Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 262128

Area Forecast Discussion
National Weather Service Cheyenne WY
328 PM MDT Wed Jul 26 2017

.SHORT TERM...(Tonight - Friday Night)
Issued at 134 PM MDT Wed Jul 26 2017

An active weather pattern is expected to continue through the rest
of the week w/ daily chances for showers and thunderstorms. Quasi-
zonal flow aloft will persist on the northern fringe of broad/flat
upper-level ridging anchored over the southern plains. We will see
numerous disturbances traverse the flow, along with periodic shots
of monsoon moisture. The timing/placement of each individual short
wave is impossible to determine, but what is certain is scattered-
numerous thunderstorms will occur in the mountains each afternoon/
evening, perhaps even continuing overnight. At this time, it looks
like the best overall chance for more widespread convection across
the high plains will be Friday as the GFS suggests a stronger wave
undercutting the ridge along w/ good low-level upslope/convergence
on the second day of deeper boundary layer moisture return. Expect
to see dew points in the 55-65 degree range, coupled with modestly
steep lapse rates to yield CAPEs around 1500-2500 J/kg which could
contribute to a severe hail threat on Thu/Fri. Deep layer shear of
at least 35-40 knots should also be supportive of strong, possibly
rotating updrafts. The main limiting factor may be an abundance of
cloud cover which could result from the monsoon pattern w/ copious
amounts of moisture in the mid levels.

.LONG TERM...(Saturday through Wednesday)
Issued at 215 PM MDT Wed Jul 26 2017

Long range models in good agreement with the overall weather
pattern, showing the upper level high across the southern plains
gradually dissipating this weekend and rebuilding further west
near the eastern Great Basin region/four corners area by early
next week. Models show PW`s between 1 to 1.5 inches in addition to
upslope flow, which suggests moisture will be plentiful across
most of the forecast area this weekend and early next week. The
GFS and Canadian show some shortwave energy moving into Colorado
this weekend and then stalling as the upper level high rebuilds
west of the Rockies. Will have to monitor this feature closely
since this pattern suggests heavy rains and flash flooding for the
Front Range. Difficult to say which day is more favorable
compared to the others, so increased POP between 20 to 50 percent
(highest in the mountains) Saturday through Monday, mainly in the
afternoon and evening hours. Depending on the track of the
shortwave activity, some of these heavy rain showers and
thunderstorms may linger through the night. Kept temperatures near
average for this time of the year with upper 70`s to mid 80`s
along and west of I-25, and near 90 across western Nebraska and
far eastern Wyoming. Expect a slight cooling trend for early next
week as a weak cool front slides south and stalls along the Front
Range Monday.

Models then show progressive northwest flow developing over
southeast Wyoming and western Nebraska by the middle of next week
as the upper level high strengthens near the four corners region
and retrogrades westward into the Great Basin. This pattern may
be more favorable for widespread rainfall across the area as an
upper level trough will move through the area late on Tuesday/Wednesday
morning. Kept POP around 30 to 40 percent along and east of the
Laramie Range Tuesday through Tuesday night.


.AVIATION...(For the 18Z TAFS through Thursday morning)

Expect IFR or near IFR conditions to persist at KLAR and KCYS
through at least early this afternoon before CIGS increase over 2000
feet AGL shortly after noon. Elsewhere, MVFR CIGS will lift this
afternoon with VFR conditions expected. Can not rule out an area of
showers and thunderstorms developing west of the Laramie this
afternoon. These thunderstorms will move into the high plains of
southeast Wyoming through 00z early this evening and may even clip
KBFF and KSNY before the thunderstorms dissipate due to a more
stable airmass.


Issued at 134 PM MDT Wed Jul 26 2017

No fire weather concerns w/daily chances for showers/thunderstorms
across southeast WY and the western NE Panhandle. Light winds will
be a general rule, and minimum RH values are expected to remain at
or above 25 percent through Sat. Gusty/erratic surface winds could
accompany some thunderstorms over the next few days.




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