347
FXUS65 KCYS 141029
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
429 AM MDT Fri Jun 14 2024

.KEY MESSAGES...

- Strong to severe thunderstorms are possible across the high
  plains of southeast Wyoming and the western Nebraska Panhandle
  on Friday. Torrential rainfall and the potential for flash
  flooding will be the primary concern. However, large hail and
  damaging winds will be possible with any storms developing early
  in the afternoon prior to storm mergers.

&&

.SHORT TERM /THROUGH SATURDAY NIGHT/...
Issued at 421 AM MDT Fri Jun 14 2024

Active weather expected over the next 18 hours with a Slight Risk
for severe weather in place for much of the Nebraska panhandle.
Current observations already show south to southeasterly winds
across much of the CWA east of the Laramie Range. Most Hi-Res
guidance is in agreement with sustaining southerly winds throughout
much of the day today. This will advect low-level moisture into the
region, steadily raising dewpoints into the afternoon hours.
Dewpoints in the 50s look possible from Cheyenne to Torrington to
Chadron and areas south of this line. Some low 60 degree dewpoints
could be possible in the southern panhandle. Various model soundings
do show evidence of a capping inversion early into the afternoon,
however, large scale lift from a strong shortwave trough moving
across the southern Rockies should be enough to initiate convection.
With the incoming trough, much of the Hi-Res guidance, including
the HRRR and the RAP, do show a convergence boundary setting up
roughly in the same vicinity as the aforementioned line from
Cheyenne to Chadron. Westerlies from the shortwave converging
with the moist southerly flow will likely lead to convective
initiation along this boundary around mid-afternoon as indicated
by Hi-Res models.

All modes of severe weather look possible with these storms. Initial
storms that develop could be discrete, lending itself to more of a
large hail threat as MUCAPE values in the panhandle approach 2000
J/kg. As storms progress and push further east, clustering looks
more likely, with isolated gusts over 60 MPH possible. Can`t rule
out an isolated tornado in the southern panhandle with these storms
as effective shear values will be in the 35 to 40 kt range. MLCAPE
values will also be elevated in the southern panhandle, around 1200
J/kg which will also contribute to the tornado potential. Lastly,
torrential rains are expected with these storms which could lead to
flash flooding concerns. GFS soundings show PWs over an inch in the
panhandle which is above climatological normal. This is backed by
NAEFS showing 90th percentile and above PWs, with as high as the
99th percentile in the panhandle. Luckily, these storms will be
quick moving as indicated by cloud layer winds, but they have
the capacity to drop large amounts of rain in a short period of
time.

The potential for strong to severe storms drops off around 9 PM
Friday night, but scattered showers and thunderstorms will linger
overnight. Behind the shortwave, conditions on Saturday will rapidly
dry out and warm up. +16C 700 mb temperatures will lead to highs in
the upper 80s and 90s for areas east of the Laramie Range.
Thunderstorm development does look possible Saturday afternoon with
a weak upper-level disturbance passing overhead. Because of the dry
low-levels, its possible that much of the precipitation with these
storms will not make it to the ground, leading to strong, gusty
winds. Showers and storms will dissipate by the evening.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
Issued at 421 AM MDT Fri Jun 14 2024

Models in reasonable agreement late this weekend through early
next week, showing broad southwest flow aloft through early
Tuesday. An upper level trough across the eastern Pacific is
forecast to remain anchored over the Pac NW into Monday, and
finally show signs of ejecting east on Tuesday. Further east,
surface high pressure will remain over the northern and central
plains, which will help in producing easterly winds for areas
along and east of the Laramie Range. Upslope easterly winds may
be enhanced by any MCS activity over the northern plains, with
models showing some signs of a mesoscale cool front moving
southwest across the northern Front Range. For Sunday and
Monday, expect a continuation of isolated to widely scattered
(10 to 30 percent coverage) thunderstorms in the late afternoon
through the late evening hours. Although forcing seems limited,
there should be enough low level moisture, jet energy aloft,
and natural terrain-induced convergence along the Laramie Range
to initiate some thunderstorm activity. Not expecting severe
weather, but a few strong thunderstorms are possible Sunday
evening through the overnight hours north of Interstate 80.
Temperatures should remain near or slightly above average with
highs in the 80s for most locations along I-80 and west of I-25.
However, with models hinting towards a backdoor cool front and
easterly winds...high temperatures will struggle to get into the
mid 70s from Douglas to Chadron Nebraska.

For Tuesday night and Wednesday, the 00z GFS is trending more
aggressive with a late-season Pacific cold front as the upper
level trough in the Pac NW tracks eastward. All models generally
have 700mb temperatures dropping between 0c to 5c above zero by
early Wednesday morning as the primary trough axis quickly moves
across the area and weakens. The ECMWF and Canadian are not as
aggressive with the cooler airmass, but are more in line with
the ensemble mean. Kept high temperatures in the mid 60s to mid
70s on Wednesday although some guidance is around 10 degrees
cooler...which is around the 10th percentile of ensemble
spreads and the NBM. Will have to monitor overnight lows with a
few locations already showing min temperatures close to freezing
early Wednesday morning. Kept low POP (15 to 25 percent) for
this time period with an unfavorable signal for widespread
convection due to the cooler temperatures near the surface.

For later next week, models do indicate a general warming trend
as we head into Thursday and next Friday with a more favorable
environment for afternoon and evening thunderstorms.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 511 PM MDT Thu Jun 13 2024

VFR conditions expected through the 00Z TAF period. Primary
aviation concerns this evening will be isolated showers
producing gusty and erratic winds. KRWL gusted to 58mph near one
of these showers, so gusty, erratic winds will be possible as
showers move over or near the terminals. Winds expected to
increase tomorrow, with showers and thunderstorms expected in
the afternoon and evening hours, some of which may be severe.

&&

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

&&

$$

SHORT TERM...SF
LONG TERM...TJT
AVIATION...AM