Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 141826

Area Forecast Discussion
National Weather Service Cheyenne WY
1226 PM MDT Mon Aug 14 2017

.SHORT TERM...(Today through Tuesday night)
Issued at 250 AM MDT Mon Aug 14 2017

The upper ridge is positioned over Wyoming per early morning
water vapor imagery, and should steadily shift to the east today
as an upper trough tracks into the northern Rockies. An embedded
shortwave ahead of the main trough will approach from the west
through the day inducing pressure falls over the central Rockies
and pulling the quasi-stationary front currently over the plains
westward. Sfc flow to the east of the front will become south-
southeast with strong convergence expected along the Laramie
Range and sfc trough zone. With the combination of the midlevel
wave and strong sfc convergence, expect storms to fire over the
mountains and along the Laramie Range/front early afternoon,
shifting east across the plains through the late afternoon and
evening. Anticipate llvl moisture across the plains to increase a
bit within the sfc flow pattern and surface-based instability
values east of a line stretching from Douglas to Kimball should
range generally from 1000-2000 J/kg by late afternoon. 0-6km bulk
shear is progged to be 30-40 kts while 0-1 km shear will be weak,
although localized pockets of higher llvl shear are possible near
terrain features such as the Pine Ridge. There will remain a weak
cap through much of the day as the thermal ridge will be
overhead, so that could inhibit storm intensification across the
plains. But, if storms can tap into that higher shear and
instability, a few could certainly produce large hail and
damaging downburst winds with perhaps a brief tornado in those
pockets of higher llvl shear. Confidence is very low on tornado
potential though due to overall marginal deep layer shear.

Convection across the plains should shift to the east after midnight
tonight but looks to redevelop over the mountains through the early
morning hours in advance of the main upper trough as it begins to
move into Wyoming early Tuesday morning. Energetic southwest flow
will keep active weather through much of Tuesday. Models develop
the attendant sfc low to our south with sfc flow across our
forecast area steadily becoming north and northwest through the
morning across the plains, and in the afternoon in the high
country. With cooler air filtering in from the north, expect the
cap to be stronger across the plains, but would not be surprised
to see a few elevated storms occur with ample energy aloft and
strong convergence along the 700 mb front evident in the models.
Threat for severe hail is not out of the question in this
environment as forecast soundings show high values of CAPE aloft,
especially in the late afternoon and evening once the 700 mb jet
increases in strength. Expect showers and storms to continue
through much of Tuesday night as the upper trough moves overhead.

.LONG TERM...(Wednesday through Sunday)
Issued at 250 AM MDT Mon Aug 14 2017

Drier and a little cooler conditions expected Weds with convection
likely hindered by subsidence behind a departing shortwave though
some still possible mainly near the mtns. next wave makes its
approach to the area Thursday with return flow ahead of it
bringing up a little more moisture so should see more convective
coverage that day especially over the plains. Minor front passes
by late in the day bringing a little cooler conditions again for
Friday along with mostly dry weather. Mainly dry and warmer
Saturday as an upper ridge shifts east across the region. Might
get a bit breezy around the mtns as lee troughing develops over
the high plains in response to the next shortwave moving into the
Pacific northwest/sw Canada. Similar pattern for Sunday though
some monsoonal-type moisture may sneak northward into the area as
the shortwave moves over the Canadian prairies.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1225 PM MDT Mon Aug 14 2017

Main concerns for aviation ops will be the timing and location of
afternoon and nighttime convection that will develop and move
across the region. Latest info suggests thunderstorms will
continue developing near KCYS/KLAR after 19z. KRWL will likely see
some convective development as well, though the drier air mass
will likely only yield high-based showers capable of producing
gusty outflow winds. This area of -TSRA/-SHRA will lift northeast
towards western Nebraska Panhandle sites 21-23Z. The bulk of the
activity will likely depart by around 05Z, with perhaps a second
round of showers and/or weak thunderstorms lingering overnight.
For now have kept VCTS confined to the location and timing above.
May need to linger convection a bit longer on the 00z/06z TAF


Issued at 250 AM MDT Mon Aug 14 2017

Unsettled weather will keep fire weather concerns to a minimum
through the near term. We expect good chances for thunderstorms
and wetting rains this afternoon through Tuesday, with drier
conditions to filter in later in the week. Afternoon humidities
could get close to critical from this weekend, but strong winds
are not expected.




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