Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 211202
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
502 AM MST Tue Feb 21 2017

.SHORT TERM...(Today through Wednesday night)
Issued at 338 AM MST Tue Feb 21 2017

Active weather will be the theme this week as winds will be the
primary forecast concern today...with a potential Winter Storm
across portions of the area the primary concern as early as
Wednesday and Wednesday night.

Strong winds have started a few hours early across the Interstate
25 corridor and the Laramie Range early this morning with gusts as
high as 75 MPH! Cheyenne reported a gust of 75 at 1:30 in the
morning due to mountain wave activity. This mountain wave activity
continues this morning before decent llvl mixing occurs after
sunrise. After sunrise...should see these strong winds spread into
the adjacent high plains and high valley locations. High Wind
Warning looks good through today. Still a little concerned that
western Nebraska may see some of these strong winds...but model
guidance has trended lower with winds speeds just across the
Wyoming/Nebraska border over the last 12 hours...so will not
extend the warning eastward. Further west...increased wind gusts
to 70 MPH since we have already observed these speeds this
morning. Lowered temperatures a few degrees this afternoon as
models were a bit too warm yesterday. However...highs in the 60`s
to low 70s are expected across the high plains with readings in
the low to mid 50`s west of I-25.

For tonight and early Wednesday...the Pacific cold front which is
currently moving across California and the Pac NW will begin to
move towards the region and even push into Carbon county after
midnight tonight. Expect an initial band of precipitation with
this front until it stalls across the area. Increase POP between
55 to 80 percent west of the Laramie Range with the highest values
in the mountains. A quick 2 to 4 inches of snow may fall in the
mountains with around one inch over the lower elevations in
rain/snow mix changing to all snow well after midnight. Believe
this band will diminish in intensity during Wednesday as the front
stalls and the Pacific upper level trough moves onshore...but will
still be well to the west of area.

For Wednesday afternoon through Wednesday night...Models show an
area of strong upper level diffluence on the eastern side of the
trough as it moves towards southeast Wyoming. This will be the
beginning of the mid-late week winter storm as Pacific energy
takes its time moving east through Wyoming. For now...most
confident that the Sierra Madre Range and Snowy Range will see
significant snow accumulations from this system.
Therefore...issued a Winter Storm Watch for these two zones
starting Wednesday afternoon. Further north and east across the
lower elevations...expect rain/snow mix by late Wednesday evening
to become all snow sometime Wednesday night into early Thursday
morning. Increased POP between 40 to 80 percent across most of the
area by late Wednesday night.


.LONG TERM...(Thursday through Monday)
Issued at 338 AM MST Tue Feb 21 2017

...SIGNIFICANT WINTER STORM LIKELY EARLY IN THE FORECAST PERIOD...

A vigorous upper-level low pressure system is likely to impact much
of southeast Wyoming and the western Nebraska Panhandle on Thursday
and Friday, with widespread heavy snow/gusty winds likely resulting
in major travel disruptions on Thursday. Medium range models remain
in excellent agreement with the overall synoptic pattern w/ a broad
and low-amplitude trough traversing across CO/WY on Thu. This storm
system continues to trend slower and stronger, probably due in part
to blocking associated a deep low progged to remain stationary over
northern Saskatchewan/Manitoba, and hints of a developing rex block
over the Gulf of Alaska in the next few days. The net result is the
stronger upper-level divergence associated with a 130+ knot H25 jet
streak from AZ/NM into TX/OK residing for a much longer period than
previous model runs were showing. This combined with persistent low
level upslope along the Laramie Range and Cheyenne Ridge should set
the stage for heavy snow accumulations somewhere in the CWA. Robust
surface cyclogenesis is expected to be well underway over southeast
CO by 12z Thu, with both the GFS/ECMWF suggesting a 988-990 hpa low
lifting across central/eastern KS through the day. We are concerned
models may still be too quick with ejecting the low, and maybe even
too far east given the aforementioned blocking, and the presence of
an increasingly difluent pattern aloft farther west. We should also
note that MSLP standard deviations in the GFS Ens are quite high to
the west of the mean low position, so a continued trend to the west
would not be surprising. Nonetheless, excellent QG ascent should be
present over the entire CWA through much of Thu/Fri along with very
strong mid-level frontogenesis and some pockets of negative EPV per
the GFS suggests the potential for convective elements. Still a bit
confused with the GFS QPF placement farther to the N, and displaced
somewhat from better dynamic support and as a result we continue to
favor the ECMWF precipitation fields as both the GFS/ECM are nearly
identical with larger scale dynamics. All models show a swath of 1+
inch of QPF from W-E across the area, but we still see considerable
variability in its placement. It is growing more likely that 12+ in
of snow will fall for some lower elevation areas, with at least 4-6
inches for most areas. Winter Storm Watches will be necessary soon,
but will let the day shift see another model cycle to hopefully get
a better idea of the location of the snow band. There may also be a
greater threat for blowing snow than originally thought, w/ a trend
toward a stronger pressure gradient over the plains. However, we do
not expect blizzard conditions. Aside from the snow, the weather is
likely to turn much colder as H7 temperatures plunge to +16C across
much of the area by 12z Friday. A fresh deep snow pack is likely to
keep highs well below guidance for a few days.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday Morning)
Issued at 500 AM MST Mon Feb 20 2017

VFR conditions will prevail today with strong/gusty winds expected
for most terminals, especially in southeast Wyoming where gusts of
45-55 knots are possible through this afternoon. Snow showers will
increase west of the Laramie Range late in the period, w/ IFR cigs
and visibilities possible at KRWL overnight. The threat exists for
severe mountain wave turbulence across southeast WY today.

&&

.FIRE WEATHER...
Issued at 338 AM MST Tue Feb 21 2017

Fire Weather concerns will be low...but elevated conditions are
possible today across portions of the area...mainly east of
Interstate 25. A period of very windy conditions are expected
with relative humidities lowering to around 15 to 20 percent
across the eastern plains of Wyoming and into western Nebraska
this afternoon. The strongest winds will be across southeast
Wyoming but very windy conditions and low humidities will stretch
into western Nebraska. Much colder weather is expected late this
week. There is increasing confidence of a Winter Storm over the
area Thursday and Friday with a high chance of accumulating snow
and gusty winds.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Watch from Wednesday afternoon through late
     Thursday night for WYZ112-114.

     High Wind Warning until 6 PM MST this evening for WYZ101-102-
     105>108-110-115>119.

NE...None.
&&

$$

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



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