097
FXUS65 KCYS 072318
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
518 PM MDT Sun Jul 7 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms will continue through the
  early evening hours, mainly in Dawes county and south of a
  Wheatland to Kimball line.

- Confidence is growing for a potentially significant heat wave
  to impact the region late in the week into the first half of
  the weekend.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 300 PM MDT Sun Jul 7 2024

Cool and somewhat unsettled weather is situated over the area today
as the axis of a shortwave trough rotates through the area. Water
vapor satellite imagery indicates the broad trough located over the
upper Midwest today with decent mid to upper level moisture in place
over our area. Current shower and thunderstorm activity is mainly
focused along a 700-mb convergence boundary which has now moved
south into Colorado. However, some post-frontal shower/storm
activity is still ongoing. The environment is pretty marginal for
storms with limited instability owing to the cool temperature. This
will mainly impact areas inside a Laramie to Wheatland to Kimball
triangle. Instability is a little greater in the north/eastern parts
of the forecast area, so we will also be watching for some showers
and thunderstorms in the Lusk to Chadron corridor. North to
northwest flow is leading to some convergence in this area in the
wake of the Black Hills. Shower and thunderstorm activity is not
expected to last long, winding down between 00 and 03z this evening.
After that, precipitation will be quite limited for a while.

The messy trough will start to shift eastward on Monday as the
strong ridge that has been locked in over the West coast over the
last few days starts to expand eastward. Between drier air moving in
and temperatures aloft climbing quickly to choke off instability,
Monday is expected to be totally dry. Even afternoon cumulus will be
mostly confined to the higher terrain and the Cheyenne ridge.
Temperatures will remain on the cool side though as the low level
cool air will be slower to evacuate. This will all produce a
very nice summer day with highs in the upper 70s to mid 80s
across the area. Highs will return to near normal values Tuesday
as the ridge gradually strengthens. The low-level warming will
eliminate the weak capping inversion, so we should see a few
isolated showers/storms go up over the higher terrain, but the
chance for decent rainfall will be low.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 300 PM MDT Sun Jul 7 2024

A potentially significant heat wave is appearing more likely for
a large portion of the region from Thursday through Sunday. 700-
millibar temperatures are expected to soar to between 18-22 C by
Friday afternoon, peaking mid-day Saturday as a highly anomalous
598-600 dam H5 ridge becomes centered over northwestern Colorado
into south central and southeast Wyoming. The deterministic GFS/
ECMWF/GEM and their ensembles are in remarkable agreement w/ the
strength & position of the ridge axis, especially on the days 5-
7 time scale. This is reflected quite well w/ the latest cluster
analysis indicating 74% of members suggesting H7 temperatures at
or above 20 deg C, above the 99th percentile & nearing the climo
max per the NAEFS. The NBM spectrum is already impressive w/ the
deterministic already painting a high of 97 deg F for Cheyenne &
105 deg F for Scottsbluff. This is particularly concerning given
the NBM tendencies to significantly undercut highs in these very
extreme scenarios. As such, have opted to go w/ the 75th %ile at
this time which paints 98/105 F for CYS/BFF the MaxT on Saturday
which will likely the hotter of the two days with the ridge axis
directly overhead. It is worth noting that that the 95th %ile of
the NBM does approach all-time record territory of 100 F For CYS
w/ the latest blend, a benchmark that has not been touched since
the 1950s. One reason that such values are so difficult to reach
in Cheyenne is the tendency for afternoon clouds to develop once
we reach the mid/upper 90s, and this could very well be the case
again as the models do show appreciable vort energy aloft, along
the immediate edge of the ridge. This does not seem particularly
convincing at this time given the extremely dry profiles, but is
a failure mode that could squash our bid at a new record & would
certainly preclude a more aggressive forecast at this time. Lows
are favored in the mid 60s, near 70 F in the Platte River Valley
which would certainly support heat-related headlines, if current
trends hold.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 515 PM MDT Sun Jul 7 2024

Convection dying off this evening with lack of instability and
cool temperatures. VFR conditions overnight. High pressure
builds into southeast Wyoming beginning Monday with warming
temperatures and clear skies. No major weather impacts from
sunset onward through Monday.

&&

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

&&

$$

SHORT TERM...MN
LONG TERM...CLH
AVIATION...GCC