059
FXUS65 KCYS 311118
AFDCYS

Area Forecast Discussion National Weather Service Cheyenne WY
516 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 /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 516 AM MDT Thu May 31 2024

Area of low stratus with patchy areas of FG are beginning to develop
across the area this morning. MVFR CIGS are already impacting the
KCYS terminal this morning and these CIGS may move into the Nebraska
terminals throughout the morning hours. Light easterly flow will
keep these low clouds around through 18z. The KCDR and KAIA
terminals my keep MVFR/IFR CIGS through the valid TAF period while
the other sites see a window of VFR conditions.

&&

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

&&

$$

DISCUSSION...MN
AVIATION...AW