462
FXUS65 KCYS 171112
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
512 AM MDT Mon Aug 17 2026

.KEY MESSAGES...

- There is a Marginal Risk (Level 1 of 5) of severe weather from
  Chadron south through Sidney. Large hail and damaging winds
  are the primary threats.

- There is a Marginal Risk (Level 1 of 5) of severe weather on
  Tuesday for most locations along and east of the Laramie
  Range. Large hail and damaging winds are the primary threats.

- Seasonable temperatures anticipated through mid-week.

- Warmer and drier weather will return to the area Thursday and
  Friday.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 149 AM MDT Mon Aug 17 2026

Upper-level ridging starts to return today with northwesterly to
westerly flow developing aloft. Northwesterly flow will keep the
door open for multiple 500mb vorticity maxima that are progged to
eject across the region this afternoon. While the monsoonal moisture
tap will be weak at best, residual moisture remains mainly in
western Nebraska. Westerly 700mb flow is expected for much of the
area this afternoon, acting to dry out some of the lower-levels. The
terrain-induced dryline will finally be able to push eastward this
afternoon, rather than remaining pinned against the high elevations.
Therefore, the I-25 corridor looks to dry out quickly this
afternoon, preventing any significant precipitation potential,
despite the multiple vorticity maxima pushing through. Western
Nebraska on the other hand will continue to see mositure. As the
dryline pushes eastward, it will lead to forcing for western
Nebraska. With residual moisture and support from 500mb vorticity
maxima, showers and storms are anticipated to develop this
afternoon. MLCAPE values increase this afternoon to around 1000 to
1500 J/kg in western Nebraska. Forecast soundings from the RAP
suggest stout Inverted-V signatures west of the dryline across
southeast Wyoming, with more favorable severe soundings in western
Nebraska, though still will substantial DCAPE values. Therefore,
some storms this afternoon could become severe and the Storm
Prediction Center has highlighted Chadron south through Sidney in a
Marginal Risk (Level 1 of 5) of severe weather this afternoon. Large
hail and damaging winds are the primary concerns, with tornadoes not
expected. Precipitable Water values remain fairly average across the
region, so while heavy rain cannot be completely ruled out, it is
unlikely due to the lost of the strong monsoonal moisture tap.

The upper-level ridge attempts to strengthen throughout the day
Monday, but a strong trough moving south out of Canada will act to
flatten the ridge for much of Tuesday afternoon. Largely zonal flow
aloft returns. Multiple lobes of 500mb vorticity look to move across
the region Tuesday afternoon into the overnight hours, due to
increased vorticity from the proximity of the upper-level trough.
Westerly flow is typically drier across the region, though some
moisture will be retained. 700mb flow becomes stronger Tuesday
afternoon behind a 700mb shortwave ejecting across the region early
Tuesday morning. This stronger 700mb flow will lead to more shear
across the region, with strong 700mb warm air advection allowing
increased forcing. The terrain-induced dryline will retreat westward
Tuesday afternoon, allowing stronger moisture advection along and
east of the Laramie Range, setting up another day with increased
precipitation chances. MLCAPE and SBCAPE look to be a smidge
stronger for Tuesday, likely due to increasing moisture in the
region. Shear will be much stronger than previous days as 700mb flow
strengthens aloft. With stronger forcing, increased moisture, and
increased shear in the region, storms will have a better chance of
becoming severe on Tuesday, with large hail and damaging winds
possible once more. The Storm Prediction Center has highlight all
areas along and east of the Laramie Range in a Marginal Risk (Level
1 of 5) of severe weather. Excessive rainfall looks unlikely Tuesday
as storms should be quicker moving and precipitable water values
remaining near average for this time of year.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 149 AM MDT Mon Aug 17 2026

Wednesday will bring a fairly similar setup, but probably with
better surface moisture due to models picking up on slightly more
organized southerly flow across the High Plains. While it is still
early, parameters look somewhat favorable for strong to severe
thunderstorms both Tuesday and Wednesday, despite lower overall
thunderstorm coverage compared to the monsoon surge our area has
been under for the last several days. The ECMWF ensemble Extreme
Forecast Index for combined CAPE-Shear is around 0.7 to 0.85 for
much of the High Plains both Tuesday and Wednesday, indicating that
a large proportion of ECMWF ensemble members have combined CAPE-
shear values that are climatologically unusual to extreme. We will
need to continue to watch this period in the coming days.

A more defined upper level shortwave trough will pass through the
area Wednesday evening. Behind this feature, the upper level ridge
over the southwest will begin to re-amplify to our west, initiating
a drier, northwest flow pattern for the remainder of the week.
Precipitable water won`t be terrible during this weather pattern, so
we should still see at least a few isolated PM showers and storms
Thursday and Friday, but expect those two days to be considerably
drier. Temperatures will also begin to climb once again. 700-mb
temperatures climb back to near the climatological 90th percentile
by Friday into Saturday, peaking around +16 to +18C across the area.
Expect high temperatures to climb above seasonal averages Thursday,
and then closer to 5 to 10 degrees above average Friday and
Saturday. Since we`ll be somewhat on the periphery of this heat
wave, current forecasts do not challenge daily record highs, but
this could still change in the coming days. Over the course of
Thursday through Saturday, the upper level ridge should start to
very slowly meander back to the east. Once this moves east of the
Four Corners, the monsoon should surge back to the north, and we may
see precipitation chances increasing again by Saturday.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 512 AM MDT Mon Aug 17 2026

Mostly clear skies early this morning with some patchy fog
developing near KCDR and KAIA. Patchy fog may continue through 14 or
15Z, but confidence is low on this fog decreasing visibility out of
VFR conditions. Partly cloudy skies develop by mid-morning as winds
start to increase. Isolated showers and thunderstorms are possible
today. Gusty and erratic winds are possible in and around any shower
or storm that develops.

&&

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

&&

$$

SHORT TERM...AM
LONG TERM...MN/AM
AVIATION...AM