272
FXUS65 KCYS 300500
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1100 PM MDT Tue Sep 29 2026

.KEY MESSAGES...

- Areas of fog will develop after midnight tonight and continue
  through Wednesday morning south of a line from Laramie, to
  Torrington to Alliance with some dense fog possible near the
  Interstate 80 Summit and the southern Laramie Range Foothills.

- A more organized rain producer will graze the southern
  Nebraska panhandle on Wednesday, but confidence remains low in
  the exact track of the storm system.

- Cooler temperatures continue through Thursday, but warmer and
  drier weather will prevail Friday through early next week.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 212 PM MDT Tue Sep 29 2026

Tonight...The closed upper low aloft moves from southeast Arizona to
the Arizona-New Mexico state line by dawn. Primary challenges during
this period will be the northern extent of the precipitation shield
associated with the low aloft, and the moisture incorporated into
the system from the tropical system Polo, as well as fog and dense
fog possibilities from rather deep upslope from the southern Laramie
Range eastward to Cheyenne and the southern Nebraska Panhandle.

Prefer the NAM handling of the mesoscale and synoptic pattern due to
trends and its rather high resolution. Expecting areas of fog to
develop after midnight from the I-80 Summit and southern Laramie
Range eastward across Cheyenne and Laramie County and to the
southern Nebraska Panhandle. Considering boundary layer progs with
wind direction and relative humidity, it looks like areas of dense
fog may develop across the southern Laramie Range and I-80 Summit
and the eastern Laramie Range foothills after midnight. With
widespread mid level cloud cover spreading northward across our
southern counties, fog may be somewhat limited in expanse and
duration, though felt confident enough to include areas of fog from
Laramie to Cheyenne to Sidney, with areas of dense fog from the I-80
Summit and southern Laramie Range to along or just west of Cheyenne
including the southern Laramie Range foothills. We may see some
drizzle or very light rain spread across our far southern counties
near the Colorado state line after midnight.

Wednesday...The wobbling upper low moves to southeast Colorado and
northeast New Mexico by suppertime. The NAM looks best with its`
depiction of the precipitation shield and thus have continued to
paint scattered showers south of a line from Laramie to Sidney,
although the best areal coverage will be near the Colorado state
line, where lift will be maximized due to 500-300 mb and 700-500 mb
quasigeostrophic forcing combined with the decent precipitable water
values and atmospheric moisture depth and mechanical upslope lift.
It looks like areas of fog will continue until noon for locations
near the Colorado state line and near the I-80 Summit. The northern
extent of the precipitation shield will be limited by rather dry low
levels and negative theta-e advection undercutting the moist mid
levels.

Wednesday night...Our closed low aloft moves slowly to the Colorado-
Kansas state line by sunrise with the northern shield of the
precipitation concentrated over northeast Colorado and will continue
to blend in with our neighboring offices grids for POPS at North
Platte and Denver/Boulder. The sharp discontinuity in the
precipitation shield will need to be watched closely.

Thursday/Thursday night...The trough aloft opens up and moves
southeastward over Kansas and Oklahoma, with west northwest flow
aloft over our counties. Although we will still see some mid and
high level cloud cover, no precipitation is anticipated due to
minimal lift.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 212 PM MDT Tue Sep 29 2026

Generally tranquil weather will persist through much of the long-
term period. Large-scale ridging will dominate the weather pattern
over the western CONUS resulting in generally mild above-average
temperatures and limited precipitation chances over southeast
Wyoming and western Nebraska. This ridging pattern is, broadly
speaking, a general result of the highly active tropical pattern
over the eastern Pacific. As the firehose of EPAC tropical storms
move west and then recurve to the north in the mid-latitudes of the
central Pacific, potent synoptic troughing is noted over the Bearing
Sea/Gulf of Alaska which thus drives downstream ridging over western
North America. These types of stable-synoptic-wave patterns can
often persist for weeks or more, locking in high-confidence
forecasts over much of the west including our region.

Regarding local details in the forecast, expect above-average temps
from Friday onward. High confidence exists in temps around 5 degrees
above average this weekend (low to mid-70s east of the mountains)
and highs 10 to 15 degrees above average through next week. Despite
the drier weather in place late this week and through the weekend,
rising heights should result in overnight lows remaining above
average as well. This should hold off any threats of frosts (or
freezes) for any populated area of southeast Wyoming outside of the
mountains. Afternoon RH values will drop to near or slightly below
20% throughout the weekend during the afternoon hours across all of
southeast Wyoming and portions of the Nebraska panhandle. However,
generally light surface flow out of the west less than 20 mph will
likely preclude the need for any fire weather headlines. Overall,
given the synoptic pattern described earlier in this discussion,
forecast confidence is quite high throughout the entire long-term
period with limited impacts expected.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1100 PM MDT Tue Sep 29 2026

VFR conditions will prevail across all terminals for the duration of
this TAF period, with KSNY and KAIA possibly being an exception.
Where VIS overnight may be reduced due to BR for both locations. In
addition, by late morning, a storm will lift far enough north in
which it will begin to affect KSNY by 16Z with CIGs lowering to MVFR
and possibly IFR by 18Z along with increased chances of showers.
Winds will pose no aviation concerns overnight into tomorrow with
speeds staying generally less than 10 knots over the next 24
hours.

&&

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

&&

$$

SHORT TERM...RUBIN
LONG TERM...MAC
AVIATION...RZ