Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 261121

Area Forecast Discussion
National Weather Service Cheyenne WY
521 AM MDT Wed Jul 26 2017

.SHORT TERM...(Today through Thursday night)
Issued at 133 AM MDT Wed Jul 26 2017

Today...Heights aloft rise slowly today as a weak ridge aloft builds
overhead. In the wake of a cold front, low level upslope east and
southeast winds will prevail. Plenty of low and mid level moisture
will remain in the atmosphere, though a more stable airmass will
reside along and east of Interstate 25, limiting areal coverage of
showers and thunderstorms. Greatest coverage of showers and
thunderstorms will be this afternoon west of I-25 in proximity to
the low and mid level theta-e ridge axis and instability axis.
Relatively cool with cloud cover.

Tonight...With the absence of a shortwave trough aloft or other
lifting mechanism, expect showers and thunderstorms to gradually
diminish and decrease in areal coverage as the night progresses. A
mild night with abundant low level moisture and cloud cover.

Thursday...Warmer temperatures as the airmass moderates and with
less cloud cover. With adequate low and mid level moisture
remaining, and low level southeast upslope remaining, will see
scattered to numerous showers and some thunderstorms develop in the
afternoon mainly from I-25 westward.

Thursday night...As is typical, will see areal coverage of showers
and thunderstorms decrease with loss of daytime heating and with the
absence of another lifting mechanism. Continued mild.

.LONG TERM...(Friday through Tuesday)
Issued at 133 AM MDT Wed Jul 26 2017

Weak shortwave will move slowly over the top of the persistent
upper ridge over the SW US on Friday. Models differ with the
timing of this wave but most have it somewhere in the vicinity of
the plains of SE WY and western NE during peak heating on Friday.
Airmass should be quite moist and unstable with SFC dewpoints well
into the 50s to low 60s. Low level SE flow will be topped by
modest W flow aloft which will produce bulk shear values around 35
kts or so. May see some strong storms especially east of the
Laramie Range where the best instability/shear overlap is located.
For Saturday through Tuesday, GFS and EC differ somewhat on the
position of the upper ridge. The GFS builds the ridge more
strongly over the Rockies while the EC tends to retrograde it
towards the US west coast. Either way, seasonable temps are
expected with at least some monsoonal moisture causing daily
chances for showers and thunderstorms. Convective details will
most likely be tied as usual to subtle waves which are
unpredictable at this time scale and/or previous days convection.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 512 AM MDT Wed Jul 26 2017

Area of light to moderate rain showers is currently tracking
eastward across the Laramie Range. These showers have caused MVFR
visibility at times near KLAR. Low stratus has at least
periodically affected all terminals early this morning with the
exception of KCDR. Expect IFR conditions to persist at KRWL and
KCYS through at least mid morning. Elsewhere, expect widespread
MVFR ceilings lifting to VFR by late morning. Late in the
afternoon expect scattered thunderstorms to evolve in a moist
atmosphere west of the Laramie Range. These storms will most
likely struggle to make it east of the Laramie Range where the
airmass will likely be much more stable especially with the
morning rain showers.


Issued at 133 AM MDT Wed Jul 26 2017

Minimal concerns based on projected winds and humidities.




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