Area Forecast Discussion
Issued by NWS Cheyenne, WY

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765
FXUS65 KCYS 261929
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
129 PM MDT Fri May 26 2017

.UPDATE...
Issued at 123 PM MDT Fri May 26 2017

Severe Thunderstorm Watch 268 now in effect for SE WY and W NE
along and south of Hwy 26 through 800 pm this evening.
Thunderstorms already firing over Carbon and Albany counties along
the nose of a plume of steep mid level lapse rates around 8.5 C/km
per RAP mesoanalysis. Deep layer shear profiles on the order of
50 kt will continue to support supercell structures as the
boundary layer destabilizes especially east of the Laramie Range
later this afternoon. Large hail and locally damaging wind gusts
will be the primary threats although an isolated tornado may be
possible especially north of the Cheyenne ridge where local
backing of the low level flow is more likely.

&&

.SHORT TERM...(Today through Saturday night)
Issued at 345 AM MDT Fri May 26 2017

A vigorous upper level short-wave was spinning along the Oregon and
Nevada border early this morning per recent GOES-16 Water Vapor sat
imagery. This feature is expected to track east-southeast across ID
into western WY through 00z Saturday, w/ increasingly difluent flow
aloft evolving across much of CO/WY. This will result in lee cyclo-
genesis/troughing along the front range, w/ moist southeasterly low
level flow developing over the high plains. The models suggest less
substantial moisture return than earlier runs, but still expect the
boundary layer will be fairly moist w/ mid 40s dew points. The main
question right now is the extent of low clouds which may develop in
the next few hours when winds become more southerly. Model sounding
data does show saturation decreasing by mid to late morning, so the
thinking is we should start to see enough cloud breaks by that time
to aid in destabilization. This will more than likely delay the on-
set of precipitation, and could be a limiting factor on the overall
threat for severe storms today if we do not get enough sun. Believe
this will not be the case though, and solar insolation, combined w/
steep mid-level lapse rates around 8 C/km will help support CAPE of
up to 1000-1500 J/kg particularly over the I80 corridor between CYS
and SNY. 0 to 6 km vertical shear around 50 knots should prove more
than sufficient for supercell storm structures once storms are able
to develop, which looks like about 21z when CIN disappears from NAM
soundings. Large hail/damaging winds will be the main threats. Low-
level shear does not appear quite as strong (on the large scale) as
it did 24 hours ago, but need to pay attention to areas from north-
central Laramie County into southern Platte/Goshen counties w/ some
terrain-induced cyclogenesis and resultant backing of sfc flow. NAM
0-1 km SRH values increase to 200-300 m2/s2 at 00z, with EHIs of 1-
3. Given LCLs less than 5000 feet, there is certainly potential for
an isolated tornado or two later today. Thunderstorm activity could
congeal into one or more clusters or organized systems w/increasing
low-level warm advection during the evening/overnight.

The upper-level trough axis is expected to swing across the area on
Saturday. Scattered to numerous showers and thunderstorms look like
a good bet once again with plenty of forcing/instability aloft, but
the overall thermodynamic environment appears too cool to support a
threat for severe weather carrying over into Saturday.

.LONG TERM...(Sunday through Thursday)
Issued at 355 AM MDT Fri May 26 2017

Medium range models/ensembles are in general agreement with a
northwest flow aloft between a upper ridge over the Intermountain
West and upper low south of Hudson Bay Canada Sunday and Monday.
The western CONUS upper ridge weakens Tuesday and Wednesday as a
upper trough digs southeast across the Pacific coast states.
Consequently, winds aloft become more westerly. The upper ridge
rebuilds over the High Plains Thursday as trough moves toward the
Four Corners, and winds aloft back to the southwest.

Convection for the most part will be widely scattered, diurnally
driven, and generally confined to the higher terrain. Any convection
that manages to spread onto the plains will be more isolated.
Convective coverage increases somewhat Thursday with increasing
moisture in the southwest flow aloft. Daily temperatures gradually
moderate to seasonal normals by Thursday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1127 AM MDT Fri May 26 2017

Showers and thunderstorms will develop after around 20Z near the
Laramie Range and become more widespread across the plains through
the late afternoon into the evening. Brief heavy rain, small
hail, and gusty winds will be possible with the stronger storms.
Shower activity should mostly be confined to the Nebraska
Panhandle after around 06Z. Low clouds and IFR/MVFR cigs will
likely develop later tonight as a front moves across the region.


&&

.FIRE WEATHER...
Issued at 345 AM MDT Fri May 26 2017

No fire weather concerns. A cool and unsettled weather pattern will
persist through the weekend w/ humidity values remaining well above
critical thresholds, and good chances of wetting rain across all of
southeast WY and the western NE Panhandle. There is potential for a
few strong to severe storms this afternoon/evening mainly along the
I-80 corridor between Cheyenne and Sidney.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...DEL
SHORT TERM...CLH
LONG TERM...MAJ
AVIATION...ZF
FIRE WEATHER...CLH



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