960
FXUS65 KCYS 022324
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
524 PM MDT Tue Jul 2 2024

.KEY MESSAGES...

- Thunderstorms are possible on Wednesday afternoon, especially
  across east central Wyoming & the western Nebraska Panhandle.
  Any storms which develop will have the potential to become
  severe with all hazards possible.

- A cool and pleasant Independence Day is expected in the wake
  of a strong cold front set to sweep across the area on
  Wednesday night.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 225 PM MDT Tue Jul 2 2024

Current Visible Satellite loop and surface observations this
afternoon show dissipating showers and thunderstorms with patchy
fair weather CU across the mountains and eastern plains. The
first upper level trough axis has pushed eastward into western
Nebraska at this hour with drier air moving into the bulk of the
forecast area with dewpoints dropping into the 20s and 30s
across most of southeast Wyoming. Surface moisture remains in
place for the northern panhandle of Nebraska, but upper level
subsidence has greatly reduced convective activity across the
area. Continued the trend of lowering POP and prob thunder late
this afternoon through this evening with clear skies expected
tonight and cool overnight lows. Surface winds are forecast to
gradually shift into the east for western Nebraska with surface
moisture increasing through midnight. This will set the stage
for the potential of severe weather Wednesday as another strong
cold front digs south out of western Canada.

All models indicate the next upper level shortwave trough and
associated cold front digging south across Montana and Wyoming
on Wednesday. Will continue to monitor moisture transport
closely, but as of right now, the best chance for strong and
severe thunderstorms will be well east of Interstate 25...and
more specifically along and east of a line from Chadron to
Kimball Nebraska. Thunderstorms may get started pretty early
depending on the speed of the cold front, with some high res
guidance showing convective initiation as early as 11 am to
noon. Thankfully, there should be a pretty small area that is
under this severe weather threat in our forecast area with
mainly Dawes, Box Butte, Morrill, and Cheyenne County in the
most likely region, but this may change if moisture transport
occurs further west towards I-25. All models of severe weather
are possible if any discrete supercells develop, including
tornadoes. High res guidance and the NAM continue to show solid
0-1km helicity of 200 to 300 within the area of peak 0-3 km max
CAPE of 3000 to 3500 j/kg in the boundary layer. Definitely
large hail and strong winds are possible threats, but a few
tornadoes can not be ruled out as well late Wednesday morning
through mid Thursday afternoon. There is additional concern for
the North Platte River valley from Scottsbluff to Bridgeport/Broadwater
and vicinity with models showing some backing of the surface winds
late Wednesday morning and strong veering of winds aloft. Expect
this activity to push east of these four counties as early as 200
to 300 PM mountain time, so a pretty quick event for our western
Nebraska counties. However, the upper level shortwave is forecast
to push across the northern zones (Douglas, Lusk, northern NE
Panhandle) late Wednesday evening. There may be a second round
of strong to severe nocturnal thunderstorms mainly north of
Interstate 80 Wednesday night through sunrise Thursday. Otherwise,
after a warm day on Wednesday, much cooler temperatures are
forecast to settle across the area late Wednesday and Wednesday
night with highs back into the 70s for all areas Thursday
afternoon. With the cooler and more stable airmass, thunderstorm
coverage will be more isolated on Thursday with mainly shower
activity. Will also have to watch the potential for fog early
Thursday morning due to the thunderstorm activity up north.
Overnight lows Wednesday and Thursday night will be on the cool
side for this time of the year with lows in the 40s to low 50s.
A few high valley locations west of I-25 may see readings in the
upper 30s just before sunrise Friday.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 208 PM MDT Tue Jul 2 2024

Near to slightly above average temperatures can be expected for this
holiday weekend into early next week. A departing shortwave trough
will eject out of the Northern Plains towards the Great Lakes region
on Friday morning. We will be under the influence of weak CAA behind
this shortwave trough, with 700mb temperatures being slow to rise
through Friday afternoon. The northern forecast zones portions of
the NE Panhandle will have 700mb temperatures only creep up to +5C
to +8C by 0Z Saturday. However, the surface high pressure will keep
skies mostly sunny, and daytime highs in the 70s to low 80s appear
favorable. Areas in southeast WY, primarily west of I-25 will see
700mb temperatures rise to +10C to +13C for Friday, leading to
warmer surface temperatures. We will remain with northwest flow
aloft at H5 and H3 as an amplifying upper level ridge will slowly
shift to the east by this weekend from the western CONUS. This
northwest flow aloft will keep our cwa on the periphery of passing
shortwave disturbances. Positively-tilted shortwave disturbances
will eject out of the Canadian Rockies toward the northern Plains on
Saturday and Sunday. This close proximity to the shortwave
disturbances combined with daytime heating will create the potential
for isolated rain showers and thunderstorms, especially Saturday
night and Sunday. Moisture advection from the Gulf of Mexico should
combine with weak atmospheric dynamics during this time. Have kept
the mention of slight chance to chance of thunderstorms in the
forecast for now, but we could see an isolated strong thunderstorm
on Saturday evening and Sunday afternoon. Daytime highs on Saturday
and Sunday will be in the 70s and 80s, with the cooler temperatures
occurring on Sunday once the weak cool front passes through our
cwa. By early next week, the aforementioned upper level ridge begins
to advect to its east from the Great Basin area. While it will
become somewhat elongated and flattened, 700mb temperatures will
creep upwards during the afternoon hours. Model guidance is trending
with +10C to +17C for an average spread across our cwa. As a general
rule of thumb, +14C is a good threshold to use for daytime highs
near or above 90 degrees F at the surface during the afternoon. This
will translate to daytime highs in the lower elevations of middle
80s to middle 90s, unless a shift in the pattern comes up in the
next several rounds of model guidance updates. Stay tuned to weather
updates as we have a gradual warmup trend on its way, making it feel
like summer once again across the region for the extended
forecast.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 521 PM MDT Tue Jul 2 2024

West flow aloft will prevail. Clear skies will be most common,
with scattered clouds near 8000 feet at times. Winds will gust
to 30 knots at all terminals except Scottsbluff until 02Z, then
gust to 30 knots at the Wyoming terminals after 15Z Wednesday.

&&

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

&&

$$

SHORT TERM...TJT
LONG TERM...BW
AVIATION...RUBIN