Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 092155

Area Forecast Discussion
National Weather Service Cheyenne WY
255 PM MST Mon Jan 9 2017

.SHORT TERM...(This evening through Tuesday Night)
Issued at 222 PM MST Mon Jan 9 2017

Strong winds continue for areas along and just east of the Laramie
Range with the sfc trough running from eastern Laramie county
northward into Platte county.  Not seeing any gusts to 65-70 mph
like this morning, however still gusting to 50-60 mph at some
locations.  Upper level wave will move across the region through the
evening with the fropa from 03-09Z.  While it will be breezy to
windy through the early night, not expecting widespread high winds
so High Wind Warning should be good to expire at 00z.  Orographic
snow has begun to pick up over the past few hours in the Sierra
Madre and Snowy Ranges.  Snow level is currently around 8500 ft but
will fall through the evening with the colder air advection.  Snow
will begin to taper off by late tonight over the mtns.  Minimal snow
accumulations are expected over lower elevations to the west of
the Laramie Range with sfc temps right around freezing.

Winds will increase by late tomorrow morning over southeast WY as
good llvl mixing combines with an strengthening llvl gradient.  Kept
the High Wind Watch going for the wind prone areas from Tuesday
morning through the night.  Cannot rule out a few gusts close to 55
mph outside of the wind prone too, however confidence was not high
enough to extend the watch to other zones.  The 850/700 mb CAG-CPR
gradient is maximized at 70-75 meters by Tuesday evening.  Decided
to extend the Winter Storm Warning through Wednesday night.
Although only light snow accumulations are expected Tuesday morning
through the evening, did not want to issue an additional Winter
Storm Watch, since moderate to heavy snow will return by Tuesday

.LONG TERM...(Wednesday through Monday)
Issued at 222 PM MST Mon Jan 9 2017

The main concern with this portion of the forecast is the snowfall
over the mountains with some of this snow possibly spreading into
the plains on Thursday.


Latest water vapor loop was showing the nose of the polar vortex
starting to move through British Columbia. This appears to be a
bit further west than what the deterministic models (ECMWF/GFS)
and ensemble means are currently depicting. The standard
deviations are a bit further west as well which may be indicating
that arctic air may be trying to build a bit further west. As a
result, we may end up having to bring this arctic air a bit closer
to Laramie Range on Wednesday. The Consensus models are showing
this trend as well, and we will plan on leaning in that direction.
If this arctic air does move a bit further west we may end up
having to drop temperatures along the Laramie Range. We will just
keep an eye on this for now. Another concern is the potential for
a brief period of freezing drizzle shortly after that boundary
moves through. Certainly appears like this boundary will have a
sharp drop in temperatures after it passes through and may see a
20+ degree drop. The timing of this boundary looks to be around
daybreak in the northern Nebraska Panhandle, but this may be too
late if this colder air seeps down quicker which is often times
the case. Meanwhile, the mountains should see snowfall rates
pickup again due to good instability...jet couplets and favorable
orographics which could bring another 6 to 12 inches of snow to
the Snowy and Sierra Madre Ranges.

Thursday-Thursday night:
The models are trying to shift the upper level energy into the
northern Plains with more jet energy filling in behind across
northern Colorado and southern Nebraska. If this jet energy
combined with isentropic lift does move through southeast Wyoming
and Nebraska Panhandle we may be looking at quick shot of
snowfall. Not expecting significant amounts at this time, but it
may cause a disruption to travel along with another day of chilly

The upper level pattern appears to become cutoff as the upper
level trof dives into the southwest states. This will favor a less
windy pattern along with warmer and drier conditions.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1039 AM MST Mon Jan 9 2017

The main concern with this taf issuance is the wind speeds this
afternoon through Tuesday afternoon across southeast Wyoming.

Latest surface analysis was showing the lee sfc. trof extending
from areas east of Wheatland to near Cheyenne. Meanwhile, the
upper level shortwave continues to track east along the
Canadian/Northeast Montana border. This shortwave will help give
the surface trof a nudge to the east this afternoon which will
help kick in the stronger winds around the CYS TAF site during the
next hour or two. Meanwhile, the west winds should continue to
spread east into the panhandle late this afternoon into this
evening. However, these wind speeds are not expected to be as
strong in the panhandle due to limited mixing and weaker lapse
rates. Another upper level shortwave will be on hits heels tonight
and once again take a track similar to the previous one and bring
more strong winds to the Wyoming TAF sites tomorrow.


Issued at 222 PM MST Mon Jan 9 2017

No fire weather concerns through the week.  It will be breezy to
windy through Wednesday across southeast WY, however minimum
afternoon humidity values will mostly be above 35 percent.


WY...Winter Storm Warning until 5 AM MST Thursday for WYZ112-114.

     High Wind Warning until 5 PM MST this afternoon for WYZ106-107-

     High Wind Watch from Tuesday morning through late Tuesday night
     for WYZ106-110-116-117.



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