134
FXUS65 KCYS 182338
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
600 PM MDT Tue Aug 18 2026

.KEY MESSAGES...

- There is a Marginal Risk (Level 1 of 5) of severe weather
  across portions of the region along and east of the Laramie
  Range. Large hail and damaging winds will be the primary
  hazards.

- Drier conditions start to return Wednesday as the upper-level
  ridge strengthens across the Intermountain West increasing
  fire weather concerns.


&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 159 PM MDT Tue Aug 18 2026

This morning the forecast area was pretty clouded over limiting the
amount of diurnal instability and decreasing the chances for the
severe thunderstorms this afternoon. The 12z and later runs of the
Hi-res guidance were far from enthusiastic about convection later on
this evening. The RAP had about of 1000 joules of SBCAPE and maybe
500 joules or less of MLCAPE depending on the area. The HRRR and the
RRFS were a little bit more generous with the CAPE and having around
a 100 more joules in each category. However, with a cloudy morning
that never really cleared out except for a few pockets, the amount
of CAPE depicted by the models may even be overdone despite how
meager those values are. Also displayed by the models were lobes of
"quick" moving vorticity and a brief window of strong warm air
advection over the Panhandle. So despite the meager environmental
conditions we may still have enough forcing to produce some embedded
thunderstorms the late afternoon/evening but outside of a brief wind
gusts over 50-60mph. No other parameters are really expected to
reach severe criteria. Even with the Bulk shear being 30 to 40 knots
this Marginal Risk of severe thunderstorms may not pan out the way
everyone thought. These showers are moving fairly quickly but they
are struggling to hold themselves together outside of the mountains
as they move east past i-25 and little precipitation makes it to the
surface.

The upper-level ridge continues to build across the Intermountain
West on Wednesday, leading to warmer and drier conditions once more.
Northwesterly flow aloft develops for Wednesday, allowing multiple
lobes of 500mb vorticity to traverse overhead, resulting in sporadic
synoptic forcing for ascent. Northwesterly flow will persist at
700mb leading to drier conditions along and east of the Laramie
Range as downslope warming and drying take hold. The terrain-induced
dryline returns Wednesday and quickly pushes well to the east,
fueled by the stronger 700mb flow advecting drier air eastward.
Precipitation chances will be limited to none for most locations
across the region. Models have really trended down the chances for
precipitation across the Panhandle tomorrow compared to the
overnight runs. Model guidance favors dry and breezy conditions as
we maintain westerly flow aloft. Elevated to near-critical fire
conditions are anticipated to develop across Carbon County where
surface winds return to more "normal" speeds and afternoon minimum
humidity values drop into the 10 to 15 percent range. Decided not to
issue any Fire Weather headlines this morning due to low confidence
in achieving 3 consecutive hours of Red Flag conditions and due to
the fairly limited coverage of any critical fire conditions.


&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 159 PM MDT Tue Aug 18 2026

Thursday...Ridging becomes more pronounced with a relatively dry
airmass, at low and mid levels becoming more prevalent, eradicating
any chances of showers and thunderstorms under northwest flow aloft.
700 mb temperatures near 17 Celsius will yield maximum temperatures
in the 80s to lower 90s.

Friday...The mid level, 500 mb, or about 18,000 feet MSL, ridge axis
moves to a Grand Junction, Colorado to Great Falls, Montana axis,
with its dominance continuing across our forecast area of southeast
Wyoming and western Nebraska, continuing the dry conditions for our
neck of the woods. Warming trend continues as 700 mb temperatures
rise to near 19 Celsius, producing maximum temperatures from the
upper 80s to mid 90s.

Saturday...With the upper ridge axis, near 500 mb, or 18,000 feet
MSL, transitioning into the western Great Plains states from the
Dakotas southward to eastern Colorado, west southwest flow aloft
will develop over our counties, helping the pseudo monsoonal
moisture to creep back into our forecast area from western Colorado
through primarily our western counties west of I-25. Afternoon
thunderstorms will be most prevalent across our Snowy, Sierra Madre
and southern Laramie ranges, with evening thunderstorms, isolated to
widely scattered, spreading across our entire forecast area, with of
course the greatest areal coverage over our mountainous zones due to
differential and orographic effects. With additional cloud cover and
precipitation coverage, high temperatures will be tempered somewhat
despite the warm mid level temperatures.

Sunday...More scattered beneficial showers and thunderstorms will
occur in a carbon copy type day compared to Saturday, with broad
northwest flow aloft over southeast Wyoming and western Nebraska,
and pseudo monsoonal moisture over the counties. Carbon copy day
temperature wise to Saturday, for the aforementioned reasons from
Saturday.

Monday...Looks like the best chance for measurable rainfall, of 0.01
inch or more, with scattered to numerous afternoon and evening
showers, and scattered thunderstorms due to increased monsoonal
moisture, light steering winds and PWATs, Precipitable water
increases. Slight cooling in maximum temperatures due to lowering of
mid level temperatures and additional cloud and precipitation
coverage.

Tuesday...Ridging aloft amplifies over the Rockies, though with
monsoonal moisture trapped under the ridge from western Colorado
through the western half of Wyoming, and to a lesser coverage over
eastern Wyoming and western Nebraska, expect to see another carbon
copy type day with scattered to numerous showers, and scattered
thunderstorms during the afternoon and evening hours. Similar
maximum temperatures compared to Monday, with clouds and
precipitation coverage tempering highs, with readings peaking i the
80s to lower 90s.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 600 PM MDT Tue Aug 18 2026

Upper level disturbance and a surface backdoor cold front will push
into the region over the next few hours. Expect showers and
thunderstorms through late this evening.

HAZARDS/WEATHER TRENDS: Low confidence thunderstorm forecast for
most of the eastern plains this evening and tonight with poor
consistency between high resolution models. Thunderstorms are more
isolated than expected due to the BKN cloud cover and lack of
surface heating. However, strong upper level jet and backdoor cold
front are expected to push into the area around sunset...which may
change the environment and result in a large coverage of
thunderstorms from 02z to 08z. Confidence is limited at this time,
so will continue to monitor for now. Kept VCTS for most terminals
this evening until 03z for the band of thunderstorms moving along
the Colorado border and across the western Nebraska panhandle.
Thunderstorms may contain strong gusty winds (including locations
well ahead of the actual line) and brief heavy rainfall/near IFR
VIS.

&&

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

&&

$$

SHORT TERM...MM
LONG TERM...RUBIN
AVIATION...TJT