181
FXUS65 KCYS 191801
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1201 PM MDT Fri Jul 19 2024

.KEY MESSAGES...

- Severe weather is possible Friday as a cold front moves across
  the region. Damaging winds and isolated large hail are
  possible as the cold front moves through. Cooler temperatures
  for Saturday.

- Cooler weather expected this weekend into Monday ahead of a
  gradual warming trend to above average temperatures through
  next week.

&&

.SHORT TERM /THROUGH SATURDAY NIGHT/...
Issued at 251 AM MDT Fri Jul 19 2024

Current Weather... Cloud cover is increasing over the region as of
08Z this morning. A few cloud pulses are visible on satellite near
the Converse/Carbon/Albany triple point and across southwestern
South Dakota. Some of the showers and storms in South Dakota could
briefly clip Dawes County in the early morning hours, but nothing
severe is expected at this time. It is a comfortable night out there
with temperatures sitting in the upper-50s to upper-60s area wide.
Chadron is the warm spot as of 08Z with a temperatures of 75F. Winds
are mostly light across the region, with the exception of Chadron
and Sidney where winds are around 10 mph.

Discussion... Upper-level ridging remains in place over the western
CONUS with the CWA under northwesterly flow aloft. A 250mb
shortwave trough that originated over the Pacific Ocean and
traversed up and around the ridge will slide past the region to the
northeast. A 700mb low associated with this shortwave trough will
support the development of a surface low that is progged to drag a
cold front across the region. At 500mb, a high is still positioned
over the Four Corners region, but will retrograde slightly to the
west throughout the day Friday. Monsoonal moisture will still be
able to advect into the region, leading to increasing moisture
during the day Friday. Precipitation has already begun to develop as
the surface low forms over western South Dakota as of 08Z.
Increasing showers and thunderstorms are likely through the morning
and afternoon hours, with the highest coverage of storms associated
with the advancing cold front. Surface winds will remain
northwesterly through the morning and afternoon hours, leading to
perpendicular orientation to the cold front as it drags through.
Deep layer, 0-6km shear vectors will be oriented perpendicular to
the cold front initially with shear values near the 50-60kt range
according to the HRRR. With MLCAPE values progged to increase
towards the 1000-1500 J/kg range over the Panhandle and increased
PWs due to monsoonal moisture advection, initially discrete storms
are expected to develop before conglomerating into a multi-cellular
cluster or semi-linear system later by the evening hours. SPC has
most of the Panhandle in a Marginal risk with the furthest east
portions of the CWA in a Slight Risk. Primary threats look to be
damaging wind and isolated large hail, as forecast HRRR soundings
suggest an inverted-V setup once more. The NAM is bullish on the
amount of additional monsoonal moisture in the region and is likely
biased towards a wetter solution. The NAM suggests PW values well
above 1 inch, closer to about 1.3in or so, while the GFS is
significantly lower, around the 1 inch mark. In most recent model
runs, the HRRR has been backing off on CAPE in the afternoon hours
while the NAMNST is showing well over 1500 J/kg in the Panhandle.
The high CAPE values in the NAMNST are likely attributed to the
wetter solution that the NAMNST is suggesting with the very high PW
values. Additionally, cloud cover is expected to increase through
the morning and afternoon hours, limiting daytime heating and
destabilization of the atmosphere. One failure mode for today would
be in the cloud cover keeps the region cooler and capped throughout
the afternoon/evening hours. At this time, it looks like there will
be enough daytime heating to increase CAPE values across the region,
but capping cannot be ruled out at this time.

Much cooler weather is expected for Saturday as the upper-level
ridge and northwesterly flow remain in place. The 500mb and 700mb
low from Friday will remain over the Nebraska/Iowa stateline and may
hover just close enough nearby to promote the development residual
showers and thunderstorms across southeast Wyoming. This low is not
forecast to move out of the region quickly, as it is being
blocked by a much stronger low over eastern Canada. Therefore,
the residual impacts of the low will be experienced for much of
the day Saturday. Afternoon showers and thunderstorms are likely
as a few 500mb vorticity maxima traverse through the ridge and
overhead. Temperatures will be in the upper-70s to mid-80s
across the region, much cooler than the mid-80s to mid-90s from
Friday.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
Issued at 251 AM MDT Fri Jul 19 2024

Cooler weather will continue on Sunday and Monday with afternoon
highs around 80, potentially staying in the mid-70s around Cheyenne
and Laramie making for very pleasant weather at this time of the
year. Scattered showers will be possible Sunday with another
shortwave passing in the northerly flow on the eastern periphery of
the western ridge. Coverage will be less on Monday as the upper
level low in eastern NE begins to progress eastward and draw away a
lot of lingering low-level moisture. Additionally, instability is
very marginal and shallow, so not expecting deep convective
development. Will most likely be dealing with showers developing
along the higher terrain with daytime heating.

Looking ahead to the middle of next week, a large scale trough
slowly approaches the PacNW attempting to breakdown this blocking
pattern. Latest cluster analysis continues to show uncertainty with
the timing of the trough passage to the north. GEFS members continue
to lean towards an earlier trough passage while EC members remain a
little bit slower, therefore increasing uncertainty with temperature
and precipitation forecast. Overall, still expecting the gradual
warming trend to continue mid-to-late next week as temperatures
climb above average for late July. Looking at 700mb temps reaching
near 18C which is above the 90th percentile value per neighboring
sounding site climatology. Latest NBM currently shows probabilities
of exceeding 100F degrees around 30-50% across the northern NE
panhandle starting Thursday. As models come into better alignment,
would not be surprised if these higher probabilities begin to spread
farther south across the NE panhandle and into portions of southeast
WY. Latest cluster analysis shows this thermal ridge of up to 18C
700mb temps moving across the central Rockies, just uncertainty on
the timing of its placement across our area.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1139 AM MDT Fri Jul 19 2024

The main concern with this portion of the forecast is the
convection potential this afternoon.

Latest satellite loop was showing lots of cumulus developing
over the Laramie and Snowy Ranges. This area of cumulus is
expected to become organized over this area during the afternoon
and develop into clusters and push southeast. The lastest HRRR
is predicting the storms to possibly affect the KCYS, KBFF and
KSNY mainly after 22Z. Would not be surprised to see some
embedded severe storms within these clusters especially as they
head into the Nebraska panhandle. Otherwise, the bulk of the
convection should exit the area by 02z.

&&


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

&&

$$

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