Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 141726
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1025 AM MST Thu Dec 14 2017

.UPDATE...
Issued at 808 AM MST Thu Dec 14 2017

We are sending out a quick update on the snowfall this morning.
Latest radar loop was showing some snow squalls developing just
east of Cheyenne. These bands are reducing the visibility to one
quarter of a mile or less this morning along with gusty wind
speeds. This will cause a threat to motorists traveling this
morning. Models are showing the potential for snow squalls this
morning into the early afternoon, due to steep low level lapse
rates and forcing from the left exit region of the upper level
jet. The best potential for these snow squalls will be mainly
east of the Laramie Range.

&&

.SHORT TERM...(Today through Saturday)
Issued at 245 AM MST Thu Dec 14 2017

Today...Windy, brisk and cold day in the wake of the cold frontal
passage, with strongest winds east of I-25, and especially over the
southern Nebraska panhandle. Leftover snow showers with the passing
shortwave trough will affect our mountain zones this morning with
snow ending this afternoon.

Tonight...Attention turns to the possible high wind event starting
after midnight for our wind prone locations such as Arlington, Elk
Mountain, Bordeaux, Vedauwoo and the I-80 Summit and foothills
between Laramie and Cheyenne. The flow aloft backs to north
northwest with the surface and low level gradients strengthening
markedly. NAM boundary layer winds increase to 50 knots over our
wind prone locations with the Craig to Casper 850 mb gradient
increasing to over 60 meters. We have opted to issue a high wind
watch for the aforementioned wind prone locations starting at
midnight.

Friday...Roller coaster like temperature swings continue as the flow
aloft continues to back to northwest, and with 700 mb temperatures
progged to be near 4 Celsius, maximum temperatures will rise into
the mid 40s at lower elevations west of I-25, with mainly 50s east
of I-25. Breezy to windy conditions will continue with progged
surface through 700 mb gradients, with high winds possible over our
wind prone locations as the Craig to Casper 850 mb height gradients
continue above 60 meters.

Friday night...Not as cold as Thursday night as the airmass has
moderated and with enough low level mixing, or winds, to limit
cooling, especially along I-25 from Wheatland to Cheyenne along the
eastern slopes of the Laramie Range.

Saturday...The NAM and GFS differ with respect to the timing of the
precipitation shield moving southward across Wyoming, with the NAM
faster than the GFS. Will stick closer to the NAM solution, which
matches up better with our neighboring offices gridded forecasts. As
the upper trough digs across western Wyoming, low level upslope will
develop over southeast Wyoming with the best chances for afternoon
snow west of I-25, and especially over our Northern Laramie, Snowy
and Sierra Madre Ranges. NAM and GFS MOS maximums vary widely
across southeast Wyoming, and have compromised between them for now.

.LONG TERM...(Saturday night through Wednesday)
Issued at 245 AM MST Thu Dec 14 2017

Going to still be dealing with snow impacts Saturday night as a
shortwave tracking west to east brings fairly widespread snow to
the area. Best chances for snow will be west over the mountains
and Carbon/Albany Counties Saturday night.

Snow comes to an end Sunday morning. Depending on snowpack...it
could be pretty cold Sunday as GFS 700mb temperatures between -8
and -10C during the day. Most locations will stay below freezing
for highs.

Arctic boundary remains north of Wyoming through the upcoming
week...laying across Montana. Could see a little warm up Monday
and Tuesday as GFS 700mb temperatures climb slightly. By Wednesday
next week we are looking at probably our warmest day as 700mb
temperatures climb back to +1 to +2C. Wednesday will probably be
our warmest day. Do not count on it lasting long though as GFS
continues to advertise a pretty strong cold front entering
northwest Wyoming Wednesday night into Thursday...eventually into
southeast Wyoming Thursday afternoon.

&&

.AVIATION...(For the 18Z TAFS through Friday morning)

Heavy snow showers have developed along and east of the Laramie
Range this morning, with brief LIFR conditions due to visibilities
near one quarter of a mile at times. This activity may last into
early this afternoon, mainly impacting KCYS, KLAR before conditions
improve by mid afternoon. VFR conditions are expected by this
evening, but will keep an eye on the potential for fog in the
valleys tonight.

&&

.FIRE WEATHER...
Issued at 245 AM MST Thu Dec 14 2017

Minimal concerns through the next week, although afternoon
humidities will dip to 15 to 20 percent Friday afternoon over far
southeast Wyoming and the southern half of the Nebraska panhandle.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...

WY...High Wind Watch from late tonight through Friday evening for
     WYZ106-110-116-117.

NE...None.
&&

$$

UPDATE...REC
SHORT TERM...RUBIN
LONG TERM...GCC
AVIATION...TJT
FIRE WEATHER...RUBIN


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