of weather across southeast Wyoming and
the western Nebraska Panhandle over the next 24-48 hours. Concerns
remain focused on strong winds through tonight, and the threat for
significant snowfall on Sunday and Monday.

A few wind gusts around 60 MPH have been observed around Arlington
this afternoon, but generally speaking most areas have remained at
or below 50 MPH with fairly modest flow aloft. We still anticipate
a dramatic up-tick in wind speeds overnight as a fairly strong sfc
low tracks across northeast WY into western SD. H7 and H85 CAG-CPR
gradients climb to between 80 and 90 meters as this happens, along
with H7-H8 flow of 55-65 knots over much of southeast WY and 65-75
knots over the N. Laramie Range. Significant downward omega should
support widespread strong wind gusts over the wind prone areas, as
well as some adjacent plains/valley areas as well. We extended our
High Wind Warnings through 16z as flow aloft remains elevated into
mid Sunday morning. After that, the high wind threat should slowly
diminish with an approaching cold front and weakening gradients. A
blustery day is likely Sunday along/w of the Laramie Range, but it
appears winds will remain below criteria at this time. Next couple
of shifts will want to re-evaluate, however.

Attention then shifts to our Winter Storm Potential. There is some
potential for snow to begin as early as mid Sunday morning, mainly
across northern Converse county w/ the initial passage of the cold
front. This initial period of snow may be fairly heavy, with model
guidance suggesting very strong frontogenesis along w/ a favorable
position of the upper jet. That said, we upgraded those areas to a
Winter Storm Warning with this package and opted to include higher
elevations in the N. Laramie Range and Ferris/Seminoe Mountains as
well. Special thanks to WFOs UNR and RIW for the coordination this
afternoon in making a decision to upgrade. Otherwise, this appears
likely to be a widespread significant snowfall as the RRQ of a 120
knot H25 jet traverses the CWA. Snow is likely to spread southeast
across the area through the day, and in general become heaviest in
the Sunday night/Monday time frame with impressive frontogenesis &
strong upper level dynamic support. Consensus guidance and WPC are
all looking very reasonable with widespread 8-12 inch amounts with
locally higher totals in the mountains. Slightly lower amounts are
expected from the Cheyenne area into the S. Panhandle, but we will
still likely have Advisory level amounts. NAM soundings also would
suggest some potential for a period of light freezing drizzle late
Sunday night for these areas, while the GFS is colder and suggests
predominantly snow. Will need to keep an eye on this as well. Kept
Winter Storm Watches generally as-is, but decided to include areas
from Torrington/Goshen county toward Alliance/Box Butte county for
expected amounts in the 4-8 inch range through Monday night.

Dangerous wind chills are also likely on Sunday night, but more so
Monday night with snow ending and deeper cold air in place.

.LONG TERM...(Tuesday - Saturday)
Issued at 252 PM MST Sat Feb 17 2018

No major forecast concerns through the remainder of next week. The
upper-level pattern favors a few passing weak disturbances, but we
expect these to be fairly dry systems with limited chances for any
precipitation.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday afternoon)
Issued at 445 PM MST Sat Feb 17 2018

Strong westerly winds with gusts 35-50 kt will continue to be the
primary aviation concern across southeast Wyoming through early
Sunday morning, especially along and west of Interstate 25. Moderate
to severe mountain wave turbulence can be expected. Introduced VCSH
at Rawlins beginning 15Z Sunday, with MVFR-IFR conditions developing
in light snow and fog from east central Wyoming to the northern
Nebraska Panhandle behind an Arctic cold front after 20Z. There will
be occasional mountain obscurations from snow shower activity.

&&

.FIRE WEATHER...
Issued at 313 AM MST Sat Feb 17 2018

Strong winds continue today ahead of an approaching arctic front
moving into the area from the north. Will see widespread strong
winds through the day today into this evening. By Sunday late
morning, we begin to see the front move into Converse and Niobrara
Counties with winds shifting north and northeast behind the front.
Widespread moderate snow and much colder air moving in behind the
front. Just about every location in southeast Wyoming and Nebraska
panhandle are going to see snow accumulations with the most
significant along and north/west of a line from Chadron to
Wheatland to Buford. Snow continues into Tuesday before ending.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Watch from Sunday evening through Monday evening
     for WYZ106>117.

     High Wind Warning until 9 AM MST Sunday for WYZ104>107-109-110-
     113-115>117.

     Winter Storm Warning from 9 AM Sunday to midnight MST Monday
     night for WYZ101-103>105.

     High Wind Warning until 9 AM MST Sunday for WYZ101-103.

     Winter Storm Watch from Sunday morning through Monday evening
     for WYZ102.

NE...Winter Storm Watch from Sunday evening through Monday evening
     for NEZ002-003-095-096.

&&

$$

SHORT TERM...CLH
LONG TERM...CLH
AVIATION...MAJ
FIRE WEATHER...GCC


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