Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 182251 CCA

Area Forecast Discussion
National Weather Service Cheyenne WY
414 PM MDT Tue Jul 18 2017

.SHORT TERM...(This evening through Thursday night)
Issued at 300 PM MDT Tue Jul 18 2017

Forecast challenges deal with PoPs and chances for heavy rainfall
through the short term.

Latest water vapor imagery showing a dry slot moving into most of
Wyoming this afternoon, which has really limited convection.
Exception being down in the southern Nebraska Panhandle where we
are seeing scattered convection developing. This band is moving at
a pretty good clip and should be out of the Panhandle within the
next hour or so.

For this evening, most locations to remain dry as this dry slot
remains over the area. HRRR guidance does show a cell developing
across the northern Panhandle that could impact Alliance and
Chadron. Does not look all that strong, so feel severe storms
should hold off.

PWATs on the increase for Wednesday. Counties west of Laramie
Range increase from .5" this afternoon up to .75-.8" by Wednesday
afternoon with the Panhandle increasing up to 1.3". Only trouble
for tomorrow is there seems to be no forcing mechanism as the weak
frontal boundary lifts northeast and lower elevation winds shift
southwest. So we may not see much in the way of convection outside
the mountains.

GFS and ECMWF still in good agreement for Thursday as upper high
remains well southeast of the area over eastern Kansas and
monsoonal surge moves directly over the CWFA. PWATs well over an
inch across much of the CWFA with near 1.3" in the Panhandle. Do
have an upper shortwave moving through the monsoonal flow Thursday
that should help develop convection. Greatest concentration of
showers looks to be along the I-80 corridor from maybe Sidney to
Arlington and in the mountains. HPC coordinated with us today for
a Slight Risk area for Excessive Rainfall across our south central
zones. That may need to be adjusted some more as the event draws
closer. Looks like a good bet we will need some flash flood
watches for some of our southern counties Thursday afternoon.

.LONG TERM...(Friday through Tuesday)
Issued at 300 PM MDT Tue Jul 18 2017

Friday...Relatively weak zonal flow aloft prevails. Deeper moisture
will be limited to Colorado and to areas along our Colorado state
line, thus will continue with isolated to scattered showers and
thunderstorms over and near our southern mountains, dry elsewhere.

Saturday...Drier air at low and mid levels will infiltrate our
western and central counties in the wake of a cold front. There may
be enough moisture left over our eastern counties for isolated late
day showers and thunderstorms, primarily over the Nebraska panhandle.

Sunday...The flow aloft backs to northwest with low level upslope
winds helping to generate isolated showers and thunderstorms along
our southern counties near the Colorado state line, with mostly dry
conditions elsewhere.

Monday...Low level upslope strengthens under ridging aloft. Looks
like enough low and mid level moisture over our southern and eastern
counties for isolated to scattered afternoon and evening showers and

Tuesday...Next shortwave trough aloft progged to move across during
peak heating, helping to spark scattered showers and thunderstorms
along and east of Interstate 25, and most numerous over western


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 412 PM MDT Tue Jul 18 2017

Wyoming TAFS...VFR, with isolated thunderstorms near Cheyenne
until 03Z. Gusts to 20 knots at Cheyenne and Rawlins after 18Z

Nebraska TAFS...VFR. Gusts to 22 knots at Chadron and Sidney after
18Z Wednesday.


Issued at 300 PM MDT Tue Jul 18 2017

Monsoonal moisture will remain over southeast Wyoming and
Nebraska Panhandle for the next several days into next week.
Result will be continued mountain showers and thunderstorms with a
general increase in coverage each day. Expect fairly high
afternoon humidity over the area. The exception being FWZs 301 and
303 where afternoon humidities fall into the mid teens both this
afternoon and Wednesday. Look for fair to good overnight
recoveries. Good chances for wetting rains in the mountains
through Friday with best chances Thursday, where many lower lying
areas stand to get wetting rains as well.




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