912
FXUS65 KCYS 021134
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
534 AM MDT Sat Aug 2 2025

.KEY MESSAGES...

- There is a Slight Risk (Level 2 of 5) for severe weather
  Saturday afternoon and evening east of the Laramie Range. All
  modes of severe weather are possible: large hail, damaging
  winds, an isolated tornado or two (southern Panhandle), and
  flash flooding.

- There is a Marginal to Slight Risk of severe weather for
  Sunday, mainly across western Nebraska. Primary hazards look
  to be large hail, damaging winds, and flash flooding.

- Drier and warmer conditions return mid-week and continue
  through the weekend.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 205 AM MDT Sat Aug 2 2025

Mostly quiet tonight after another round of severe weather pushed
through the CWA this afternoon and evening. Skies are clearing
nicely, though some low clouds have begun to move over Cheyenne and
portions of the South Laramie Range. These clouds will likely stick
around through the morning hours and may spread eastward into
portions of the Nebraska Panhandle. Temperatures are cooler tonight,
in the low-50s to low-60s over in the Panhandle. Winds a light in
most locations, except Bordeaux which is gusting around 40mph due to
weak, 700mb disturbance.

Yet another round of severe weather is possible this afternoon
across much of western Nebraska and into southeast Wyoming, where
the Storm Prediction Center has highlighted a Slight Risk (Level 2
of 5) for the southern Panhandle and a Marginal Risk (Level 1 of 5)
for the remaining areas east of the Laramie Range. A broader, upper-
level trough will push into the region from the west with a moisture
fetch back west well into the Pacific Ocean. Strong 500mb cyclonic
vorticity advection will enable the eastern branch of the upper-
level trough to deepen, becoming better defined, especially as the
anticyclonic vorticity advection to the west enhanced the developing
upper-level ridge. Ahead of this trough, storms are expected to
initiate along and east of the Laramie Range once more. 700mb flow
remains relatively weak, though a hint of a low-level jet developing
in the early afternoon across the western regions may lead to
strengthening of the overall environmental potential as that subtle
jet move eastward throughout the evening hours. A weak, 700mb cold
front will push in from the north during the afternoon and evening
hours with 700mb temperatures dropping from around 12 to 13C down to
9 to 10C this evening. At the surface, winds will remain mostly
southerly throughout the night before turning westerly around 12Z
for locations just east of the Laramie Range. This will result in
the terrain-induced dryline to propagate eastward throughout the
late morning and early afternoon hours, which is helping push much
of the best severe threat into the Panhandle. However, a brief
period of easterly flow may exist later this afternoon, prior to the
dryline moving eastward, that will help storms initiate along the
Laramie Range. This may also lead to a couple rounds of showers and
storms today, with the early easterly flow initiating storms along
the Laramie Range, then the dryline initiating storms as it moves
east off of the Laramie Range throughout the afternoon and evening
hours. Therefore, a longer duration of showers and storms may be
possible this afternoon and evening. Temperatures will warm into the
low-80s to low-90s across the Panhandle, providing ample atmospheric
destabilization for storm initiation later this afternoon.

As far as severe parameters goes for this afternoon, MLCAPE values
will increase into the 1800 to 2200J range. The NAM suggests the
highest CAPE values, but also has a bit of a moisture bias within
the model, likely artificially increasing forecast CAPE values.
However, the HRRR is a bit dry resulting in lower CAPE values the
afternoon, further increasing model disagreement. 0-6km Bulk Shear
values are decent for the afternoon, sitting around 40 to 45 kts at
this time, per the HRRR with slightly lower values (35 to 40 kts) in
the NAM and RAP. Some preference goes towards the HRRR solution
today as the HRRR performed well with yesterday`s convection.
Forecast soundings from the NAM and HRRR suggest drier low-level,
but ample CAPE in the hail growth region across the Panhandle. DCAPE
values in the 1200 to 1500J range also indicate the potential for
strong to damaging winds gusts across much of the Panhandle this
afternoon. 0-1km Storm Relative Helicity on the other hand sits only
around 25, depending which model is being interrogated, with 0 to
3km only around 30 or so. These values are a little low for tornadic
potential, but an isolated tornado cannot be completely ruled out
today with some turning evident in the lowest-levels. Hodographs are
fairly straight, not matter which model is considered, leading to an
increased chance of storm splitting and upscale growth across the
region. With bulk shear vectors oriented more perpendicular to the
dryline and incoming cool front, initially discrete storms are
possible. However, weak steering flow favors storms then growing
upscale throughout the afternoon and evening hours into a more
linear system. In addition to the severe threat, the Weather
Prediction Center has highlight much of the region in a Marginal
Risk for excessive rainfall, with a small sliver of the southern
Panhandle under a Slight Risk for excessive rainfall. With
precipitable water values increasing towards the 1 inch mark across
the Panhandle and fairly weak steering flow, heavy rain and flash
flooding will also pose a risk today. Therefore, all modes of severe
weather are possible this afternoon: large hail, damaging winds, an
isolated tornado or two, and flash flooding from heavy rainfall and
slow moving storms growing upscale. Despite this, western portions
of the region, generally west of the Laramie Range, will see
elevated fire weather conditions this afternoon with little to no
precipitation chances and little to no wetting rainfall chances.

