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831
FXUS65 KCYS 261021
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
421 AM MDT Wed Jun 26 2024

.KEY MESSAGES...

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- Scattered to numerous thunderstorms are expected to develop
  this afternoon and evening with the potential to become strong
  to severe. Large hail, strong winds, and flash flooding are
  the primary hazards today.

- Another round of fairly widespread thunderstorms is expected
  Thursday afternoon and evening. The main hazards will be
  strong winds and heavy rainfall.
-- End Changed Discussion --

&&

.SHORT TERM /THROUGH THURSDAY/...
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Issued at 259 AM MDT Wed Jun 26 2024

A more active weather day is expected across the area as low-level
moisture finally returns to the High Plains. A weak surface high
pressure system moving into the Northern Plains this morning is
pushing the dryline back westward today. Surface observations show
dewpoints in the 50s for most of the I-25 corridor, with the higher
moisture starting to creep into Albany county. Current water vapor
satellite imagery shows a region of much higher mid to upper level
moisture rotating over the top of the 594+dm 500-mb high
currently centered over New Mexico. A vort-max moving in
between the ridge and a strong trough pushing into the Pacific
northwest has tapped into good monsoonal moisture and is now
pulling it north and east towards our area. The combined push of
low-level moisture from the east and mid to upper level
moisture from the west will lead to some pretty impressive PW
values by this afternoon. Expect this to exceed 1.0" along and
east of the Laramie range, with much of the area exceeding
1.25". These values will be easily above the 90th percentile of
climatology, and possibly approaching the 99th. Low clouds
developing this morning will lead to a modest capping inversion,
but a fairly unstable environment will be setting up by the
early afternoon.

Vorticity advection ahead of the approaching vort-max combined with
positive theta-e advection and isentropic lift should be more than
enough to overcome any capping inversion, and lead to convective
initiation in the early afternoon hours over the higher terrain.
HiRes models show a few interesting features on the surface wind
field analogous to the typical "Chugwater cyclone" set-up, but
not exactly in the right place since this dryline will be about
25 to 50 miles further west than in those scenarios. Two areas
of concern for the first initiations are present where models
show a cyclonic convergence zone developing. The first is around
Rock River to Bosler, and the second is in the northern Laramie
range north of Garrett and west of Esterbrook. While these are
the most likely locations to see initial development of
thunderstorms with supercellular characteristics (assuming said
cyclonic convergence zones indeed develop), the potential is not
limited to here. Expect storms to begin around 2-4PM and then
spread eastward and grow upscale through the afternoon and
evening hours. 0-6km shear values in the 40 to 50 knot range and
SBCAPE above 1500 J/km will lead to the potential for storms to
become strong to severe. Lapse rates will be modestly steep,
between 7 and 8 C/km for most of the High Plains. Nearly
completely saturated soundings may push things more towards
large amounts of small hail, but the potential certainly exists
for large hail. The main hail threat will be during the initial
phase of the storms, when supercells are more likely. An
isolated tornado can`t be ruled out either. However, with the
abundance of lift, expect storms will grow upscale fairly
quickly and linearize, transitioning to more of a strong wind
and heavy rain threat. How quickly this happens is still the
main uncertainty for today`s severe weather potential. HiRes
models remain split, with some showing several hours of
supercell potential, while others linearize much faster.
Regardless of storm mode, these storms will be efficient rain-
producers, leading to a local flash flood threat despite fairly
progressive storm motion. While overall storm motion will be
generally west to east, we will have to watch for right- moving
supercells which may carry a more NW to SE vector. The severe
threat will last through about 9-10PM, at which time the vort-
max aloft should clear the eastern boundary of the forecast
area. However, the radar may not totally clear behind, with a
jumbled mess of 500-mb vorticity continuing to rotate around the
ridge in the monsoonal-ish flow. A few showers and some rumbles
of thunder may linger through the night and into early Thursday
morning.

