Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 172052

Area Forecast Discussion
National Weather Service Cheyenne WY
252 PM MDT Sat Mar 17 2018

.SHORT TERM...(This evening through Monday)
Issued at 245 PM MDT Sat Mar 17 2018

Main forecast concern will be the next snowfall event expected to
begin sometime around noon Sunday west of I-25. For this
afternoon, pleasant weather after a cold morning with a few areas
dipping into the single digits due to a remnant surface snowpack.
Mostly sunny skies will continue this afternoon with increasing
clouds tonight. Can not rule out some fog forming across the North
Platte River valley and areas north which still have a decent
snowpack still on the ground. Some of this fog could be dense
tonight, but do not expect it to linger into late Sunday morning.

For SUNDAY, upper level trough will slowly move east and become
negatively tilted as it moves across the central and southern
Rockies. With the lack of a substantial ridge across the Great
Plains, this storm is expected to take a track further south
compared to the Thursday/Friday storm. However, there will be more
cold air with this system over portions of the forecast area, so
impacts may be similar through Sunday night. All models show
decent upper level diffluence, although not as strong nor as long
compared to the previous storm, along with upslope flow and llvl
convergence. Before the front moves across the area, can not rule
out some thunder showers along the I-80 corridor before rain
changes to snow. Otherwise, any rainfall should quickly change to
all snow west of I-25 Sunday afternoon. With increasing north to
northeast winds becoming northwest in the evening, believe this is
a good set up for heavy snowfall along and near the Snowy Range,
so upgraded the Watch to a Warning. Issued Winter Wx Advisories
for the surrounding valleys and foothills for 3 to 6 inches of
snow through Sunday night. Did not include the Saratoga area, but
that may need to be added shortly depending on how much of a snow
shadow effect is observed in the upper North Platte River valley.

For Sunday night and Monday, will have to monitor the track and
evolution of the upper level low. Right now, models continue to
show it progressing eastward along the Kansas/Oklahoma border.
Further north, all models indicate a deformation axis with
convergence along and east of I-25 for a time as a shortwave
disturbance moves east behind the main storm. Will have to keep an
eye on snowfall rates, especially across Converse and Niobrara
county as well as the southern Nebraska panhandle, Sunday night.
for now, went with 1 to 3 inches of snow along and east of the
I-25 corridor. May see some locally higher amounts across the
southern panhandle and there is the potential for Winter Wx
Advisories to be extended eastward into this region as well for
Sunday night.

.LONG TERM...(Monday night through Friday)
Issued at 317 AM MDT Sat Mar 17 2018

A trend to drier and milder weather will occur by midweek as upper
ridging builds over the western CONUS Tuesday and moves east to
the plains states by late Thursday. A weak impulse ahead of the
building ridge may bring a few snow showers mainly to the mtns
Tuesday. Max temps a little above seasonal averages Weds and
Thursday. The next upper trough looks to affect the region late
Thursday through Friday with pcpn returning mainly over western
parts of the CWA then.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1256 PM MDT Sat Mar 17 2018

Main aviation impacts will be the potential for fog development
again at western Nebraska Panhandle airfields later tonight and
the development of -SN at KRWL and KLAR after 12z Sunday. Fog has
finally lifted at all terminals here early this afternoon, giving
way to VFR conditions everywhere. Satellite images do show some
high clouds inching their way closer to the region. These VFR
conditions will prevail at least through midnight. Abundant
moisture from snow-melt, light winds, and cooler daytime temps
today will set the stage again for another round of fog for
western Nebraska sites, developing likely in the 10-12z timeframe.
Some question if increasing clouds will help mitigate that some,
but trended TAFs that direction for now. Further west, approaching
system will begin to produce some snow at KRWL and KLAR at some
point 12-18z Sunday. Timing of onset will need to undoubtedly be
refined with the latest information.


Issued at 207 PM MDT Sat Mar 17 2018

No Fire Weather concerns this weekend and into next week with
high relatively humidities and another chance for rain and snow
Sunday and Sunday night. Warmer and somewhat drier conditions are
expected Tuesday through Thursday.



WY...Winter Storm Warning from 9 AM Sunday to 6 AM MDT Monday for

     Winter Weather Advisory from 9 AM Sunday to 6 AM MDT Monday for

     Winter Weather Advisory from noon Sunday to 6 AM MDT Monday for

     Winter Storm Watch from Sunday afternoon through late Sunday
     night for WYZ116.



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