266
FXUS65 KCYS 110923
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
323 AM MDT Thu Jul 11 2024

.KEY MESSAGES...

- Very hot temperatures are likely across all of southeast
  Wyoming and the western Nebraska Panhandle this weekend
  through early next week.

- A potential monsoonal pattern may develop through into the
  early to middle portion of next week, giving way to increased
  chances for precipitation and some relief to the extreme
  temperatures.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 310 AM MDT Thu Jul 11 2024

Forecast remains on track through the end of the week and into
the weekend as a long duration heat wave is forecast over the
area. There were some slight tweaks to POP and weather made to
reflect convective potential this afternoon and Friday
afternoon. The strong area of high pressure aloft, currently
centered near the Nevada/Arizona/Utah border, will continue
drifting eastward across Utah today and eventually into western
Colorado and southern Wyoming late Friday and Saturday.
Afternoon temperatures will trend another 3 to 6 degrees warmer
today compared to yesterday, with highs generally in the low to
mid 90s across southeast Wyoming and the southern Nebraska
Panhandle. Highs will likely climb into the upper 90s to around
100 degrees along and north of the North Platte River
valley...including east central Wyoming this afternoon.

For POP and probability of thunder today. Models underperformed
yesterday with the potential and coverage of convection, which
makes this part of the forecast a little more uncertain. Surface
moisture this morning is a little higher than expected with
dewpoints solidly in the 50s across most of western Nebraska.
Light southeast upslope winds observed around the western fringe
of a surface high across the eastern Great Plains this morning
should keep some low level moisture in place across far eastern
Wyoming and western Nebraska for most of today as a dry line
forms near the Wyoming/Nebraska border. With increasing
subsidence and dry air aloft, not expecting much activity along
and west of the I-25 corridor. Maybe a few weak thundershowers
will develop with very little, if any, rainfall. Further east,
high res guidance shows isolated to widely scattered
thunderstorms developing around mid afternoon and continuing
through the early evening hours. Kept POP around 15 to 18
percent for isolated coverage, but also not terribly confident
that much activity is going to form with each run of the HRRR
and NAMnest showing little run-to-run consistency. For any
storms that do form, gusty outflow winds will be the main threat
with hardly any rainfall expected at the surface.

Very hot temperatures expected on Friday and Saturday as the
center of the upper level high pushes into western Colorado. All
models show 700mb temperatures between 17c and 21c, which
translates to surface temperatures in the middle 90s to near
100 for most locations and as high as 103 for western Nebraska.
Some locations will be flirting with their daily record highs
each day. Models continue to show two upper level disturbances
digging southeast around the northeast quadrant of the upper
level high pressure on Friday. This continues to add
uncertainty to the forecast for east central Wyoming and most of
western Nebraska. GFS and ECMWF are now initializing these
features a bit further northeast compared to yesterday. Added a
mention of thunderstorms to the forecast to be on the safe
side...but not terribly confident about it. However, SPC does
show another Marginal Risk area for western Nebraska on day 2
for this general area. Highs on Saturday will be similar to
Friday...and likely a few degrees warmer in a few locations but
a few degrees cooler in other locations due to variations in cloud
cover and wind direction. Thought about issuing Heat Advisories
for far eastern Wyoming and northern/central Nebraska
Panhandle, but believe the hottest temperatures will mainly be
this weekend with the warmest overnight lows (mid 60s to mid
70s), so will hold off for now.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
Issued at 212 AM MDT Thu Jul 11 2024

Much above average temperatures are still on track for the majority
of the extended forecast period. Clockwise flow from a broad dome of
high pressure will advect monsoon moisture from the Gulf of Mexico
by early next week, bringing an increased potential of much needed
rain showers and thunderstorms. By midweek, a weak cold front will
bring daytime highs down closer to average for this time of year.
However, it will still be slightly above average for daytime highs
across the forecast area.

The upper level ridge and its associated ridge axis will be located
along the Central Rockies by Saturday evening, and become elongated
or more broad by early Sunday. As this occurs, hot temperatures will
continue to remain present across the region, with sufficient WAA
keeping the daytime high forecast well above average. 700mb
temperatures are expected to range from +15C to +22C for each day
through early next week. Sunday will see daytime highs approximately
15-20 degrees above normal for mid-July. All areas east of the
Laramie Range will be approaching the century mark or higher for its
daytime high. Very slow-moving thunderstorms are anticipated to
become more widespread across the Central Rockies by Sunday evening,
with our western half having the highest chance for convection. The
0Z Canadian deterministic model is the most aggressive with
widespread to numerous thunderstorms, whereas the GFS and ECMWF are
less aggressive with thunderstorms developing across the majority of
our cwa.

Monday will see hot temperatures once again, although they will be
slightly cooler by approximately 5-7 degrees Fahrenheit. A stronger
surge of monsoon moisture will saturate the atmospheric column
across our cwa, and we will see an earlier onset of cloud cover
development. This cloud cover will limit daytime highs slightly.
PWATs will be in the 75th to 90th %ile by Monday evening, and there
will be sufficient heating in the lower boundary layer to assist
with convection. This will be the first day where a wetting rainfall
greater than 0.10 inches for some areas will be favored.

Tuesday is when we will begin to see the upper level ridge axis
shift to the east, and as it does so, we will transitions to a more
quasi-zonal flow aloft at H5/H3. A potent upper level shortwave
disturbance will trek to the east towards the Great Lakes region,
and the upper level flow will transition to a weak northwest
gradient. We will also have the moisture advection from the Gulf of
Mexico (GoM), so there will be another opportunity for scattered
rain showers and thunderstorms. Cloud cover is favored to be
prevalent across the cwa, so daytime highs will not be as warm for
the diurnal maximum. Would expect another decrease of a few degrees
compared to Monday. 80s/90s are still expected, but will be about 10-
20 degrees cooler compared to the weekend daytime highs.

The active weather pattern is expected to continue through
Wednesday, and a weak cool front will advect colder air aloft from
Canada. Slightly above average temperatures and the continued slug of
monsoon moisture should provide one more round of thunderstorm
activity from daytime heating. Model guidance then begins to keep
the area warm and less active once we transition towards the end of
next week. Overall, the forecast remains on track for hot weather
and multiple days of afternoon thunderstorms to be present for the
extended forecast.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 929 PM MDT Wed Jul 10 2024

Northwest flow aloft will continue. Mostly clear skies will
prevail, with occasional periods of scattered to broken clouds
from 10000 to 12000 feet. Winds will be mainly at or below
12 knots.

&&

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

&&

$$

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