Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 181154

Area Forecast Discussion
National Weather Service Cheyenne WY
554 AM MDT Thu May 18 2017

.SHORT TERM...(Today through Friday night)
Issued at 330 AM MDT Thu May 18 2017

Very challenging forecast with this package. Heavy snow, amounts
and snow levels are the forecast challenges.

Currently...Potent upper low seen on water vapor imagery
currently over northern Utah near Salt Lake City this morning.
Very diffluent flow over southeast Wyoming with fairly widespread
rain across the southern half of the CWFA. Surface low over east
central Colorado with inverted trough bisecting the CWFA up into
northwestern Wyoming early this morning. Freezing levels are
fairly low this morning. Earlier, F.E. Warren was reporting snow
with a temperature of 34 degrees while here at the airport,
temperatures were 39 in rain. Looking at webcams, its snowing
pretty heavy up by Vedauwoo all the way down to mile marker 353
(Warren Interchange). Wyo Hill WYDOT site on I-25 just south of
Cheyenne was showing snow with a temperature of 33 degrees. Now
they are back to 35 and it looks like the snow turned back over to

Cold air advection continues this morning with 700mb temperatures
falling from -3C currently to -5C around 15Z. Look for slowly
falling temperatures this morning. NAM MOS showing down to 31F at
15Z, falling to 29F by 00Z. This rain should change over to snow
by mid morning and continue through the day. GFS and ECMWF
continue in good agreement on movement of the upper low into
central Colorado this afternoon. CWFA stays in diffluent upper
level flow as this low slowly tracks eastward. Moderate to heavy
precip band on the northern side of this low remains in place over
the Laramie Range. SOuthern edge of this band anchored over the
south Laramie Range while the northern part of this band pinwheels
north in Converse and Niobrara Counties.

Went ahead with an advisory for Converse County and warnings for
the Pine Ridge in Niobrara and northern Sioux Counties. Getting
snow amounts of 10-12 inches for the Pine Ridge above 4800 feet.

Heaviest snow still looks to be tonight as the upper low tracks
into northeastern Colorado. Could see snow amounts close to 3
feet over the south Laramie Range.

.LONG TERM...(Saturday through Wednesday)
Issued at 330 AM MDT Thu May 18 2017

The strong low pressure system will push out to the northeast on
Saturday with drier air filtering in from the northwest. Outside of
a few mountain showers, precip should end by the afternoon.
Temperatures will warm 10 to 15 degrees from Friday. Models keep us
within cyclonic northwest flow through the early part of next week
with showers and afternoon thunderstorm chances expected each day
especially for the mountains. A vigorous shortwave will dive south
from Canada on Monday bringing a more widespread shot of precip with
a few thunderstorms as well. Winds will turn to the northwest behind
a cold front progged to move southeast across our counties midday
Monday with gusts around 35 mph likely. Tuesday will be a few
degrees cooler and not as windy. Looking toward midweek, models are
showing a fair amount of disagreement with the next system in
line for Wednesday and Thursday. GFS keeps cyclonic flow over the
northern CONUS while the ECMWF develops a shortwave ridge over
this area. For now, trended the forecast toward the consensus
blends to account for stark differences in the models.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 554 AM MDT Thu May 18 2017

MVFR/IFR conditions are widespread this morning in rain, snow and
stratus. Snow levels have dropped to about 6500 ft, and should
continue to drop through the day to about 5000 ft. Expect
periodic LIFR in snow today especially along and west of the
Laramie Range. North to northeast winds will be gusty through much
of the period as well, with gusts of 20 to 30 kts common across
the forecast area. Conditions will remain poor in rain and snow
through tonight.


Issued at 330 AM MDT Thu May 18 2017

Minimal fire weather concerns with a widespread wetting rain/heavy
snow event continuing through Friday. Cold temperatures and very
wet conditions to preclude any chances for fire starts or spreads.
Cold temperatures and high humidities to continue through the


WY...Winter Storm Warning from 3 PM this afternoon to 3 PM MDT Friday
     for WYZ102.

     Winter Storm Warning until 3 PM MDT Friday for WYZ103-105-106-

     Winter Weather Advisory until 3 PM MDT Friday for WYZ104-109-111-

     Winter Storm Warning from 9 AM this morning to 3 PM MDT Friday
     for WYZ107-118.

     Winter Weather Advisory from noon today to 3 PM MDT Friday for

NE...Winter Storm Warning from 3 PM this afternoon to 3 PM MDT Friday
     for NEZ095.



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