Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 131632
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1032 AM MDT Thu May 13 2021

.SHORT TERM...(Today - Friday Night)
Issued at 259 AM MDT Thu May 13 2021

A warming trend continues today w/ rising heights aloft associated
with broad & flat upper-level ridging. This will definitely not be
a dry pattern as the CWA will remain on the northern fringe of the
building ridge, leading to continued active west-to-northwest flow
in the mid and upper levels. Numerous weak disturbances are likely
to traverse the flow today w/ the bulk of the vort energy slipping
just off to the north and east. This should be sufficient for some
shower activity, but forecast soundings suggest the boundary layer
should be quite dry with deep inverted-v profiles, so most of this
might end up being virga. Deep unidirectional flow aloft w/ around
50 knots at the top of the mixed layer suggests potential for some
very strong downdrafts especially considering the depth as well as
dryness of the sub-cloud layer. More widespread convection will be
likely on Friday as the upper-level ridge slowly breaks down ahead
of a deepening western US trough. Low-level moisture return should
become more established with dew points climbing into around 45-50
deg F, leading to more widespread CAPEs exceeding 500 J/kg with 40
to 50 knots of H5 flow leading to appreciable shear. A few strong/
locally severe storms appear possible on Friday afternoon. Primary
hazards would be large hail and gusty winds.

.LONG TERM...(Saturday - Wednesday)
Issued at 259 AM MDT Thu May 13 2021

The second day of moisture return on Saturday should be the signal
for a higher likelihood of strong to severe storms for the eastern
plains with the GFS showing 48-52 F dew points for the majority of
the area along/east of the Laramie Range. Closed low drifting into
central California, coupled with a northern stream short wave over
the Intermountain West will lead to increased difluence aloft with
good support for widespread convection. Cooling aloft will lead to
steepening mid-level lapse rates to around 7-8 deg C/km, which may
support MLCAPEs up to 1500 J/kg beneath continued strong, 50+ knot
northwesterlies aloft. 40+ knots of effective bulk shear should be
supportive of organized, locally intense updrafts capable of large
hail. There could be enough near-surface directional shear for the
development of a brief/weak tornado as well, but time will tell w/
how that environment eventually evolves. Daily chances for showers
and thunderstorms (some possibly severe) should persist into early
next week as the upper low drifts eastward across the western US.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1033 AM MDT Thu May 13 2021

VFR conditions expected to prevail through the forecast period for
all terminals. Main concerns remain during the afternoon hours
with a chance for scattered showers and isolated thunderstorms
east of the Laramie Range. Went ahead and extended the VCTS and
VCSH through 03z for Southeastern Wyoming and the Nebraska
Panhandle. Still expecting high storm cloud bases and uni-
directional wind, which may support gusty conditions in the
vicinity of the storms. After 03z, expected conditions to calm
down with lifting ceilings and very light wind in the early
morning hours.


&&

.FIRE WEATHER...
Issued at 259 AM MDT Thu May 13 2021

Significant precipitation over the past 72 hours will keep general
fire weather concerns low during the next few days despite warming
temperatures. Isolated to widely scattered showers and storms will
be possible today w/ a threat for gusty & erratic winds. Strong to
severe storm chances increase Friday through Sunday, mainly across
eastern areas.

&&

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

$$

SHORT TERM...CLH
LONG TERM...CLH
AVIATION...MD
FIRE WEATHER...CLH


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