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303
FXUS65 KCYS 141915
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
115 PM 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.

&&

.MESOSCALE...
-- Changed Discussion --
Issued at 114 PM MDT Fri Jun 14 2024

Initial thunderstorm development over the southern NE panhandle
early this afternoon was likely elevated convection associated with
700mb frontogenesis as indicated by SPC mesoanalysis. Low-levels
based on latest RAP soundings remain capped with 1500 J/kg of MLCAPE
available aloft. Observed mostly heavy rainfall with these storms as
MRMS estimates suggested 1-2" of rainfall between Bushnell and
Kimball over a short duration. Only 1 HREF member suggested this
early development (and it was a time-lagged member). More widespread
CI will commence over the next 1-3 hours near a line from Cheyenne
through Chadron as well as further south in CO moving northeast into
portions of the NE panhandle.

Latest GOES imagery continues to show clearing across Laramie Co
along with southern Goshen and Platte counties in WY as the
atmosphere continues to destabilize today. Stable billow clouds still
appear to be present over portions of the South Laramie Range while
latest trends from GOES EMeso2 shows a narrow Cu field trying to
materialize east of Chugwater. This is positioned along the moisture
convergence boundary noted in surface observations with ~35-40F
degree dew points to the west and moist ~55-60F degree dew points
eastward towards the NE border. This boundary will be the focal
point for additional convection later this afternoon along with any
storms that develop over the higher terrain as the low-levels
continues to destabilize. Latest research sounding taken at CSU
around 17z still showed over 100 J/kg of SBCIN that needs to be
overcome, but PW near 1" will support heavy rainfall with these
storms headed into this afternoon and evening.
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&&

.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 /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1139 AM MDT Fri Jun 14 2024

Numerous showers and thunderstorms are expected across the area
this afternoon. Thunder should start to become more prevalent in
this activity over the next 1-2 hours as storms develop and move
from SW to NE. Storms will have the potential to produce very
heavy rainfall producing IFR conditions, frequent lightning,
gusty winds, and some hail. Most likely impacts are around KSNY
and KAIA, but all terminals could see storms move directly
overhead. Expect conditions to begin to improve after about 03z,
but lingering scattered showers with some isolated thunder may
continue through much of the night. Look for clearing skies by
Saturday morning.

&&

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

&&

$$

MESOSCALE...MB
SHORT TERM...SF
LONG TERM...TJT
AVIATION...MN