288
FXUS65 KCYS 311001
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
401 AM MDT Fri May 31 2024

.KEY MESSAGES...

- Daily chances for isolated to scattered showers and
  thunderstorms continue through the weekend. A few storms may
  have the potential to become strong to severe each day.

- Warm and dry weather is expected most of next week, aside from
  an uncertain potential for a cold front which could bring some
  shower/storm activity on Tuesday or Wednesday.

&&

.DISCUSSION...
Issued at 400 AM MDT Fri May 31 2024

It`s an active morning across the area with an unusual round of
nocturnal thunderstorms pushing across the southern Nebraska
panhandle. Low level moisture wrapping around the south side of
a surface high near the MT/SD/ND triple point has been enough to
bring in a narrow tongue of elevated instability into the
southern panhandle. Overrunning isentropic lift over the surface
frontal boundary associated with the leading edge of a
shortwave trough has provided enough lift to get storms going in
this area. The environment is marginal for severe weather, but
can`t rule out another storm briefly producing severe hail or a
quick wind gust. These storms are producing near constant
lightning though, and a good dose of moderate to heavy rainfall.
So far, Kimball has received over 0.80 inches of rain. As of
now, these are progressive enough to reduce flooding concerns,
but there is the potential for training storms which would
increase the flash flood potential. Showers and storms should
clear the area by mid morning. Elsewhere, a deck of low clouds
is westward behind the surface cold front. This may lead to some
areas of fog through mid morning near the I-80 summit and parts
of Laramie county eastward along the Cheyenne ridge.

Areas along and east of the Laramie range can expect a cooler
day today in the wake of the overnight cold front. Low stratus
will also keep things considerably cooler especially this
morning. Expect clouds to start to break up from west to east
starting late in the morning through the afternoon hours. The
rate of clearing will set the stage for a conditional
thunderstorm threat this afternoon. Coverage is not expected to
be very high, but if the inversion erodes enough to break the
cap, storms will have a decent amount of instability to tap into
in the presence of sufficient wind shear. If this occurs, it
will probably be around the Cheyenne area towards the southern
Laramie range once again, but once formed may persist eastward
along the I-80 corridor. Later this evening, a weak ripple in
the flow aloft is expected to drive a brief period of isentropic
lift once again, this time focused more on the northern NE
panhandle. HiRes models are depicting a few showers and
thunderstorms developing in a narrow tongue of instability after
midnight and lasting through the early morning. As a result,
expanded some low end PoPs for Sioux, Dawes, and Box Butte
counties late tonight.

The flow aloft transitions to more zonal flow this weekend, but
a broad shortwave is expected to traverse across the west. A
vorticity maximum on its leading edge will provide some enhanced
lift Saturday afternoon. The main uncertainty for the
thunderstorm threat is the dryline position once again. Some
models, such as the GFS and HRRR, push this feature eastward
rapidly, bringing dry air across the area. This would mean
storms would mostly just be a gusty wind threat with inverted-v
soundings in place. However, a slower departure of the surface
high to the north may keep southeasterly moist flow further
west, which would mean a chance for more organized convection.
It`s too early to say which will end up verifying, but will need
to monitor model trends over the next few cycles. Rinse and
repeat for Sunday as the main axis of the upper level trough
moves through our area during the afternoon. Again, the
uncertainty is in the dryline position, but a few strong to
severe storms could occur east of the dryline, with gusty winds
to the west.

Behind this shortwave, a stout but potent ridge starts to grow
over the western CONUS. This ridge will bring fairly significant
warming aloft, stabilizing the atmosphere into the early part of
next week. Monday thus looks dry and warm to hot. The midweek
trough will be the main forecast challenge for next week as
another shortwave swings over the top of the ridge. The GFS
continues to show a shallow and progressive trough that gives
way back to the dominant ridge by Wednesday. The ECMWF has held
with its solution of a much deeper and slower trough that would
bring some shower and thunderstorm chances back into the picture
for Tuesday and Wednesday. Overall, ensemble guidance has
trended slightly towards the ECMWF solution over the last 24
hours, with about 45% of members now showing a much more
pronounced cool down on Tuesday and Wednesday. The inter-
quartile range in ensemble 700-mb temperature forecasts for
Wednesday morning ranges from 3 to 10C, which is a pretty large
spread for this lead time on the cusp of summer. The remaining
55% of members is mainly driven by the GEFS, which shows the
ridge rebounding quickly, leading to just a brief pause in the
hot temperatures.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1050 PM MDT Thu May 30 2024

A complex forecast has become more complex over the past few
hours. Primary aviation concern will be thunderstorms east of
the Laramie Range, but KRWL and KLAR will likely not see any
storms overnight. KCYS, KBFF, KCDR, KSNY, and KAIA will likely
see isolated to scattered thunderstorms beginning within the
next 1-2 hours and likely coming to an end between 11 and 13Z.
One the backside of the storms, KCYS will likely see fog moving
in, dropping the terminal to IFR to LIFR in the densest fog.
KCYS should be the only terminal that sees fog overnight, but
cannot completely rule out KSNY at this time. Winds will be calm
overnight before increasing into the afternoon hours tomorrow.
Expect gusty and erratic winds near any thunderstorms that
develop.

&&

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

&&

$$

DISCUSSION...MN
AVIATION...AM