The Storm Prediction Center has indicated a Marginal Risk (Level 1
of 5) for severe weather on Sunday as northwesterly flow moves
overhead. Similarly, far eastern regions of the CWA have a Marginal
Risk for excessive rainfall on Sunday as well. As the upper-level
ridge develops over the Intermountain West, northwesterly flow will
develop overhead, opening the door for subtle disturbances to pass
through the flow. A very subtle upper-level shortwave will be the
primary synoptic forcing for severe weather potential Sunday as it
moves through the upper-level ridge into the Southern Plains. Ample
500mb CVA will precede this shortwave, leading to additional, though
weaker, synoptic lift associated with these vorticity maxima. 700mb
low once again remains weak, but some models suggest the development
of a 700mb low over eastern Nebraska, which would act to strengthen
winds across the CWA and a very weak 700mb cold front to push
through, so weak that temperatures will likely not react much to the
frontal passage. However, this will still be enough to get showers
and storms going again Sunday afternoon. Bulk shear will be stronger
on Sunday with values around 50 to 55kts across portions of the
region, likely as a result of the developing 700mb low over
Nebraska. MLCAPE values will return to the 1500 to 1800J range
Sunday afternoon with PWs increasing towards the 1 inch mark once
more. Forecast soundings suggesting fairly dry low-level and more
moist mid-levels than previous days. DCAPE values around 1000J will
be present with the drier low-levels and with decent CAPE in the
hail growth layer, large hail and damaging winds will be possible
once again. However, forecast HRRR soundings suggest much less CAPE
and DCAPE, increasing the likelihood of smaller hail than the NAM and
RAP. Low-level turning is nearly non-existant, so tornadoes are not
expected at this time for Sunday. Short range models should begin to
come into better agreement later today into the overnight hours, so
forecast updates will be implemented as a clearer picture is painted
for Sunday. Despite the severe risk to the east, fire weather
concerns will once again be present west of the Laramie Range,
though winds should be below criteria.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 205 AM MDT Sat Aug 2 2025

Warmer and drier weather is still expected in the long term forecast
for Monday onwards. However, there is currently a Marginal Risk for
severe weather across eastern portions of the region on Monday, due
to northwesterly flow aloft allowing a weak disturbance to push
through the flow and initiate enough synoptic lift to support a few
showers and storm, especially as monsoonal moisture attempts to
return the region. Storm chances are expected to drop off Tuesday as
an upper-level ridge builds over the southwestern CONUS and
subsidence returns to southeast Wyoming and western Nebraska. This
high is expected to remain firmly in place, leading to more zonal
flow across the region throughout the week. With the ridge so close
by, its influence will be felt as temperatures once again rise into
the 90s to near 100s. The warmest day looks to be Thursday at this
time, though Wednesday through Friday will be quite hot. The next
best chance for precipitation will be Saturday as an upper-level
trough attempts to dive south out of Canada. However, this is still
a week away, so things will change. This potential trough will
continue to be monitored throughout future forecast updates.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 533 AM MDT Sat Aug 2 2025

Mostly clear skies continue across southeast Wyoming and western
Nebraska. Some low stratus have begun encroaching on KSNY as of 11Z.
These low clouds look to continue for the next 1 to 3 hours for this
terminal. There is a slight chance for patchy fog at all terminals
east of the Laramie Range. Confidence is low on this fog developing,
but included 2 hour TEMPO groups for patchy fog this morning, ending
at 14Z. As the afternoon progresses on, another round of strong to
severe storms are expected. Gusty, erratic winds and large hail will
be possible in and around any of the storms that develop this
afternoon. Expecting a northerly wind shift at most terminals,
though timing on this shift varies.

&&

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

&&

$$

SHORT TERM...AM
LONG TERM...AM
AVIATION...AM