Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 201126
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
426 AM MST Mon Feb 20 2017

.SHORT TERM...(Today through Tuesday night)
Issued at 415 AM MST Mon Feb 20 2017

Very windy conditions have been observed through most of last
night and early this morning. However...winds have struggled to
reach High Wind criteria in most locations. Will keep the High
Wind Warning out for now across SW Platte county and the southern
Laramie Range since we still expect winds to become very strong
later today and into tonight. A few sites have observed wind gust
around 60 MPH mainly near the I-80 summit but the northwest
component to the wind direction has limited strong winds across
the Bordeaux area to less than 40 MPH.

Today will be dry across the area with winds spreading into the
High Plains and even western Nebraska with breezy conditions
expected at the very least during the afternoon. Typically...
temperatures would lower behind the front...but due to the
downslope westerly wind today temperatures will rebound into the
mid 50`s to mid 60`s along and east of I-25 with mid 40`s further
west from Laramie to Rawlins. Expect any precipitation to remain
north and west of the forecast area today and tonight as the
Pacific storm system takes its time moving into the west coast. By
tonight...the primary wind event will get started and become
widespread by early Tuesday morning. Believe the wind prone areas
will see gusts around 65 mph as early as late this evening...so
continued the warning across the Bordeaux area and upgraded the
Watch to a Warning for the Arlington area due to high confidence.
The main concern will be across the valleys and high plains on
Tuesday with winds potentially getting strong as early as sunrise
Tuesday. All models show a strong jet moving into the area with
700mb winds between 50 to 60 knots and very good llvl subsidence.
Matter of fact...this strong subsidence extends towards the
Nebraska border by early Tuesday afternoon. Given recent trends
with local in-house models as well as MOS...decided to extend the
High Wind Watch to include the whole Interstate 25 corridor from
Douglas to Cheyenne and areas east of Interstate 25 towards the
Nebraska border due to good llvl mixing and strong winds just off
the surface. Added the Laramie Valley into this watch as well with
very strong subsidence just off the surface. There are also
concerns that these winds may make it into western Nebraska given
recent model runs...which show strong llvl pressure falls through
the afternoon with pressure rises further west. Thankfully...High
temperatures Tuesday will be pleasant with readings in the mid
60`s to mid 70`s across the high plains into western
Nebraska...and generally in the mid 50`s west of the Laramie
Range.

For Tuesday night...the Pacific cold front will begin to move
towards the area and may even push into Carbon county after
midnight. Increase POP between 50 to 70 percent along and west of
the Laramie Range with the highest values for the mountains. The
lower elevations will see rain/snow mix changing to all snow well
after midnight. High Wind headlines may need to be extended for
portions of the area due to gusty winds associated with fropa.

.LONG TERM...(Wednesday through Sunday)
Issued at 351 AM MST Mon Feb 20 2017

The main forecast concern/highlight in the extended forecast period
continues to be a potential winter storm during the Wednesday night
through Friday time frame. The GFS/ECMWF are in excellent agreement
with a vigorous upper-level system digging along the W. Coast early
in the period. This system is expected to continue to deepen on Thu
w/ notable surface cyclogenesis somewhere in the vicinity of the S.
high plains, likely extreme SE CO or SW KS. A 110+ knot 250 hpa jet
and good low-level upslope will likely provide support for a period
of accumulating snow over much of the CWA, heaviest Thu. We do have
a few negative factors, including the somewhat progressive and open
wave nature of the system and jet energy well to the south over TX/
OK. Progged surface low is close enough though to be concerned. The
GFS continues to be very fragmented with its QPF compared to the EC
but both models show very similar synoptic setups, so tend to agree
with the ECMWF for this model cycle for a larger precip event, with
the main uncertainty in the exact placement. One thing is for sure,
and that is much colder temperatures will arrive by Fri as H7 temps
plunge to -15 deg C or so. We trended colder than guidance with the
troughing and the anticipation of at least moderate snow accums for
much of the area.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1025 PM MST Sun Feb 19 2017

Band of showers over the western Panhandle will continue to move
northeast through the early this morning. VFR conditions expected.
Strong winds expected across southeast Wyoming airports.

&&

.FIRE WEATHER...
Issued at 351 AM MST Mon Feb 20 2017

Fire Weather concerns will be low...but elevated conditions are
possible on Tuesday across portions of the area. A period of very
windy conditions are expected Monday and Tuesday with relative
humidities lowering to around 15 percent each afternoon. The
strongest winds will be across southeast Wyoming but windy
conditions and low humidities will stretch into western Nebraska
as well with elevated Fire Weather concerns possible. Colder
weather is expected late this week along with a good chance of
accumulating snow by Thursday and Friday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...

WY...High Wind Warning until 5 PM MST this afternoon for WYZ116-117.

     High Wind Watch from this evening through Tuesday afternoon for
     WYZ116-117.

     High Wind Warning from 5 PM this afternoon to 6 PM MST Tuesday
     for WYZ110.

     High Wind Watch from late tonight through Tuesday afternoon for
     WYZ101-102-105-107-108-115-118-119.

     High Wind Warning until 6 PM MST Tuesday for WYZ106.

NE...None.

&&

$$

SHORT TERM...TJT
LONG TERM...CLH
AVIATION...CLH
FIRE WEATHER...TJT



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