690
FXUS65 KCYS 181121
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
521 AM MDT Thu Jul 18 2024

.KEY MESSAGES...

- Severe weather potential for Thursday and Friday, with Friday
  looking to be the better day.

- Cooler weather expected this weekend with daily chances of
  storms through early next week.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 316 AM MDT Thu Jul 18 2024

Current Weather... Mostly clear skies as of 08Z for most of the
region this morning. Some clouds remain over northern portions of
the CWA and southern Albany and western Laramie counties.
Precipitation in the northern regions has come to an end over the
past few hours, leaving fairly quiet conditions overnight. Fairly
light winds are ongoing across southeast Wyoming and western
Nebraska with temperatures in the mid-50s to low-60s at this time.
Clear skies, light winds, and lowering dewpoint depressions over the
southern Panhandle should lead to some patchy fog developing within
the next few hours.

Discussion... Upper-level ridging continues over the western CONUS
for Thursday with northwesterly flow over the region. Several 500mb
vorticity maxima are progged to traverse through the ridge during
the day Thursday, promoting localized, but weak, synoptic lift over
southeast Wyoming. The stout 500mb high over the Four Corners region
will remain in place throughout the day, ushering monsoonal moisture
into the region and increasing PW values into the 90th percentile
for western Nebraska. PW values approaching 1 inch are expected for
much of the Panhandle, with some higher values creeping into far
southeast Wyoming/Laramie County. A convergence boundary looks to
set up along the Laramie Range, with easterly winds to the east of
the mountains and westerly to southwesterly winds west of the
mountains. With increased moisture in the region and sufficient
surface lift provided by the surface convergence zone and terrain-
induced lift across the southern Laramie range, showers and
thunderstorms are expected to develop across the higher terrain and
propagate into the adjacent zones. With daytime heating across
across the region and increasing moisture from monsoonal flow,
MLCAPE values are progged to be between 500 and 1000 J/kg across
southeast Wyoming and closer to 1000 to 1500 J/kg in the Nebraska
Panhandle. While surface moisture will be on the rise throughout the
day, mostly dry low-level sounding profiles are expected with
inverted-V HRRR forecast soundings across southeast Wyoming.
Panhandle soundings are more moist in the lower-level, but still
suggesting strong mid-level lapse rates. Bulk shear values will be
fairly weak, ranging from about 30-35kts with weak steering flow.
Overall, isolated showers and thunderstorms are expected to develop
over the higher terrain this afternoon and propagate easterly into
the Panhandle where the environment will be more favorable for
severe weather. Some of these showers and thunderstorms will likely
be heavy rain producers due to the high PW values and the weak
steering flow aloft. SPC has the CWA in a Marginal Risk for severe
weather, with gusty winds and isolated large hail being the primary
threats. Right now, looks most likely that the gusty wind threat
will be dominant over southeast Wyoming, with the hail threat
increasing into the Panhandle where the environment is more
conducive to more severe weather types.

Friday will feature another day with predominantly northwesterly
upper-level flow across the region. Several 500mb vorticity maxima
will traverse through the ridge, with the 500mb high over the Four
Corners region retrograding back to the west. While this shift to
the west will lead to weaker monsoonal moisture advection into the
region, lingering moisture from Friday will allow ample moisture to
remain across the CWA. The main instigator for convection on Friday
will be a 700mb low that is progged to eject to the southeast out of
the Dakotas and across central Nebraska. This 700mb low will be
accompanied by a surface low and attendant cold front. As the cold
front is dragged across southeast Wyoming and western Nebraska it is
expected to initiate convection across our region. MLCAPE values
over 1000 J/kg are expected across western Nebraska. Bulk shear will
increase with the surface low to the east and the cold front moving
towards the south, leading to shear values in the 35 to 45kt range.
Steering flow will be stronger Friday, so while PW values wil be
around an inch, heavy rain producers will be more difficult to get as
storms will not remain over the same area for a long time. Forecast
HRRR soundings suggest dry low-level across the Panhandle with
strong low-level to mid-level lapse rates. However, storms should
still be surface based and SBCAPE values are well over 1500 J/kg.
Overall, expecting storms to develop along the advancing cold front
with isolated storms possible initially before likely conglomerating
into a multi-cellular structure as the evening goes on. SPC has the
CWA in a Marginal Risk for severe weather with strong winds and
large hail as the primary threat. Minimal turning with height will
limit any tornado potential on Friday.

Temperatures for Thursday and Friday will be warm towards hot.
Thursday will be the warmest day, with highs in the mid-80s to mid-
90s. Friday will drop a few degrees as the 500mb high shifts
slightly further west. Highs will be in the low-80s to low-90s area-
wide. Winds will be fairly calm for the end of the week, with no
strong gusts expected across the region.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 316 AM MDT Thu Jul 18 2024

Overall, looking at cooler weather this weekend with afternoon highs
around 80 on Saturday and potentially dropping into the mid-70s for
Sunday. This will be all while much of the western CONUS is
expecting above average temperatures developing under the vast upper
level ridge, however much of our CWA will be positioned well east of
the ridge axis and more under the influence of the large scale
trough across the central and eastern CONUS allowing for cooler
weather. Daily chances for showers and thunderstorms continue with
multiple disturbances passing over and along the eastern periphery
of the ridge. Saturday and Sunday look to have the best chance for
more widespread coverage as stronger upper level support will be
present associated with the upper low over the central CONUS mixed
with post-frontal east/northeasterly flow up against the Laramie
Range.

Looking ahead to the middle of next week, a large scale trough
approaches the PacNW attempting to breakdown this stale upper level
pattern. Latest cluster analysis shows growing uncertainty with the
timing of the trough passage to the north. One cluster mainly
comprised of GEFS members shows a quicker trough progression across
the Canadian border that shifts the thermal ridge overhead while
other clusters more comprised of EC members are on the slower side.
Will need to continue to monitor model trends as this will impact
how quickly we return to above normal temperatures for the later
portions of next week.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 519 AM MDT Thu Jul 18 2024

VFR conditions expected for the 12Z TAF period. A thin layer of
fog has developed near KAIA, dropping visibility to 7-8 SM as of
11Z. This fog is not expected to become dense and visibility
should not be reduced below 6SM. Primary aviation concern today
will be isolated showers and thunderstorms over southeast
Wyoming and the far southern Nebraska Panhandle. Could see
thunderstorms near the KLAR, KCYS, and KSNY terminals, but
confidence is lowest for KSNY at this time. KRWL will likely see
some showers nearby throughout the afternoon hours. Light winds
throughout the day with skies remaining mostly clear.

&&

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

&&

$$

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