Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 131249

Area Forecast Discussion
National Weather Service Cheyenne WY
649 AM MDT Fri Apr 13 2018

.SHORT TERM...(Today and tonight)
Issued at 420 AM MDT Fri Apr 13 2018

**Blizzard conditions possible from Chadron southward to Alliance
  and Sidney today and tonight

**Still a bit of uncertainty remains for locations near and along
  the Interstate 25 corridor

Winter storm currently developing across the Front Range and High
Plains early this morning as the upper level trough appears to
have slowed a bit and is likely closing off from the mean flow
somewhere across western and central Colorado. It is still raining
across western Nebraska with some locations reporting heavy
rainfall and even some thunderstorms across the southern
panhandle. However, the cold air is not far behind as snow has
developed across Converse and Niobrara counties in the last hour
or so. Expect this cold air to gradually move across the area as
the low to midlevels become saturated. Expect rain to change to
snow relatively quick in the next 2 to 4 hours, sooner up north
but after sunrise around Sidney and Scottsbluff. Winds aren`t too
bad right now, but these winds will rapidly increase through the
morning. For western Nebraska, did not make many changes to snow
accumulations, especially up north with a general 5 to 10 inches
of snow expected with amounts up to 14 inches possible along and
near the Pine Ridge. Further south, added Banner and Kimball
counties to the Winter Storm Warning since models are now in
better agreement and have initialized better this morning with
higher snowfall rates towards the Wyoming border. Dropped the High
Wind Warning for these two counties, but not because we expected
lower wind speeds, but because snow and blowing snow will likely
be the primary travel impacts instead of just wind. For the rest
of the western Nebraska panhandle, the main forecast concern is
blizzard conditions. Believe the winds will hit criteria, but a
prolonged 3+ hour visibility criteria will be tough to meet since
no model shows a persistent area of heavy snowfall with these
winds. Held off on upgrading to a blizzard warning, although there
is higher than normal confidence for blizzard conditions. Will
keep a strongly worded Winter Storm Warning for now.

Further west, there is still quite a bit of uncertainty towards
the I-25 corridor as models have trended a little further west
with the moderate to heavy snow in addition to blowing snow. Some
models such as the SREF, HRRR, and even the NAM are more bullish
and aggressive from Douglas southward to Wheatland and Cheyenne.
Even the 00z ECMWF has trended west with the overall shield of
precipitation. Other models maintain the drier but very windy
solution, but these are becoming fewer with every operational run.
Decided to increase accumulations and issue a Winter Weather
Advisory for the zones bordering the current Winter Storm
Warnings. It is possible these advisories may be upgraded or
extended further west into the Laramie Range today.

Expect conditions to gradually improve late tonight, mainly after
midnight. However, considerable blowing snow may still be a
problem across the western Nebraska panhandle into early Saturday
with winds still gusting over 40 mph. With low temperatures in the
teens, may see wind chill values as low as -5 degrees.

.LONG TERM...(Saturday through Thursday)
Issued at 300 AM MDT Fri Apr 13 2018

Medium range models/ensembles are in good agreement with the progressive,
amplified large scale flow. Vigorous late season storm over the eastern
Great Plains Saturday morning tracks slowly east toward the Middle MS
Valley through Sunday. Meanwhile, a upper ridge slides eastward across
the Rockies and into the Great Plains Sunday through Monday
resulting in return to warmer than normal temperatures and dry
conditions. Temperatures rebound to above normal by Monday, with
highs back into 60s to lower 70s. Next upper trough and cold
front affect the CWA Tuesday with scattered rain/snow showers and
cooler temperatures.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 649 AM MDT Fri Apr 13 2018

IFR to LIFR conditions to become widespread along and east of
I-25 today as the winter storm system moves through...with MVFR
and brief IFR conditions possible for Rawlins and Laramie today.
Conditions should improve during the evening hours as the main
band of snow shifts east into the central Plains.


Issued at 300 AM MDT Fri Apr 13 2018

Minimal Fire Weather concerns this weekend due to much colder
than normal temperatures, widespread snow across the eastern
plains and the mountains, and high humidities. Warmer temperatures
and lower relative humidities are expected early next week, but
critical conditions appear unlikely at this time.


WY...Winter Storm Warning until midnight MDT tonight for WYZ102.

     High Wind Warning from 10 AM this morning to midnight MDT
     tonight for WYZ106>108-117>119.

     Winter Weather Advisory until midnight MDT tonight for WYZ101-

NE...Winter Storm Warning until midnight MDT tonight for NEZ002-003-



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