263
FXUS65 KCYS 082047
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
247 PM MDT Mon Jul 8 2024

.KEY MESSAGES...

- Confidence is growing for a potentially significant heat wave
  to impact the region late in the week through the weekend.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 300 PM MDT Mon Jul 8 2024

Quiet weather is expected through the next several days as the
strong heat wave to our west begins to expand eastward. Current
satellite imagery shows nearly clear skies over most of the area
aside from a few shallow cumulus clouds developing. Temperatures
aloft have warmed significantly over the last 24 hours, which
has lead to a capping inversion around 500-mb. This, combined
with a dry boundary layer will prevent convection today, aside
from a virga shower over the mountains at most. While pleasant
temperatures are still in place today, significant heat is now
just around the corner. The ridge axis will shift a touch
eastward for Tuesday leading to a range in 700-mb temperatures
from +10C in the northeast to +15C in the southwest. Highs
should be near normal for most of the area. The cap will be
weaker tomorrow, which may allow for a few isolated showers and
storms to develop over the higher terrain. Wednesday`s 700-mb
temperatures will nudge upward about 2C, pushing afternoon highs
a few degrees above average for this time of year.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Issued at 300 PM MDT Mon Jul 8 2024

A potentially significant heat wave is appearing more likely for
a large portion of the region from Thursday through Monday. 700-
millibar temperatures are expected to soar to between 18-22 C by
Friday afternoon, peaking by midday Sunday 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 83% 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 highs of 95+ F for Cheyenne and
101+ for Scottsbluff. This is particularly concerning, given the
NBM tendencies to significantly under cut highs in these extreme
scenarios. As such, have opted to go w/ the 90th percentile with
this package, suggesting 98/106F for CYS/BFF for MaxTs on Sunday
which should be the hottest day w/ the thermal ridge axis moving
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 shot at a new record, which
largely precludes a more conservative forecast at this time. Low
temperatures are favored in the mid 60s to near 70 F, warmest in
the Platte River Valley. Heat Advisories, and possibly Excessive
Heat Watches may need to be strongly considered in the future if
current trends hold and cloud cover appears any less likely.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 204 PM MDT Mon Jul 8 2024

VFR. Expect FEW/SCT coverage of fair weather cumulus clouds with
bases around 10k feet AGL this afternoon. Otherwise, SKC.

&&

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

&&

$$

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