Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 152048

Area Forecast Discussion
National Weather Service Cheyenne WY
248 PM MDT Thu Mar 15 2018

.SHORT TERM...(This evening through Friday night)
Issued at 200 PM MDT Thu Mar 15 2018

Active weather expected over the forecast area for the next 24
hours, with rain changing to snow, heavy snow late tonight, and
even the potential for some sleet and/or freezing rain across the
panhandle of Nebraska. Forecast for the I-80 corridor is still
low/moderate confidence, mainly due to timing uncertainties of
rain changing to snow which will greatly impact snowfall

shows rain showers increasing in coverage along and just east of
the Laramie Range across southeast Wyoming. No lightning has been
detected yet, but believe there is still a good chance for
isolated thunderstorms with gusty winds and small hail through
this evening. Axis of peak upper level diffluence will move over
the area over the next 3 to 6 hours, so expected this area of
precipitation to expand in coverage and become heavier in
intensity. Models continue to show impressive QPF amounts along
and east of the Laramie Range including the I-25 corridor this
evening. Increased POP near 100 percent for this area with
moderate to heavy rain initially. Snow levels will lower to 6500
to 7500 feet around sunset, which will result in accumulations
beginning across the high valleys and mountains. Will have to
closely monitor the rate of cooling this evening and adjust the
forecast as needed since dynamic cooling will be the primary
mechanism to switch rainfall to snowfall down to around 5500 to
6000 feet later this evening near the I-25 corridor. With
dewpoints hovering around the mid to upper 20`s this afternoon,
and heavy precip expected with very good dynamic forcing, believe
there is a good chance for Laramie, Cheyenne, and even Pine bluffs
and Kimball to see snowfall before midnight.

For TONIGHT, models are in better agreement showing a large area
of precipitation, initially in the form of rain, spreading north
and eastward overnight. Rapid cyclogenesis is expected as the
initial shortwave stalls and forms a close low over far east
Colorado or NW Kansas. This evolution will pull in colder air from
the north, resulting in snowlevels lowering below 4000 to 4500
feet. Highest confidence for heavy snow accumulations will
continue to be along/north and east of a line from Lusk to
Alliance. So no changes made for the Winter Storm Warning. The
only slight uncertainty is to introduce some sleet and perhaps
freezing rain from Chadron southward to Alliance and Sidney. Model
soundings do show a warm layer just above 800mb with temps above
freezing for several hours. Depth does not appear to be enough to
warrant large amounts of sleet. In addition, the elevation of the
warm layer increases through the night, so expect more of a sleet
threat rather than freezing rain. However, only a tenth or two
tenths of an inch of ice accumulation is expected at best. Further
south and west, lower confidence with snow accumulations although
these areas will receive precip and some of it will be snow. Went
with a consensus for the change over time to snow, with a change
over to snow around 9 to 11 PM around 6000 feet, and just after
midnight for areas down to 3500 feet. With this timing in mind,
along with the western fringe of the precip shield pushing east of
I-25 sometime early Friday morning, decided to issue a Winter
Weather Advisory for the remaining areas along and east of the
Laramie Range, including I-25, beginning at midnight. Snow will
likely start before midnight above 5500 feet, but should take a
while to accumulating on road surfaces. Again, there is greater
confidence in this snowfall forecast as you go towards the western
Nebraska panhandle, and less confidence along I-25. With
potential significant travel impacts due to heavy snow, decided to
issue the Advisory for the I-25 area as well.

For FRIDAY, most of the moderate to heavy snowfall will be located
across western Nebraska with periods of sleet and maybe freezing
rain for the Alliance and Sidney areas. Snow will likely be
diminishing in intensity for the eastern Wyoming plains, but kept
the Advisory going just in case the storm system slows down a bit
more, which is typical this time of the year. Most of the snow
should be over by the afternoon, even across western Nebraska.
Winds will then become more of a concern for southeast Wyoming,
but winds have been trending lower over the last day or so.
Expected wind gusts around 45 MPH from Elk Mountain to the
southern NE panhandle, by late morning and into afternoon. Some
blowing and drifting snow is expected, but any blowing snow should
be patchy and shouldn`t last too long due to the relatively warm

.LONG TERM...(Saturday through Wednesday)
Issued at 250 AM MDT Thu Mar 15 2018

Models in better agreement with the next system that will be
moving across the area later on Sunday, closing off a small upper
low as it moves across southern Colorado and Kansas through Sunday
night. his will bring another round of scattered mainly snow
showers to the CWA Sunday afternoon and evening as well as the
next push of cooler air. Pcpn should move out of the CWA Sunday
night/early Monday but a few snow showers should still hold on
through the day Monday primarily over the mtns as another weak
impulse drops southeast across the region. Tuesday and Wednesday
look to be mainly dry with warming temperatures as upper ridging
builds into the region.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1154 AM MDT Thu Mar 15 2018

Wyoming TAFS...Occasional MVFR at Rawlins until 22Z, otherwise
VFR. IFR from 00Z to 10Z, then VFR. VFR at Laramie and Cheyenne until
01Z or 02Z, then IFR until 12Z, then VFR.

Nebraska TAFS...VFR at Chadron and Alliance until 03Z, then IFR
until 15Z, then MVFR or VFR. VFR at Scottsbluff until 06Z, then
IFR until 13Z, then VFR. VFR at Sidney until 05Z, then IFR until
13Z, then MVFR.


Issued at 113 PM MDT Thu Mar 15 2018

No Fire Weather concerns through this weekend with a wetting
rainfall expected along with widespread snow accumulations across
the southeast Wyoming plains, the Laramie Range, and western
Nebraska through Friday. Another low pressure system will impact
the region this weekend bringing another chance of rain and snow.



WY...Winter Storm Warning from midnight tonight to 3 PM MDT Friday
     for WYZ102.

     Winter Weather Advisory from midnight tonight to noon MDT Friday
     for WYZ101-106>108-117>119.

     Winter Weather Advisory until noon MDT Friday for WYZ103-110-112-

NE...Winter Storm Warning from midnight tonight to 3 PM MDT Friday
     for NEZ002-003-095-096.

     Winter Weather Advisory from midnight tonight to noon MDT Friday
     for NEZ019>021-054-055.



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