Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 221052 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Cheyenne WY
409 AM MDT Fri Sep 22 2017

.SHORT TERM...(Today through Sunday)
Issued at 400 AM MDT Fri Sep 22 2017

A combination of satellite and upper atmospheric observations shows
an extensive upper-level trough encompassing much of the western US
early this AM. This will be the dominant weather feature during the
next several days as the models suggest little eastward progression
through Sunday night. Moist southwesterly flow is increasing, along
with strong upper difluence in advance of this system. We expect to
start seeing the effects of this storm by afternoon, in the form of
scattered showers and isolated thunderstorms. The greatest coverage
is expected after 21z over east central Wyoming and the northern NE
Panhandle, within the exit region of the 110-120 knot H25 jet along
w/ several smaller scale disturbances embedded within the flow. The
forecast for Saturday still looks miserable to say the least, given
continued difluent flow aloft and northeasterly low-level upsloping
becoming better established. Widespread cloud cover / precipitation
will undoubtedly be the result, so expect little/no opportunity for
solar heating with highs well below guidance. High peaks within the
Snowy Range above 10,000 feet could see a change over to snow by 21
Z or so if precip rates are high enough, but forecast soundings are
still showing temperatures of 37+ F invof Medicine Bow Peak and the
WBZ heights remain above 11,500 through the day, so it appears this
will not be too much of a problem. NCAR ensembles also suggest pcpn
will stay as rain through at least 00z Sun. GFS/NAM/ECM show colder
air finally spreading eastward through the day on Sunday, but more-
so on Sunday night, so again we are tending to favor higher terrain
for an accumulating snow threat, but even that is questionable with
H7 temperatures barely below 0C unlike earlier runs. That said, the
snow accumulation forecast is low-confidence. However, there is the
possibility of significant, warning-level snow amounts above 10,000
feet or so. We contemplated a Winter Storm Watch for the Snowy Mtns
but several inches of our total accumulation forecast assumes a Sat
afternoon change-over, and that just doesn`t seem likely enough for
the time being. Widespread precipitation will continue on Sunday w/
the continued dynamic support, and highs will most likely fall into
the 45-55 degree range, warmest far southeast near KSNY.

.LONG TERM...(Sunday night through Thursday)
Issued at 400 AM MDT Fri Sep 22 2017

Good consensus from the 00Z medium range models/ensembles progging
the closed upper low over southwest WY to lift northeast into
eastern MT Monday night, with a trough axis extending southwest
through the Four Corners and Baja CA. A north-northwest flow aloft
develops by Thursday as a upper ridge builds from the Great Basin
into southwest Canada.

With the track of the upper low across central and northern WY
Sunday night and Monday, overall snow accumulations will be lower.
Three to six inches will be possible above 8000 feet over the Snowy
and northern Laramie ranges, with lesser amounts for the surrounding
valleys. Elevations below 6500 feet can expect scattered rain
showers. The precipitation will end Monday night with the departing
low. Monday will be chilly with highs 45-55F lower elevations and 35-
45F high country. Milder temperatures return Tuesday along with
drier conditions as flow aloft backs to the southwest. Highs will
range from 55-65F with 40s for the mountains. On Wednesday, some
moisture sneaks northward from CO ahead of the upper trough, with
isolated showers developing over the southeast WY mountains, plains
and southern NE panhandle. Expect highs in the 60s east of I-25,
with 40s and 50s west. Showers will taper off from north to south
Wednesday night as the lift and moisture shifts into CO.
Dry and seasonal mild temperatures are forecast for Thursday.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 400 AM MDT Fri Sep 22 2017

Clouds will increase and lower through the day today in advance of a
Pacific storm system. A cold front over northern Colorado early this
morning will move northward as a warm front today, but remain south
of Alliance and Chadron where winds will be northeasterly 10-20 kt
and gusty. South to southwest winds will prevail south of the front,
with gusts of 25 to 35 kt west of the Laramie Range. Scattered SHRA
and isolated TSRA will develop west of the Laramie Range by early
afternoon, then spread across the plains late this afternoon and
early evening. Winds will shift to northeasterly and remain breezy
tonight as a cold front moves east across the terminals.


Issued at 400 AM MDT Fri Sep 22 2017

Fire weather concerns will be very low over the next few days w/the
anticipation of colder temperatures and widespread rain & potential
mountain snow. One more dry day today w/RH values around 20 percent
or so, but better chances for afternoon showers and thunderstorms.




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