634
FXUS65 KCYS 151146
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
545 AM MDT Wed Oct 15 2025

.KEY MESSAGES...

- Dense Fog Advisory remains in effect for all of the eastern
  plains of southeast Wyoming and the Nebraska Panhandle this
  morning. Visibility below one quarter mile can be expected.

- A storm system will move through the area today and Thursday,
  first bringing showers and thunderstorms, followed by
  mountain snow and strong winds across the wind prone areas of
  southeast Wyoming.

- A few of the thunderstorms this afternoon may become strong to
  marginally severe, of which they may produce hail and gusty
  winds along with heavy rainfall.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 345 AM MDT Wed Oct 15 2025

Forecast remains on track today and tonight with models
continuing to indicate 99th+ percentile PWATs across most of the
eastern high plains. This is evident with the widespread dense
fog early this morning with visibility down to one eighth of a
mile at times along and east of the I-25 corridor. Expanded the
Dense Fog Advisory further west to include most of the
Interstate 25 corridor up to Converse county. Current surface
observations show dewpoints above 55 degrees across western
Nebraska, which is going to set up a potentially active period
of weather the next 24 to 36 hours.

Fog is forecast to slowly lift by noon today as a warm front
slowly lift northeast across the forecast area. IR Satellite
loop shows the strong upper level low pressure center starting
to track eastward across the Great Basin region early this
morning. This feature, along with the stationary front, will be
responsible for late season thunderstorm activity over the next
24 to 30 hours. Thunderstorms have already been observed late
last night across Carbon and Converse counties well ahead of the
upper level low. There remains a conditional risk for strong to
severe thunderstorms for areas just east of the Laramie Range
eastwards across all of western Nebraska. However, much of the
thunderstorm activity isn`t expected to form until the evening
hours and especially overnight due to the remaining stratus deck
and arrival of dynamic forcing...with model soundings showing
conditional instability and increasing lapse rates tonight as
the cold pool aloft moves over the area. This is going to lead
to a tricky forecast after noon today for this time of the year.

With MLCAPE near 1000 j/kg and MUCAPE closer to 1500 j/kg late
this evening, worded forecast for strong to severe thunderstorms.
The most favorable locations are northeast of a line from
Casper Wyoming to Bridgeport Nebraska where forcing should be
the strongest north of the developing surface low. However,
locations along the I-80 corridor have a decent chance at seeing
some activity as well, especially if the forecast location of
the surface low is just a little off. High res CAMs have been
consistent, so increased POP and prob thunder across the most
favorable areas. Multiple lines of convection are expected
through midnight tonight with some activity lingering into early
Thursday morning. Strong gusty winds, hail, and brief very
heavy rainfall are the most likely threats with these storms.
Variables that may bust this forecast include mid to upper level
clouds moving into the area early, a stubborn stratus deck
and/or fog lingering through the afternoon, and the upper level
low slowing down today. Pretty confident that mid to upper level
cloudiness may not be much of a problem given current trends on
IR Satellite early this morning.

For Thursday, all models show the upper level low lifting
northeast from the Salt Lake City area to central and northeast
Wyoming by Thursday afternoon. This position is a little further
south and east compared to 24 hours ago. Expect another round
of precipitation Thursday with strong jet energy out ahead of
the trough axis. Any thunderstorm activity looks to be in the
morning, with rain shower activity along the surface cold front,
which could stall across the eastern plains in the afternoon
and evening. High temperatures will return to slightly below
average on Thursday for most areas with highs in the mid 40s to
upper 50s across southeast Wyoming, and low to mid 60s across
western Nebraska. Given the storm track, winds are also
expected to increase with windy conditions looking more likely
across southeast Wyoming Thursday afternoon and evening.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 222 AM MDT Wed Oct 15 2025

Heading into Thursday night, the next round of high winds will
arrive. The cold front will cause MSLP gradients to tighten west of
the Laramie Range, which will in turn cause 700 mb winds to increase
up to 60 kts over the usual southeast Wyoming wind prones. Strong
subsidence will easily push these winds down to the surface. 60+ MPH
winds will be possible at Arlington, Bordeaux and the South Laramie
Range late Thursday night into Friday morning. Winds will gradually
decrease throughout the day Friday as the trough pushes away.

For the weekend, models are struggling with the shortwave energy
digging southeast out of western Canada. All models show a
strong cold front, with the potential of some lower elevation
rain/snow mix Saturday morning. However, models are out of phase
with this trough axis with the GFS the most progressive...showing
it clipping the area but mainly moving across the Dakotas. The
Canadian and ECMWF show the trough digging further south with
possible snow down to near 6000 feet as 700mb temperatures lower
to -4c to -8c. Ensemble spreads amplify this disagreement with
low confidence with POP and temperature forecasts. Similar
pattern unfolds for early next week with large temperature and
POP spreads as another progressive shortwave moves east across
the area. Decent model agreement with the ECMWF and Canadian,
but the GFS shows a ridge axis dominating the weather pattern on
Monday. Lowered Max and Min temperatures Saturday through
Monday, and kept 15 to 30 percent POP for now.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 545 AM MDT Wed Oct 15 2025

Dense fog will impact the eastern plains terminals this morning with
VIS below one half mile at times. Fog should lift by this afternoon
as a Pacific storm system tracks eastward across the Great Basin.
Several rounds of thunderstorms are possible beginning late this
afternoon through tonight.

HAZARDS/WEATHER TRENDS: IFR or LIFR conditions will prevail for all
western Nebraska terminals and KCYS until 16z to 20z today. KCYS
will fluctuate between VFR and IFR in fog, being on the western
fringe of the low stratus deck. KCDR may remain in light fog and IFR
CIGS through the day, with only brief breaks during shower or
thunderstorm activity.

Low confidence with timing of thunderstorm activity today and
tonight. Models have trended later with convection initiation,
holding off until late this evening and tonight. Added PROB30 groups
to the sites with higher confidence in timing (KRWL, KBFF, KAIA, and
KCDR).

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Dense Fog Advisory until noon MDT today for WYZ102-107-108-118-
     119.
NE...Dense Fog Advisory until noon MDT today for NEZ002-003-019>021-
     054-055-095-096.

&&

$$

SHORT TERM...TJT
LONG TERM...TJT/SF
AVIATION...TJT