Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 261144

Area Forecast Discussion
National Weather Service Cheyenne WY
544 AM MDT Wed Apr 26 2017

.SHORT TERM...(Today through Thursday night)
Issued at 514 AM MDT Wed Apr 26 2017

Main forecast concern in the short term will be snowfall in the
mountains and nearby high valleys. Water vapor imagery shows fast
northwest flow across the PacNW and into the Great Basin, and 00Z
models remain consistent showing this stream of moisture and
energy moving overhead the next couple of days. The main jet will
remain to our west and south though, keeping us under cold
cyclonic flow. A vigorous shortwave trough currently just offshore
OR/WA will approach the forecast area today, along with a leading
vort max. These features will move overhead tonight and stall over
the high plains while the strong upper jet carves out a deep
trough along the spine of the Rockies late Thursday. For today,
expect mainly dry conditions through the morning with rain/snow
showers developing over the mountains by early afternoon. Sfc
pressure falls in advance of the shortwave and vort max are
expected along the mountains, with sfc convergence looking to
strengthen just to the lee of the Laramie Range. Therefore, could
see a few showers and storms develop along this convergence
boundary in Wyoming. East of the boundary looks relatively stable
in the llvls, but models do show perhaps sufficient elevated
instability along with the midlevel forcing to maintain a few
showers into the Nebraska panhandle as well.

The sfc trough will shift a bit westward and strengthen tonight as
the more vigorous wave edges closer, inducing llvl upslope flow
tonight through Thursday. Maintained chance to likely PoPs across
the plains through this time for the combination of increasing
upper forcing and upslope flow. Rain/snow showers expected tonight
and THursday will give way to mainly all snow on Thursday night.
Along and west of the Laramie Range, expect orographic
showers/storms this afternoon with a brief break in the early
evening before the stronger shortwave brings increasing QG forcing
and orographic lift tonight through Thursday. Should be a good
burst of snow over the higher terrain later this evening and
overnight, with the potential for additional snowfall in
convection Thursday afternoon with the shortwave overhead. Total
snow accumulation in the Snowy and Sierra Madre Mountains through
this time should be roughly 6 to 10 inches with perhaps locally
higher amounts in locations that experience convection Thursday
afternoon. Went ahead with a Winter Weather Advisory for these
mountains from this evening through THursday afternoon. The nearby
lower elevations could also see a period of snowfall as the
shortwave moves overhead and sfc trough shifts east back towards
the plains, with accumulations here looking to be in the 1 to 3
inch range mainly tonight. Warming temps through the day Thursday
could keep precip all rain in the high valleys. As mentioned
already, the sfc trough will shift east into the plains with
shortwave passage, but looks to stall along the lee of the Laramie
Range through Thursday night. Therefore, best sfc and upper level
dynamics will translate to the plains, although a few lighter
orographic snow showers remain possible as well.

.LONG TERM...(Friday through Tuesday)
Issued at 514 AM MDT Wed Apr 26 2017

Strong 160kt upper jet will dive SE`ward into the N`ern and central
Rockies by early Friday, carving out a deep trough over the W`ern
US. Models are in pretty good agreement in closing off an upper low
over NW CO on Friday afternoon and having it sink slowly S`ward
through early Saturday.  At the low levels, an inverted trough will
set roughly from the N Laramie Range SE`ward into extreme NE CO. N
of this trough, moist E`ly flow along with good overrunning will
allow ample amounts of precipitation especially early Friday as
there will still be some upper diffluence and remnant mid level
energy from the Thursday wave hanging around. 700 mb temps range
from -7 to -8 Friday morning which combined with decent
precipitation rates should lower snow levels to 4500 to 5000 ft.
Think the areas from the N Laramie Range to Douglas/Lusk could see
several inches of snow by mid day Friday.  Areas to the south such
as Laramie and Cheyenne eastward to Sidney may be in a local precip
minima based as by the time the flow switches to a more favorable
upslope direction, the best forcing for ascent is sagging to the
south and east and focused more into CO.  Could still be some light
amounts of snow over the plains E of the Laramie Range Friday

Upper trough shifts E`ward Saturday into Sunday as transitory
ridging moves overhead.  Early next week looks active once again.
Weak shortwave passing to the N may spark some afternoon showers
Monday before yet another trough digs into the northern/central
Rockies on Tuesday


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 540 AM MDT Wed Apr 26 2017

MVFR ceilings have been slow to lift over the NE sites this
morning. Based on satellite trends think things will begin to lift
into VFR by around 15Z. Breezy west winds with gusts 25 to 30kt
will affect areas from KCYS west this afternoon. Scattered showers
will develop by 18z over the west and progress slowly eastward
through the afternoon hours. MVFR ceilings and gusty winds are
possible near any showers.


Issued at 514 AM MDT Wed Apr 26 2017

No fire weather concerns are anticipated through the end of the
week due to cool temperatures and daily chances for rain and snow


WY...Winter Weather Advisory from 6 PM this evening to 6 PM MDT
     Thursday for WYZ112-114.



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