Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 190124

Area Forecast Discussion
National Weather Service Cheyenne WY
624 PM MST Sun Feb 18 2018

Issued at 615 PM MST Sun Feb 18 2018

Cancelled the High Wind Warnings a few hours ago as wind speeds
west of I-25 diminished. The Arctic cold front pushed southwest
through the eastern Wyoming plains and western Nebraska late this
afternoon. Since 455 PM, the temperature at Cheyenne dropped 30
degrees. Similar temperature changes were observed in the wake of
the frontal passage, as well as wind shifts to the northeast,
gusting to 40 mph, and low stratus with patchy freezing drizzle
and fog. Added patchy freezing drizzle and fog from the south
Laramie Range east and northeast into the southwest corner of the
Nebraska Panhandle this evening. Looking at I-25 webcams and
surface observations, light snow was falling at Douglas and Lusk,
with visibilities as low as a mile. Snow will continue to develop
and gradually spread south and east tonight. No changes to the
winter weather headlines at this time.


.SHORT TERM...(Tonight - Monday Night)
Issued at 254 PM MST Sun Feb 18 2018

Low-level gradients are finally starting to diminish w/the passage
of a strong cold front across southeast Wyoming and the western NE
Panhandle. Wind gusts have finally decreased below 50 MPH for much
of our High Wind Warning area, but will leave the Warning in place
through 00z as some sensors are still showing sustained winds near
35-40 MPH.

A widespread, potentially significant winter storm episode remains
on track for the region from tonight through Monday night. Highest
confidence in 12+ inch snowfall amounts still exists roughly along
a RWL-DGW line which will be directly in the RRQ of a strong (120+
knot) H25 jet. Significant frontogenesis is progged here, and both
NAM and GFS forecast soundings suggest an extremely deep/saturated
dendritic growth layer for several hours at Casper. SREF plumes do
have a few members well in excess of a foot for those areas, which
makes sense with high potential for excellent snow growth. Snow is
expected to begin for northern Converse county shortly, w/ a large
part of Carbon county not far behind this evening. Snow will begin
to spread slowly southeast across the area through Monday, w/areas
from far southeast WY into the southern NE Panhandle likely seeing
impacts by late afternoon. The heaviest snow here will fall during
the evening hours, and local forecast soundings also show a period
of deep dendritic growth. It is worth noting that several 15z SREF
members show 5-10 inch amounts for CYS, and this is quite a jog in
the upward direction from previous runs. Changes to highlights are
restricted to primarily timing. Spatial extent of inherited Winter
Storm Warnings remains unchanged. We added a Winter Wx Advisory to
the remainder of Laramie County and a few zones in the southern NE
panhandle as well. It is possible that these areas could require a
Winter Storm Warning eventually once confidence increases. We will
also probably need to consider adding the rest of the I80 corridor
in the southern Panhandle for accumulations of 3-6 inches. Primary
uncertainty is for Laramie, as easterly surface flow may yield the
dreaded dry downsloping despite good overall dynamics.

One other potential concern. The models, especially the GFS, still
show a dry slot over southern Carbon county early Monday. Do think
this will limit snow chances for a time, and could even yield some
temporary sunshine over the high mountains into Arlington/Laramie.
This occurs as cold front stalls over southern Wyo, resulting in a
rapid increase in low-level gradients and 0.5 km flow of 45-55 kts
over these same areas. If there is enough sunshine, we expect low-
level mixing to become quite strong with potential for very strong
wind gusts. Hopefully, this is not juxtaposed with any snow but it
will get extremely nasty pretty quick if it does.

.LONG TERM...(Tuesday - Sunday)
Issued at 302 AM MST Sun Feb 18 2018

Brutally cold Monday and Monday night as 700mb temperatures fall
to -17C over the Panhandle to -22C out by Carbon County. Snow
looks to slowly end from northwest to southeast by late morning
Tuesday. We may need to extend the end time of our warnings to
reflect the slower clearing. Windchills well below warning level
Monday night/Tuesday morning, so expect Windchill headlines after
the Winter Storm Warnings come to an end.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 513 PM MST Sun Feb 18 2018

Arctic cold front now moving southwest of Cheyenne with broken low
clouds at MVFR gradually lowering to IFR from northeast to southwest
across SE Wyoming and the western Nebraska Panhandle through morning.
Snow will gradually also LIFR visibilities across the Scottsbluff,
Alliance, and Chadron areas through tomorrow morning with Cheyenne
and Sidney not seeing significant widespread visibility reductions
until tomorrow afternoon as the upper jet energy swings across.
Rawlins will lower to LIFR by daybreak.


Issued at 302 AM MST Sun Feb 18 2018

Strong winds this morning will ease across southeast Wyoming by
early afternoon as an arctic front moves into the area. Widespread
wind gusts in excess of 60 mph this morning, turning north and
northeast with gusts to 20-25 mph this afternoon. Widespread snow
event to unfold from north to south over the next 36 hours behind
the front with very cold temperatures moving in behind the front
as well. Significant snow accumulations across entire area of
southeast Wyoming and Nebraska Panhandle before snow ends Tuesday
morning. No fire weather concerns expected.


WY...Winter Storm Warning until midnight MST Monday night for

     Winter Storm Warning from 6 AM Monday to midnight MST Monday
     night for WYZ106>108-115>117.

     Winter Weather Advisory from 2 PM Monday to 6 AM MST Tuesday for

NE...Winter Storm Warning until midnight MST Monday night for NEZ002-

     Winter Storm Warning from 6 AM Monday to midnight MST Monday
     night for NEZ003-096.

     Winter Weather Advisory from 2 PM Monday to 6 AM MST Tuesday for



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