395
FXUS65 KCYS 101107
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
507 AM MDT Wed Jul 10 2024

.KEY MESSAGES...

- A potentially significant heat wave is likely to impact the
  region from Friday through Monday.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 325 AM MDT Wed Jul 10 2024

Forecast remains on track through the end of the week with a few
minor tweaks to POP and weather. The strong upper level high,
currently over Nevada, will continue drifting eastward into Utah
today and Thursday. Should be a noticeable increase in afternoon
temperatures today as 700mb temperatures climb towards 15c to
18c, which will likely translate to surface temperatures in the
upper 80s to mid 90s for most of the forecast area including
western Nebraska. The heat wave will begin on Thursday though as
700mb temperatures increase between 16c to 21c with forecast
highs well into the 90s for most locations. Chadron,
Scottsbluff, and Torrington will likely be a degree or two
around the century mark (98 to 102) Thursday afternoon. Kept
high temperatures close to the previous forecast, which is
generally near the 75th to 90th percentile of both GEFs
and the NBM. Believe MOS guidance and raw model output is too
low with daytime temperatures with a clear signal for super-
adiabatic lapse rates near the surface and a very dry boundary
layer that extends up to 500mb each afternoon.

Although models are in good agreement with the general pattern
and temperature trends, some degree of uncertainty exists with
convection initiation and thunderstorm potential. Expect today
to be similar to yesterday with high-based showers and a few
thunderstorms possible over the mountains and adjacent valleys
on the eastern fringe of the upper level high. A little more
uncertainty for Thursday as a dry-line like feature develops
across the eastern high plains with increasing south to
southeast winds ahead of the boundary and some moisture
advection into the western Nebraska Panhandle. Some high res CAM
guidance shows a few strong thunderstorms developing along this
boundary and moving southeast through the afternoon hours.
Thermodynamic profiles look marginal in this case with MLCAPE
hovering around 500 to 800 j/kg. The upper level ridge axis,
which will extend northeast from the main upper level high, will
amplify northeast over the high plains during this time with
increasing subsidence aloft noted on the GFS and NAM soundings.
This will make a highly conditional forecast for thunderstorms
across western Nebraska where there is quite a bit of
uncertainty. With SPC adding a Marginal Risk for strong to
severe thunderstorms...decided to add a slight chance of
thunderstorms with possible strong wind wording. A small chance
for showers and thunderstorms will continue for the mountains,
but chances are not quite as high as today due to the increasing
subsidence aloft associated with the strong upper level high to
our west.

Hot day expected on Friday as the center of the upper level high
pushes into western Colorado and southwest Wyoming. All models
show 700mb temperatures between 17c and 21c...with surface
temperatures in the mid to upper 90s for most locations and as
high as 102 for western Nebraska. There is one subtle difference
compared to yesterday...models now show two distinct shortwave
features digging southeast around the northeastern quadrant of
the upper level high during Friday afternoon and evening hours
across the high plains. These two features were much further
north in Montana yesterday and models showed the upper level
ridge axis remaining over the area through most of the daylight
hours. Will have to watch this closely since coverage of
thunderstorms may be higher than what is currently in the
forecast. For now, increased POP up to 20 percent for portions
of far eastern Wyoming and western Nebraska...but mainly for the
northern and central Nebraska Panhandle.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Issued at 235 AM MDT Wed Jul 10 2024

Hot temperatures continue to be well advertised in the long term
forecast. The CPC 6-10 day outlook favors above average, to even
much above average temperatures through the third week of July for
portions of our cwa. The potential for daytime highs to meet or
exceed 90-100+ degrees for areas east of the Laramie Range for up to
a week in a row also remains. The upper level ridge and its
associated ridge axis will be entrenched across the Four Corners
area by Friday evening. As this occurs, stagnant airflow from a
stacked upper level High, and a surface High will continue to impact
the Great Basin and Intermountain West with hot temperatures. WAA
beneath this upper level ridge will provide sufficient heating for
our cwa, as 700mb temperatures are expected to range from +15C to
+22C for each day through the extended forecast. Deterministic
output from model guidance has been consistent for several days
regarding the high likelihood of 90s/100s being the forecast daytime
highs for most locations east of the Laramie Range through Tuesday
of next week. Areas across the NE Panhandle this weekend will see
daytime highs easily reach 100+ degrees. The persistent hot
temperatures with little relief during the overnight hours does
cause concerns for possible heat advisory headlines beginning this
weekend. At this point of inspection, the overnight lows across
areas east of the Laramie Range are forecast to remain in the 60s,
but that may need another look once we approach the weekend with
better hi-res model data. If overnight lows aren`t forecast to dip
below 70 degrees, then heat related headlines would be highly
favored. There is the potential for isolated rain showers and a
rumble of thunder to occur over the afternoons of this upcoming
weekend, but the amount of subsidence and dry air would be hard to
overcome. GFS modeled sounding data depicts LCLs at approximately
500mb, or 10,000 to 15,000 feet, so any thermals that rise aloft and
create potential towering cumulus clouds will have very high cloud
bases. The better timeline for high-based clouds/thunderstorm
activity is expected by Monday and Tuesday of next week. Will
continue to recommend those that expect to be outdoors this weekend
and early next week where near-record high temperatures are forecast
to take frequent breaks and hydrate. The return of monsoon moisture
toward early next week should help with limiting daytime highs,
although they are still expected to be above average. The earliest
break in the stagnant upper level regime will arrive by mid-week and
beyond. However, the varying model consensus leaves forecast
confidence at low/medium for an end to the much above average
temperatures. We will continue to monitor the trends regarding the
potential for the monsoon moisture to bring a wetting rainfall
greater than 0.10 inches to the area, as the very hot temperatures
will quickly dry the region out, and any moisture we receive will
be much-needed.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 505 AM MDT Wed Jul 10 2024

VFR is expected for all terminals through the forecast period.
High clouds will become present this afternoon. Have gone with
wind gusts up to 20 knots for KRWL and KLAR this afternoon, but
the wind gusts will taper off after 0Z. All other terminals are
likely to see winds/gusts of 12 knots or below through the
forecast period.

&&

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

&&

$$

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