With high moisture and cloud cover remaining in place, tonight looks
fairly mild across the area. Continued southwest flow over Montana
between now and Thursday will eventually spawn a surface cyclone in
the lee of the northern Rockies. As this develops, look for a modest
low-level jet to develop tonight into Thursday morning. While this
will pull the wind shift boundary along the dryline further east,
areas to the west won`t actually be that dry Thursday morning.
Instead, this will be a boundary between exceedingly moist (60+F
dewpoints) over the high plains, and dewpoints in the upper 40s to
50s over Carbon/Albany counties. This will edge eastward Thursday
morning, bringing those 40s and 50s dewpoints across most of Wyoming
and turning the low-level flow westerly. Even though there will
probably be only a brief window of clearing skies, look for a quick
warm-up in that short period. Then, the approach of the axis of the
more significant northwest CONUS trough will kick off another round
of scattered to numerous thunderstorms in the afternoon hours. Lapse
rates will be less steep, around 6-7 C/km, but there will still be
sufficient instability and shear. Models show storms linearizing
quickly Thursday though, mainly along the pressure-trough that
swings through the area during the late afternoon and early
evening hours trailing south of the aforementioned surface low
to our north. Thursday looks more like a heavy rainfall and
strong wind threat, with plentiful moisture, skinny CAPE
profiles, and the quick linear development.
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&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
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Issued at 415 AM MDT Wed Jun 26 2024

A cooler weekend is expected with temperatures quickly ramping by up
by Monday afternoon. Upper-level troughing is progged to be over
northern Montana and southern Canada Friday morning, with strong
southwesterly flow out ahead. Flow down at 700mb will be more
westerly as the 700mb low passes north of the region across Montana
and North Dakota. 700mb heights will tighten briefly across the
region, leading to a short period with elevated wind potential,
especially in locations west of the Laramie Range. These winds will
quickly be decreased as the attendant cold front sweeps across the
region, with gusty northwesterly winds likely after the passage. The
frontal passage looks to be mostly dry, as moisture values are not
overly impressive across southeast Wyoming. A better chance for
precipitation will be across the Panhandle where dewpoints increase
into the 50s ahead of the cold front. Temperatures are expected to
be in the upper-70s west of the Laramie Range and upper-70s to upper-
80s east of the Laramie Range.

Saturday will be a few degrees cooler behind the passing cold front.
High temperatures will be in the upper-70s to low-80s west of the
Laramie Range and mid-70s to low-80s east of the Laramie Range.
Renewed warm air advection across western portions of the CWA will
increase temperatures for Saturday, despite the cold frontal passage
the day before. 700mb temperatures increase back into the 15-17C
range west of the Laramie Range, further promoting these warmer
temperatures. Zonal upper-level flow will return for Saturday as the
upper-level trough pushes off to the east and a ridge begins to
build over the central CONUS once more. With the zonal flow expected
Saturday, could see continued chances for showers and thunderstorms
as disturbances push through the upper-level flow.

Upper-level ridging returns in full for the CWA on Sunday, with
700mb temperatures increasing into the 16-18C range throughout the
day. Southerly flow at the surface will enhance the already warm
temperatures during the day, with highs expected to be in the upper-
80s to low-90s west of the Laramie Range and low- to upper-90s east
of the Laramie Range. A moisture fetch from the Gulf of Mexico will
be advected into the region with the strong southerly flow, leading
to potentially slightly muggy conditions for the Panhandle and
potentially southeast Wyoming. With additional moisture being
advected into the region, isolated showers and thunderstorms will be
possible in the afternoon Sunday with additional support from
daytime heating.

The remaining long term looks to be very warm again, with highs
consistently in the upper-80s and 90s across the region. The 4th of
July looks to be rather toasty, with the GFS and ECMWF suggesting
700mb temperatures to approach 20C! This would likely result in
surface temperatures approaching 95F or higher for the Cheyenne
area. However, this is still over a week out at this time. Attention
will need to be paid to how warm the holiday will be and if that
heat could turn dangerous.
-- End Changed Discussion --

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 927 PM MDT Tue Jun 25 2024

West northwest flow aloft will continue, with increasing low and
mid level moisture and a weather disturbance helping to increase
cloud cover and the coverage of showers and thunderstorms
Wednesday afternoon and evening.

Clear skies will prevail until 15Z, except for occasional fog
reducing visibilities to 4 miles and ceilings to 1500 feet from
09Z to 13Z at Chadron, Alliance and Scottsbluff. After 15Z,
scattered to broken clouds from 5000 to 10000 feet will occur,
with thunderstorms in the vicinity at all terminals except
Rawlins after 00Z. Winds will gust to 25 knots at the Wyoming
terminals after 15Z Wednesday, and to 30 knots at the Nebraska
terminals after 15Z to 21Z Wednesday.

&&

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

&&

$$

SHORT TERM...MN
LONG TERM...AM
AVIATION...RUBIN