000
FXUS65 KCYS 242122
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
322 PM MDT Wed Apr 24 2024

.KEY MESSAGES...

- Above average temperatures with isolated to scattered showers
  and thunderstorms continuing into this evening. Patchy dense
  fog expected near the I-80 corridor from Cheyenne to Sidney
  late tonight and early Thursday.

- Scattered to numerous showers and scattered thunderstorms
  expected Thursday afternoon and evening, with some
  thunderstorms possibly becoming severe east of the I-25
  corridor. Large hail and damaging wind are the primary
  hazards, but an isolated tornado can not be ruled out.

- Localized moderate to heavy rainfall possible on Friday
  morning across the Nebraska Panhandle.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 243 PM MDT Wed Apr 24 2024

Latest radar and satellite shows numerous showers and
thunderstorms that have developed early this afternoon mainly
along the I-80 corridor from Rawlins through Cheyenne with
leading shortwave energy ahead of the larger scale upper level
trough approaching the CA coast. Marginal instability (250-500
J/kg MLCAPE) and shear (25-30 kt) are currently present over
the area that have resulted in a few brief 50 dBZ cores
developing quickly and then collapsing leading to gusty winds. A
recent storm that passed over KCYS gusted to 41 kt. Main
hazards will continue to be gusty winds and occasional lightning
extending eastward into the southern NE panhandle through early
this evening.

Looking at another warm day on Thursday before increasing cloud
cover and storm development occurs in the afternoon with the
large scale trough moving into the four corners region. Lee
cyclogenesis over eastern CO will enhance south/southeastward
into the southern NE panhandle. SPC has portions of the CWA
under a marginal risk for severe thunderstorms. Latest CAM
guidance shows numerous strong thunderstorms developing mainly
over the NE panhandle and portions of eastern WY with the
potential for a few to turn severe, which is right on track for
this time of the year. The the first Severe Thunderstorm Warning
in our CWA is on average April 25. MLCAPE values around 1000
J/kg with 30-40 kt of 0-6 km shear present will be sufficient
for severe storms to develop with strong low-level shear
possible just north or the surface low.

Areas of uncertainty include portions of extreme southeast WY
and the placement of the moisture boundary impacting location of
CI in the afternoon. While mean MLCAPE entering Kimball Co is
around 1000 J/kg as mentioned, HREF 10th percentile values drop
below 200 J/kg suggesting a potential failure mode with the lack
of westward moisture transport/eastward dryline propagation with
downsloping winds off the Laramie Range. Additionally, strong
ascent with the approaching trough will lead to numerous storms
with the potential for embedded stronger cores rather than
discrete cells. Storms will likely continue through the mid-
evening with upscale growth developing off the east.

Headed into Friday, this deepening surface low will continue to
propagate into central NE with colder air wrapping around the
back side of the surface cyclone into our CWA. 700mb temps will
begin to dip below 0C with wrap around moisture continuing over
the NE panhandle early Friday. While temperatures will be cooler
with afternoon highs dropping down into the 50s, the colder air
mass behind this system does not arrive until Friday night, so
ptype will likely remain as rainfall through Friday afternoon.
Additionally, gusty northwest winds with the strong mslp
gradient are likely.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Issued at 400 AM MDT Wed Apr 24 2024

Medium-range deterministic models and various ensembles continue
to display excellent agreement w/ regard to an energetic mid and
upper-level short wave over the Four Corners region early in the
forecast period, pivoting around the base of larger scale upper-
level troughing encompassing much of the western & central CONUS
late in the week. The GFS/ECM/GEM all suggest similar timing and
placement of this disturbance ejecting north and east across the
southern and central high plains on Saturday, giving way to very
strong/robust lee cyclogenesis over southeastern Colorado with a
sub-998 millibar surface cyclone tracking across central/eastern
Kansas from Saturday night through Sunday. Widespread stratiform
precipitation is likely to develop on the back side of this low,
as an impressive TROWAL & deformation axis evolves by mid-day on
Saturday. Latest ensemble cluster analysis indicates substantial
model agreement with QPF exceeding 0.75 inch across a large part
of the CWA from Saturday through early Sunday. Perhaps this will
come as no surprise, given widespread PWs of 0.5+ inch, near the
90th percentile of climatological norms for late April and early
May. Overall, this appears to be a fairly warm system w/ most of
the lower elevation areas likely to see rain. However, as 700-mb
temperatures fall to near -2 to -4 deg C, elevations above about
7500 feet could see notable accumulations of heavy and wet snow,
including the I-80 Summit between Laramie & Cheyenne. The Snowy/
Sierra Madre ranges could see over 12 inches for the event. Will
need to watch areas into the I-25 corridor in central Laramie Co
as the CAD signature along the front range could suggest current
models are too warm w/ their thermal profiles. Although it seems
to be an unlikely scenario at this time, a change over from rain
to snow cannot be entirely ruled out as far east as Cheyenne. No
headlines at this time given we are still looking at 5th period/
beyond. Temperatures should be seasonably cool due to the clouds
and precipitation, so have trended toward lower %iles of the NBM
spectrum through Sunday. Temperatures quickly rebound by Monday/
Tuesday w/ the return of upper-level ridging.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1128 AM MDT Wed Apr 24 2024

VFR conditions prevail through at least 06Z/Thursday. Convection is
beginning over the higher terrain and tracking east-northeast at
midday, a trend that will continue through the afternoon. Scattered
showers and a few thunderstorms appear most likely at KLAR and KCYS
during the mid-afternoon before reaching the Nebraska Panhandle
between 23Z-01Z. There could be brief MVFR with the strongest cells,
but otherwise VFR prevails. Return flow on the backside of surface
high pressure in the Plains keeps gusty southerly surface winds of
15-30kts across the Panhandle terminals through much of Wednesday
evening. These speeds decrease around 06Z/Thursday and low-level
moisture begins to creep north. A deck of IFR/MVFR low clouds and
fog reach KSNY around 08Z/Thursday and KAIA around 11Z. Have hinted
at the brief potential for lower clouds at KBFF, but KCDR appears to
avoid this deck. Clouds retreat toward the end of the period with
VFR conditions returning.

&&

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

&&

$$

SHORT TERM...MB
LONG TERM...CLH
AVIATION...Jones