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000
FXUS65 KCYS 171800 CCA
AFDCYS

Area Forecast Discussion...CORRECTED
National Weather Service Cheyenne WY
1200 PM MDT Sun Sep 17 2017

.UPDATE...
Issued at 1042 AM MDT Sun Sep 17 2017

Morning GOES-R water vapor loop showed the next upper trough tracking
east across ID/NV/UT, with showers developing and spreading northeast
across northwest CO and south-central WY. Stratus deck over far
southeast WY is gradually eroding per visible satellite trends. This
will allow afternoon destabilization ahead of the aforementioned
trough, with scattered showers and thunderstorms developing across
southeast WY this afternoon, and spreading east into western
Nebraska by this evening. Tweaked sky and temperature grids for
this morning/s update.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 335 AM MDT Sun Sep 17 2017

Current observation show mostly clear skies with some mid to high
level cloudiness beginning to push into the area. With
temperatures hovering around the middle 30`s across the valleys,
kept the frost advisory going through sunrise this morning. Some
low stratus is beginning to develop along and east of I-25 as
well, but do not expect any fog at this time. A flat ridge of high
pressure aloft will quickly move across the area today, resulting
in slightly warmer temperatures compared to yesterday. However,
current GOES 16 watervapor loop shows the next upper level
disturbance moving eastward out of the Great Basin region this
morning. This feature will likely impact southeast Wyoming later
this afternoon and then impact western Nebraska this evening
through tonight. Although instability tends to be marginal this
time of the year and sbCAPE values will generally be between 500
to 1000 j/kg, models show a potent upper level jet with ample
upper diffluence due to the position of the right-entrance quad of
the jet streak across the high plains this evening. With llvl
moisture in place and 0-6km shear between 40 to 50 knots, believe
there is a good chance of scattered thunderstorms with a few
becoming strong across far eastern Wyoming and western Nebraska.
Activity should be east of the area by midnight with a few left
over showers over the high plains. There is some uncertainty with
the timing of this since the ECMWF is slightly less progressive
with the disturbance, so kept slight chance POP through midnight
across western Nebraska. Can not rule out some patchy fog by
early Monday morning, especially in locations where there is
decent rainfall amounts.

Drier weather is expected on Monday with warm southwest flow at
all levels of the atmosphere. Temperatures will respond by the
afternoon with highs returning to the 70`s and low/mid 80`s
across the area, warmest below 4500 feet. It will be breezy in the
afternoon west of the the Laramie Range, due to an increasing
pressure gradient to the west as a strong Pacific cold front
moves southeast. This cold front will push across the area on
Tuesday as early morning highs around 60 will rapidly drop into
the 50`s and even into the 40`s during the day west of I-25.
Further east, high temperatures will remain in the 70`s and low
80`s ahead of the front. Windy conditions will spread across
southeast Wyoming and into the High Plains adjacent to the I-25
corridor by Tuesday afternoon with gusts between 40 to 50 mph
possible. The base of the upper level trough will be too far north
and progressive to produce any significant precipitation across
the area, but some showers activity is likely north of a Saratoga
to Chadron Nebraska line. The airmass will be cold enough to
change rain over to snow above 9000 feet, mainly across the Sierra
Madre, Snowy Range, and the northern Laramie Range.

.LONG TERM...(Tuesday Night through Sunday)
Issued at 246 AM MDT Sun Sep 17 2017

A vigorous mid-level short wave will likely sweep across central Wyo
early in the forecast period, w/ scattered showers and mountain snow
expected on Tue night and early Wed. Low-level height gradients will
be fairly strong during this time with the models showing 45-55 knot
westerlies aloft over the CWA along with notable CAA aloft. This may
be enough for an early season bora event, so will definitely need to
keep an eye on this for possible high wind headlines. A cool, mainly
unsettled pattern persists through Thursday as upper-level troughing
dominates the Pacific NW. The next substantial cold frontal surge is
progged for Thu night or Fri as the aforementioned trough digs south
into the 4 Corners by 00z Saturday. An interesting scenario may very
well evolve this weekend as recent model guidance has suggested some
potential for a closed low to develop somewhere over the central and
southern Rockies. This has mainly been the ECMWF, but other guidance
including the GFS suggests a very cold system regardless w/ H7 temps
as low as around -2 deg C over much of the CWA by 12z Sun. Snow is a
possibility over low elevations of southeast Wyo, but w/ significant
model variability in evolution and overall dynamics, it is difficult
to say what the impacts (if any) will be. For now, we increased PoPs
slightly and lowered daytime highs by 2-5 F for Sat/Sun.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday morning)
Issued at 1117 AM MDT Sun Sep 17 2017

VFR will be prevalent over all terminals through the period. Sctd
showers and tstms expected to develop by late this afternoon over se
Wy mainly east of the mtns and move across the Panhandle early this
evening.  Stronger storms may produce gusty winds and MVFR
visibilities for a brief period of time.

&&

.FIRE WEATHER...
Issued at 246 AM MDT Sun Sep 17 2017

An upper level disturbance will move across Wyoming and western
Nebraska through this evening resulting in scattered showers and
thunderstorms. Thunderstorms may also linger through midnight
tonight. Fire Weather concerns will increase early in the week as
temperatures moderate and winds increase out of the west and
southwest. By Tuesday, a strong cold front will push across the
area from the Pacific NW which will bring some mountains snows to
the area along with cooler temperatures.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...MAJ
SHORT TERM...TJT
LONG TERM...CLH
AVIATION...RE
FIRE WEATHER...TJT