Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 171249
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
649 AM MDT Tue Apr 17 2018

.SHORT TERM...(Today through Wednesday night)
Issued at 415 AM MDT Tue Apr 17 2018

Main forecast concern will be the potential for strong Bora-like
winds today through this evening across mainly the southern two
thirds of the forecast area including the whole I-80 corridor from
Rawlins to Sidney Nebraska. IR Satellite loop and obs show the
surface cold front moving across western Wyoming early this
morning and should be entering central Wyoming in the next few
hours. It has been very windy at times last night, but expect
winds to increase and become more widespread as the surface
pressure gradient rapidly increases through 15z. Models are in
much better agreement with this event compared to 24 hours ago, so
went ahead and issued the High Wind Warning and expanded it
eastward to include the eastern plains, mainly south of a
Wheatland to Alliance line. Strong winds are possible north of
this line, but just not as confident since all models show the
best llvl pressure gradient and peak winds aloft are further
south along I-80. With 600-700mb winds around 60 to 65 knots,
there is a good chance at most sites in the warning area hitting
the higher gusts as the cold front and upper level cold pool
aid in increasing steep surface to near 500mb lapse rates for a
time late this morning through early this afternoon. Even after
that, there is a good chance the llvl pressure gradient along the
front itself will result in sustained winds over 40 mph for a
decent amount of time through this evening. So confidence is now
pretty high on this wind event, even though it may not last all
that long. The only question is cloud cover, which may prevent
some mixing. However, believe the very strong gradient along the
front and the strong subsidence will help overcome any lack of
mixing due to cloud cover. Gusts up to 70 MPH can not be ruled
out, but did not go quite that high in the official forecasts. As
for precipitation, precip will begin mostly in the form of rain
across the area, with a quick change over to snow above 6000 feet.
However, the snowfall won`t be consistent enough to warrant much
in the way of accumulation, especially since it will fall during
the daytime hours. Up to one inch of snow is possible on grassy
and elevated surfaces, mainly along and north of a Rawlins to
Chadron line. The front and trough axis move through the area so
fast that the mountains will even struggle to pick up more than 3
or 4 inches of snow. With decent low to midlevel instability
through early this afternoon, some heavy snow showers/squalls are
possible, especially over 6000 feet. Thunderstorms are also
expected as well through early this afternoon with a few
containing small hail, mainly over the eastern plains.

Winds should gradually diminish late this evening through tonight
with some chilly overnight lows for mid April, generally in the
low to mid 20`s. A few locations may drop into the upper teens if
winds decrease enough in the more sheltered valleys. Winds will
continue to dissipate on Wednesday as low and upper level
gradients relax through the day. Wednesday will be dry and cooler
than average with highs in the mid 40`s to mid 50`s.

.LONG TERM...(Thursday through Monday)
Issued at 242 AM MDT Tue Apr 17 2018

The main forecast concern/highlight in the extended period remains
the potential for a significant spring storm affecting the Central
Rocky Mountain region from Thu night through early Sat. The EC and
GFS remain in excellent agreement w/ a vigorous upper low tracking
across the Four Corners at 12z Fri. We have seen some changes over
the last 24 hours with regard to the track, as all guidance is now
quite a bit further south. Still quite a bit of uncertainty though
with respect to how much this system will dig. Standard deviations
per the GEFS to the west & east of the 500 mb low track would help
to confirm the possibility of continued variability. We have still
been looking at a trend toward warmer thermal profiles as well, so
snow levels are still a major question. We expect to have somewhat
of a better idea by Wednesday AM as the system approaches the west
coast and becomes better sampled by those networks. Until then, we
do not have a lot of confidence in a high-impact winter event, but
we do acknowledge the possibility despite recent trends in a less-
favorable direction for our CWA. Climatology says this is the type
of system we need to watch closely.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 645 AM MDT Tue Apr 17 2018

In general, VFR prevails with locally MVFR cigs/vsbys possible for
KRWL/KCDR/KAIA later today. Strong wind & mountain wave turbulence
will be the primary aviation impacts across the region through the
morning and afternoon hours. Wind gusts of 45-55 knots could occur
over several terminals today. Winds will likely remain gusty after
03z, but will decrease some after that time.

&&

.FIRE WEATHER...
Issued at 242 AM MDT Tue Apr 17 2018

Fire Weather concerns will be low for the remainder of the week. A
strong cold front will quickly push through the region late this
morning through this afternoon. Very windy conditions with strong
winds gusts are expected through the afternoon. Thankfully,
relative humidities will increase as the front moves across the
area. Cooler than normal temperatures and relative humidites above
25 percent are expected Wednesday and Thursday before the next
Pacific storm impacts the area late this week.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning from 11 AM this morning to 9 PM MDT this
     evening for WYZ107-108-118-119.

     High Wind Warning until 4 PM MDT this afternoon for WYZ105-106-
     109-117.

     High Wind Warning until 4 PM MDT this afternoon for WYZ110-113-
     115-116.

     High Wind Warning until noon MDT today for WYZ111.

NE...High Wind Warning from 11 AM this morning to 9 PM MDT this
     evening for NEZ019>021-054-055.

&&

$$

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



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