Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 192213

Area Forecast Discussion
National Weather Service Cheyenne WY
313 PM MST Fri Jan 19 2018

.SHORT TERM...(This evening through Saturday night)
Issued at 226 PM MST Fri Jan 19 2018

Forecast discussion will mainly concentrate on the Winter Storm
set to impact southeast Wyoming starting Saturday morning, and
western Nebraska late Saturday afternoon. Clouds will continue to
increase and lower tonight ahead of the storm system. Models show
some snow developing across the far northern and western zones as
early as midnight across far western Carbon county, including the
mountains, and far northwest Converse county. Timing looks on
track for most of Carbon county, northern Laramie county, and
Converse county with snow developing in the morning or just before
noon. Upper level trough will begin to close off the from mean
flow aloft by Saturday afternoon, which will result in slower
propagation of snowfall developing across the plains. Decided to
upgrade the Winter Storm Watch to a Warning across most of Carbon
and northern Albany counties. Added the northern Laramie Range as
well since over a foot of snow is expected. Believe most areas
along and north of I-80 towards the lower North Platte river
valley, east of the Laramie Range, will be mainly dry through
early Saturday afternoon with highs in the mid 30`s to mid 40`s
before temperatures quickly fall through the afternoon.

As the upper level low nears the four corners area late Saturday
evening, snow is then expected to develop and move northeast into
the northern Nebraska panhandle for locations east of I-25. Models
have continued to be consistent in showing better dynamic forcing
for this area and higher snowfall rates/amounts. All models show
impressive upper level difluence along and east of I-25 through
Saturday night and into early Sunday morning. In addition to warm
air advection just above 700mb, saturated conditions, and upslope
flow, expect moderate to heavy snowfall rates for a 3 to 6 hour
time frame. Believe one half inch to one inch snowfall rates are a
good bet during this time. Expanded the Winter Storm Watch to
include all of the southern NE panhandle, but will leave the rest
of the Watch alone for now even though confidence is increasing
with snowfall amounts and Winter Storm conditions.

A few concerns that need to be monitored:

the slower ECMWF, which continues to trend slower compared to the
other models. This solution may keep snow going across the
eastern plains through early Sunday night with higher snow totals.
Increased snow totals a bit, especially further east, to account
for this trend. Will have to monitor how this system ejects into
the plains in the event that it slows down even more.

Very windy conditions are expected on the backside of this storm,
arriving Sunday night and possible continuing through Monday. This
might result in a prolonged blowing and/or drifting snow issue
along and east of the Laramie Range well into Monday. Therefore,
started adding blowing snow to the forecast late on Sunday.

.LONG TERM...(Saturday night through Thursday)
Issued at 328 AM MST Fri Jan 19 2018

Latest model guidance suggests we could see an uptick of stronger
winds on the backside of the system Sunday across the Plains, so
we upped wind speeds for the eastern Plains which would suggest
potential for blowing snow issues during the day Sunday along and
east of I-25...something we will need to keep an eye on for
additional refinements to messaging as details become more clear.

Clearing out Sunday night with generally cool conditions Monday
through Thursday. Weak impulses should bring some periods of snow
showers to mainly the mtns early next week.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1021 AM MST Fri Jan 19 2018

Wyoming TAFS...VFR. Wind gusts to 45 knots through 00Z,

Nebraska TAFS...VFR, with MVFR at Chadron, Alliance and
Scottsbluff from 12Z to 14Z Saturday, then IFR after 14Z Saturday.


Issued at 226 PM MST Fri Jan 19 2018

No Fire Weather concerns through this weekend and into next week
as a winter storm will move through the region resulting in
widespread snowfall, windy conditions, and cold temperatures.
Slightly warmer temperatures are expected during the middle of
next week.



WY...Winter Storm Watch from Saturday afternoon through Sunday
     afternoon for WYZ102-107-108-117>119.

     Winter Storm Warning from 2 PM Saturday to 2 PM MST Sunday for

     Winter Storm Watch from Saturday afternoon through Sunday
     afternoon for WYZ101-106-115.

     Winter Storm Warning from 8 AM Saturday to 2 PM MST Sunday for

     Winter Weather Advisory from noon Saturday to 2 PM MST Sunday
     for WYZ113.

NE...Winter Storm Watch from Saturday afternoon through Sunday
     afternoon for NEZ002-095.

     Winter Storm Watch from Saturday evening through Sunday evening
     for NEZ003-019>021-054-055-096.



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