Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 162132
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
332 PM MDT Tue May 16 2017

.SHORT TERM...(This evening through Thursday)
Issued at 325 PM MDT Tue May 16 2017

Active weather pattern this week across Wyoming and eastward
across the high plains. Scattered to numerous showers with widely
scatter thunderstorms have developed over the eastern plains of
Wyoming and Colorado this afternoon. Believe the potential for
severe thunderstorms is less compared to the last few days as CAPE
values and instability are slightly lower due to cooler
temperatures and substantial cloudiness. Still expect
thunderstorms to contain small hail, gusty winds, and frequent
lightning into tonight east of I-25. The most favorable area for
severe storms will be from Chadron southward towards Alliance,
due to warmer temperatures and higher dewpoints. This activity
will likely continue through most of tonight as the initial upper
level trough near the four corners region lifts northeast across
Kansas and Nebraska.

For Wednesday and Thursday, attention shifts towards the potent
Pacific storm forecast to move southeast from Washington state to
a position around northern utah/far western Wyoming. All models
then show this storm stalling near the Colorado border Wednesday
night and Thursday before lifting northeast later this week. For
starters, in terms of the overall pattern this is a favorable
location and evolution of a storm system to produce widespread
moderate to heavy precipitation amounts across most of southeast
Wyoming and western Nebraska due to ample upper level dynamics
(diffluent flow aloft), upslope flow and llvl convergence, and
moisture advection. There is always concern with these systems
that deep convection over the central and southern plains may rob
the area of moisture, but that does not look to be the case with
this event, at least through late Thursday. With that said, Wednesday
will start off pleasant with late morning/early afternoon
temperatures in the mid 50`s to mid 60`s. The only exception will
be Carbon county which will trend cooler in the 40`s as the cold
front associated with the Pacific storm moves eastward. Rain
showers and thunderstorms will rapidly develop in the afternoon
along and just ahead of the cold front. Models then show the area
of showers expanding and becoming steady rainfall as dynamic lift
and some conditional instability is released. All models show an
area of heavy precipitation from far eastern Carbon county
extending eastward toward the WY/NE border. Increased POP over 80
percent Wednesday night through Thursday, with the main forecast
concern becoming snowlevels. Models show the upper level low and
associated cold front stalling as it drifts south into Utah, with
the bulk of the cold air remaining over the interior of Wyoming.
It should be cold enough to get snow levels down to 5500-6500
feet, so will have to closely monitor the high valleys west of the
Laramie Range. As for the mountains, confidence is increase for
heavy snow amounts above 7500 feet, so issued a Winter Storm Watch
for the Laramie Range, Snowy Range, and the northern Snowy Range
foothill. A foot or more of snow is not out of the question in
these areas, although most of the I-80 corridor between Elk
Mountain and Laramie should only see a few inches by Thursday
morning. Another forecast challenge is the I-25 corridor. If the
snowlevel was only 500 feet lower with temps a few degrees cooler,
this stretch from Douglas to Cheyenne may be impacted as well.
Precipitation should be moderate to heavy, so it will not take
much dynamic cooling to make this area all snow. However, with
the time of the year in mind and the fact that a lot of this
precipitation will fall during the daytime, not very confident for
continuous accumulating snow below 7000 feet. Expect all rain
further east including the Nebraska panhandle through Thursday
evening.

.LONG TERM...(Thursday night through Tuesday)
Issued at 244 PM MDT Tue May 16 2017

The main concern with this forecast package is the timing and
placement of closed low.

Thursday night-Friday:
There is still quite a bit of uncertainty on the evolution of
this closed low beyond Friday. The ECMWF is tracking this feature
to the northeast towards eastern Nebraska, the GFS is tracking the
low through western Kansas into central Nebraska, while the
Canadian is tracking the closed low further south and then tracks
into northwest Kansas. The NAEFS/GEFS seems to follow a track
similar to the GFS solution and slows this system down and wraps
up moisture into it. With these things in mind, there is also
still quite a bit of uncertainty on the depth of the moisture and
the potential for any trowal signature. However, there is one
theme that this system is showing that the models do agree on.
They are all showing good upper level diffluence. This may give us
a prolonged period of snowfall. Not confident on how much
accumulation will take place, due to good melting and warmer
ground temperatures, and uncertainty where the highest snowfall
rates will be. However, we will most likely see quite a bit of
snowfall with the main impacts in the higher elevations above
6000ft between Cheyenne and the Snowy Range. Meanwhile, the
southern Nebraska panhandle, rainfall amounts could exceed an inch
in some spots.

Friday night-Saturday: The snowfall should begin taper off on
Friday night with a chilly day in store for Saturday. Afternoon
highs are expected to only climb to the 45-55 degree range in
Wyoming with mostly 50s in the Nebraska panhandle.

Sunday-Tuesday:
A northwest flow regime will be in place during this time which
will favor periodic shortwaves transversing through along with
cool temperatures. Afternoon highs will only reach the 50s and
60s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1137 AM MDT Tue May 16 2017

The main concern with this TAF issuance is the thunderstorm
chances and location.

Latest satellite loop was showing several cumulus clusters
developing in southeast Wyoming. We will need to keep an eye on
things today on which cluster will turn into convection this
afternoon. Local models are showing very limited cap today, so
convection could get off to an early start. However, the
instability will be quite limited until later in the afternoon
especially over CDR and AIA. The main threat from any
thunderstorm that does develop in the northern Nebraska panhandle
will be wind gusts up to 60 mph and quarter size hail. The bulk
of these storms will move northeast at 20 to 25 mph. The remainder
of the TAF sites may have the potential to see a thunderstorm or
two, but are not expected to be severe. The convection should
begin to diminish after sunset with mostly VFR conditions expected
overnight.

&&

.FIRE WEATHER...
Issued at 239 PM MDT Tue May 16 2017

No Fire Weather concerns this week and this weekend due
to widespread precipitation, much colder temperatures, and high
relative humidities. Scattered thunderstorms this afternoon will
gradually become more isolated tonight and Wednesday as the
storm system moves towards the area.

&&

.HYDROLOGY...
Issued at 239 PM MDT Tue May 16 2017

Although rivers and streams are on the low side across most of the
area, the potential for persistent moderate to heavy rain across
the high plains Thursday and Friday may result in elevated flows
late this week and this weekend.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...

WY...Winter Storm Watch from late Wednesday night through Friday
     afternoon for WYZ103-110-114-116.

NE...None.
&&

$$

SHORT TERM...TJT
LONG TERM...REC
AVIATION...REC
FIRE WEATHER...TJT
HYDROLOGY...TJT



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