Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 211204

Area Forecast Discussion
National Weather Service Cheyenne WY
604 AM MDT Tue Mar 21 2017

.SHORT TERM...(Today through Thursday)
Issued at 315 AM MDT Tue Mar 21 2017

Overnight water vapor loop depicted a weakening short wave moving
smartly eastward into the north central high plains, with the back
edge of shower activity over the Nebraska panhandle. Lighter, more
isolated showers were detected over south central Wyoming, in closer
proximity to a stalled front which extended west into southern
Idaho and south along the Colorado Front Range. Skies were partly-
mostly cloudy with temperatures from 35-45 degrees. Winds were
variable 5-15 mph, with gusts 15-20 mph between Laramie and Rawlins.

The unsettled weather pattern will continue through much of the
short term under an increasingly moist and energetic west-southwest
flow aloft. Weak short wave ridge will quickly move across the CWA
this morning, followed by a another upper level disturbance this
afternoon and evening. Despite the low CAPE, there will be modest
elevated instability for isolated thunder along and west of I-25,
similar to what occurred late yesterday. Showers will continue
through late this evening across the high plains before exiting
east with the disturbance. It will be breezy and cooler today,
with high temperatures in the 40s and 50s.

The front over southeast Wyoming will shift east into the plains
Wednesday due to a stronger west-southwest flow aloft. Downslope
winds east of the Laramie Range will push the mercury into the
60s-lower 70s. Showers will develop across south central Wyoming
Wednesday afternoon with the arrival of another short wave.
Breezy to windy conditions will develop Wednesday as 700mb winds
increase to 40 kt along the Laramie Range.

Rain will become more widespread Wednesday night and Thursday as
a vigorous Pacific low pressure system tracks across the Four
Corners. Surface cyclogenesis occurs over eastern Colorado
Thursday afternoon, which will enhance the upslope flow and
precipitation. Temperatures will turn much cooler, especially west
of the Laramie Range.

.LONG TERM...(Thursday night through Monday)
Issued at 300 AM MDT Tue Mar 21 2017

We continue with a challenging forecast early in the period w/ the
potential for an ongoing winter storm on Friday AM. The models are
still in excellent agreement w/ a vigorous upper low tracking over
Colorado after 00z Friday, and surface cyclogenesis focused across
east central or southeast Colorado. The GFS/ECMWF both continue to
suggest a strong TROWAL feature over the Nebraska Panhandle, which
likely will favor high precipitation rates and possibly even a few
convective elements over the eastern half of the CWA. There is not
much of a question whether or not precipitation will fall, as most
available guidance suggests well over 1 inch of QPF as a result of
this storm. Precipitation type is going to be a huge challenge for
this system given a lack of cold air. The pattern actually becomes
very cut off by 12z Friday, not really allowing H7 temperatures to
get much colder than about -3C. In fact, the GFS/NAM/GEM/ECMWF are
all showing +2C to +6C over much of the plains at 06z Friday, when
precipitation should be well underway. Given this uncertainty, the
best course of action is to remain cautious with snow amounts, but
continue to advertise potential for winter travel impacts. This is
especially true for higher elevations along/west of I-25. A short-
wave ridge offers a brief break on Saturday before another mid-lvl
disturbance traverses the region on Sunday and re-inforces chances
for rain showers. We will also need to keep a close eye on another
Pacific system in the Mon/Tue time frame, but it is much too early
to go into any details there.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 600 AM MDT Tue Mar 21 2017

VFR conditions prevail with SCT-BKN coverage of clouds around 10k
feet AGL through this afternoon. Wind gusts of 20 to 30 knots are
expected to develop today, mainly after 18z. Showers, and perhaps
a few thunderstorms will develop along and west of I25 during the
early to mid afternoon, and spread northeast in the evening. MVFR
conditions may briefly occur with any shower activity.


Issued at 225 AM MDT Tue Mar 21 2017

A stalled front over southeast Wyoming interacting with upper level
disturbances will produce scattered showers through tonight. Isolated
thunderstorms will be possible this afternoon along and west of
Interstate 25. The front will shift east into the plains Wednesday
as west to southwest winds increase ahead of a stronger Pacific
low pressure system. This low will track east across the central
Rockies and plains Thursday through Friday bringing widespread
rain and snow to the districts. Minimum relative humidities the
next few days will range from 20 to 30 percent, with fair to good
nighttime recoveries.




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