Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 262042

Area Forecast Discussion
National Weather Service Cheyenne WY
242 PM MDT Sat May 26 2018

.SHORT TERM...(This evening through Sunday)
Issued at 141 PM MDT Sat May 26 2018

Latest regional obs showed mostly clear skies and very warm
temperatures across western Nebraska and southeast Wyoming. Highs
should max out in the upper 80s to mid 90s especially across the
Nebraska Panhandle. GOES 16 vis imagery did show a few cumulus
popping up along the higher terrain to the west of Interstate 25
with an area of high clouds further south across colorado and New
Mexico. Most of the region will stay dry this afternoon and
evening but a few high-res models show a shower or thunderstorm
developing later this evening. High clouds will advect north as an
approaching upper level low moves in from the Great Basin area.
This upper level low will continue its movement east through the
overnight into Sunday. An area of low pressure at the surface will
begin to take shape, to the south, along the lee of the Rockies.
As this happens, rich moisture, pooled across the Great Plains,
will surge westward. Mid and high level clouds will develop
overnight Sunday with the warm advection, helped in part by a
strengthening low level jet.

Sunday should start mostly cloudy over the plains with a few
breaks further west. Clouds will become scattered through the
afternoon as the atmosphere mixes. The extra cloud cover means
high temperatures will be 5 to 7 degrees cooler across the area
than the past several days. The breaks in the cloud cover should
still allow ample solar heating to combine with the surface
moisture and steep mid level lapse rates to produce moderate
destabilization by the afternoon. MLCAPE values of 1500-2000 j/kg
will be more than adequate for thunderstorms to develop in the
early afternoon hours. The SPC Day 2 outlook has all of the
Nebraska Panhandle and a good chunk of se Wyoming included in the
slight risk. This seems appropriate given the destabilization and
the combination of moderate deep layer shear. 0 to 6 km shear on
the order of 50 kts will be more than able to support supercell
structures capable of damaging winds and large hail. Model
soundings show two areas of favored initiation, with one along
the Laramie range west of interstate 25 and another at the edge
of a lee trough/pseudo dryline cross northeastern Colorado and the
southern Nebraska Panhandle. Storms should mature and strengthen as
they head eastward. LCLs are still quite high with T/TD spreads
approaching 30 degrees F. This in combination with modest low
level shear indicates more of a damaging wind and large hail
threat. While a high based tornado cannot be ruled out, it seems
unlikely given the lack of low level CAPE and modest shear.
Regardless, the setup is still very conducive for severe weather
across southeastern Wyoming and western Nebraska.

.LONG TERM...(Sunday night through Saturday)
Issued at 141 PM MDT Sat May 26 2018

A few stray showers and thunderstorms will continue overnight
Sunday and into Monday as the moisture hangs around. Advection
monday morning should be fairly stout with a 40 kt low level jet
expected to develop across the Great Plains. This will help
transport even richer low level moisture into southern Nebraska by
Monday morning. Outflow and left over convection from Sunday will
likely play a key role in determining exactly where the threat
for severe weather will develop on Monday. A weak frontal boundary
will move over southeast Wyoming from the northwest early Monday.
This should push the severe weather threat mainly over the
Nebraska Plains. Cloud cover may impact heating on Monday keeping
temperature much cooler and limiting the instability. The exact
details of the mesoscale environment remain a mystery for now but
will be crucial in determining the severe threat for Monday. A few
strong storms with threats for damaging winds, large hail, and an
isolated tornado will be possible.

Monday night showers and storms will fill in over the area and
continue through Tuesday. Things will start to dry out early
Wednesday as the clouds erode from the west. Wednesday will start
another warming trend with highs back in the middle 80s to near 90
by the first weekend of June.

Active weather is likely to continue through the first part of the
new month as the upper level jet stream meanders south across the
lower 48. Periods of storms will be possible with GLobal models in
good agreement on several chances for rain in the next couple of


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

VFR conditions expected through tonight. Breezy conditions can be
expected at times, with gusts around 20-25 knots possible. The
highest gusts are expected over SE WY in the afternoon, and over
western NE during the overnight hours. The potential also exists for
a few evening thunderstorms, mainly over southeast Wyoming after
00z. Although south to southeast winds will continue into tonight,
the probability of fog across the eastern plains appears rather low
at this time.


Issued at 141 PM MDT Sat May 26 2018

Fire weather concerns will be low through the period as an
approaching area of low pressure will bring unsettled weather to
the area over the next several days. Relative humidites will
increase as as moisture moves its way westward. Widespread showers
and thunderstorms will develop Sunday through Tuesday bringing
wetting rain to much of the area.




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