429
FXUS65 KCYS 112329
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
529 PM MDT Sun Aug 11 2024

.KEY MESSAGES...

- Near critical fire weather conditions return to Carbon county
  today along with areas of smoke and haze due to distant
  wildfires.

- Much lower thunderstorm coverage is expected this afternoon,
  but storms that manage to form may quickly become strong to
  severe.

- An active pattern continues through the week ahead with near
  normal temperatures and daily chances for afternoon and
  evening showers and thunderstorms.

&&

.MESOSCALE...
Issued at 1211 PM MDT Sun Aug 11 2024

A few cloud streets have begun to crop up in the past hour
across portions of WY, showing signs of increased daytime
instability in the atmosphere via GOES satellite imagery. A few
towering cumulus have also started to appear on satellite in
Laramie County. SPC Mesoanalysis depicts low-level lapse rates
between H7-H5 at 7.5 to 9.5 degrees C/km as of 18Z. The upper
level shortwave axis is presently situated across northern WY,
and that is where the current majority of convection and
thunderstorms reside. Additionally, MUCAPE of 500 to small
pockets of 2000 J/kg have popped up via SPC mesoanalysis for
portions of our cwa thanks to the morning sunshine. 0-6km
effective bulk shear is sufficient for rotating thunderstorms
this afternoon, as it is analyzed at at 25-50 kts, especially
east of the Laramie Range. This is where SPC has updated their
Day 1 SPC severe weather risk to a Slight/Marginal, instead of
just a Marginal Risk east of I-25. DCAPE values exceeding 1000
J/kg also have been analyzed by the 18Z mesoanalysis page. All
of this weather jargon being mentioned overall is suggesting
isolated to potentially scattered severe thunderstorm initiation
in the next 1-3 hours, particularly off the North Laramie Range
and eastern Laramie County near the CO/WY/NE state line. Hi-res
model guidance suggests that any development should be all but
done near 0Z for our cwa. The main hazards will be brief, gusty
winds up to 60mph, large hail, and heavy downpours. However, the
coverage is expected to be isolated in nature as discrete
thunderstorm activity may become right-moving supercells for
what turns up to occur this afternoon.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 330 AM MDT Sun Aug 11 2024

Another round of nocturnal showers is moving across the area this
morning, fueled by modest synoptic lift ahead of the next upper
level shortwave trough. That features is moving across western
Wyoming at this hour, with its axis expected to cross through the
forecast area late this morning or early this afternoon. Once the
axis passes through, we`ll see synoptic scale subsidence take over
which will significantly limit today`s thunderstorm coverage.
Meanwhile, plentiful moisture remains in place in the lower
atmosphere with weak upslope flow supporting another round of low
clouds and fog over the High Plains. This is less thick and
widespread than the last few mornings, but travelers should
still prepare for patchy low visibility in fog this morning. Any
lingering fog will clear by midday.

Today will be a dry outlier in the middle of a cool and wet weather
pattern. A developing surface low pressure system over northeast
Colorado by this afternoon is expected to pull in much drier air
along the I-80 corridor. This dry region is visible on GOES
satellite water vapor channels moving across southern Idaho at this
hour. Breezy westerly winds and low RH will return near critical
fire weather conditions to Carbon County, while forming a sharp
dryline in the vicinity of I-25. The progressive dryline and a
convergence zone along the WY/CO border will provide the best
potential to produce thunderstorms this afternoon, but coverage will
be much lower than Saturday. However, any isolated storms that
manage to develop over the High Plains in the rich moisture
environment will exist in a fairly potent environment with decent
vertical wind shear, lapse rates, and instability. Thus, we are
looking at a conditional threat for severe thunderstorms today,
contingent on thunderstorm updrafts overcoming the synoptic
subsidence in the wake of the shortwave trough aloft. Large hail,
strong winds, and locally heavy rainfall will also be possible.
Directional shear in the lowest 1-2km also means that an isolated
landspout or tornado can`t be totally ruled out. On the bright side,
the window for storms will be fairly short today with synoptic
subsidence and dry air winning out by around 6PM. In addition to the
drier air making intrusions today, this trough may bring another
round of smoke and haze from distant wildfires.

The dry air break of Sunday will be short lived though as the ridge
axis shifts back to eastern Wyoming during the day Monday.
Southwest flow across the Great Basin will allow for monsoon
moisture to rapidly fill back into the area for Monday. With a
continuous train of vorticity maximums also returning, we`ll
kick off another round of scattered to numerous afternoon and
evening showers and thunderstorms Monday afternoon, which may
carry well into the evening hours once again. Monday`s storms
will once again bring the chance for heavy rainfall and isolated
strong winds and large hail.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 330 AM MDT Sun Aug 11 2024

An active extended forecast period for southeast Wyoming and the
western Nebraska Panhandle w/ numerous mid and upper-level short
wave disturbances embedded within quasi-zonal flow aloft. Expect
daily chances for showers and thunderstorms as several pieces of
energy continue to emanate from the slow moving parent trough in
the Pacific Northwest. Multiple rounds of strong/severe thunder-
storms appear possible given substantial low-level moisture with
dew points in the 50s and 60s and occasional mid-level jet maxes
contributing to adequate deep layer vertical shear for organized
storms. Still too early to determine any specifics of which days
may feature a more organized severe risk as this will be heavily
dependent on the eventual timing of individual waves. Otherwise,
expect temperatures to be near seasonal norms with daytime highs
mainly in the 80s.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 529 PM MDT Sun Aug 11 2024

Quiet this evening as storms have moved out of the forecast area.
Southeast flow will lead to the return of low stratus later tonight
across western Nebraska terminals. Expect IFR CIGs through at least
mid-morning at most of these terminals. Dense fog will likely also
develop at KSNY, KAIA, and possibly KBFF, leading to significant
visibility reductions overnight and into Monday morning. Scattered
showers and thunderstorms are once again expected Monday
afternoon.

&&

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

&&

$$

MESOSCALE...BW
SHORT TERM...MN
LONG TERM...CLH
AVIATION...SF