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000
FXUS65 KCYS 241729
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
Issued by National Weather Service Riverton WY
1129 AM MDT Wed Apr 24 2024

.KEY MESSAGES...

- Above average temperatures with isolated to scattered showers
  and thunderstorms this afternoon and evening. Patchy dense
  fog expected near the I-80 corridor from Cheyenne, WY to
  Sidney, NE 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 across
  the area due to persistent rain showers.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 400 AM MDT Wed Apr 24 2024

An active weather forecast pattern is taking shape across the
area in the upcoming period of the short term forecast package.
Our region will be under the influence of an amplifying upper
level ridge today. This will lead to a quick surge in daytime
highs compared to yesterday. We will see partly cloudy skies
once the light rain showers push further to the east later this
morning. Expect high temperatures approximately 7-12 degrees
warmer than yesterday, topping out in the upper 60s to upper 70s
for most of the lower elevations. As the ridge amplifies, and we
have subsequent WAA occurring in the lower levels of the
atmosphere today, shortwave disturbances embedded within the
upper flow aloft will propagate across the region. Convective
temperatures will be achieved for several areas, and light rain
showers and isolated thunderstorms will be the result by the
late afternoon hours. The rain shower activity will move from
west to east. At this time of inspection, our convection today
looks disorganized.

The main weather event to discuss for the short term forecast
will begin to take shape early Thursday morning. An upper level
longwave trough will propagate over the Four Corners of the
desert southwest by Thursday morning. Further downstream, a
deep fetch of moisture will advect northward from the Gulf of
Mexico and southern CONUS early Thursday toward the Central
Plains, courtesy of a 30-40 knot nighttime LLJ. The weather
setup for our cwa east of the Laramie Range will include a deep
swath of upslope flow. Dense fog and low-level cloud ceilings
are expected to ensue, which may evolve into a short-fused
Dense Fog headline for Wednesday night and Thursday morning.
PWATs are expected to surge to 90-99th percentile per
climatological sounding data on Thursday/Thursday night
(between 0.5 and 0.8 inches). This will happen due to a surface
low over Colorado that is anticipated to undergo rapid lee-
side cyclogenesis. The rapid deepening of the surface low over
northeast CO will approach near-bombogenesis thresholds: 15-18mb
decrease in just over 12 hours on Thursday. The aforementioned
upper level longwave trough will eject toward the Intermountain
West/Central Plains during this same time period, becoming
negatively tilted. Model guidance is beginning to come into
solid agreement for convective initiation to occur in several
locations, including our cwa. Steep mid-level lapse rates,
SBCAPE and MUCAPE values in excess of 1000-1500 J/kg for areas
east of the Laramie Range are progged, with even some members
showing readings as high as 2000-2500 J/kg by 0Z Friday along
the WY/NE state line. This weather system will shift from a
mesoscale to synoptic-induced convection Thursday evening and
overnight into Friday morning. Before it does so, the potential
for a couple of discrete supercells exists. However, there are a
couple of discrepancies to note. Model soundings for southeast
Wyoming show DCAPE values of 750-1000 J/kg, with slightly less
readings in the NE Panhandle. 0-6km effective shear parameters
are not the strongest, but there will be a small and narrow
corridor east of I-25 near the WY/NE state line that could
be just the right amount thanks in part to lobes of vorticity
maximums propagating through the region. After taking a look at
sounding analogs, there is a slight concern for a tornado or two
to occur. The primary reason for this is because of isolated
pockets of storm-relative helicity in excess of 200 m2/s2, and
very low cloud bases near or slightly below 1 km being possible
east of I-25. Overall, damaging straight line winds in excess of
60-70mph, hail between one and two inches in diameter, and an
isolated tornado can not be ruled out. This is further evidenced
by the Storm Prediction Center expanding the Marginal Risk for
isolated severe weather for the majority of our cwa east of
I-25 during the Day 2 overnight forecast update.

Thursday night, the aforementioned upper level trough is
anticipated to become cut-off or "stacked". The attendant 700mb
low and surface low will be semi-stationary in approximately in
the same position for several hours. This will create a narrow
window of elevated to potentially strong wind gusts along the
Interstate 80 corridor east of the Laramie Range. At this time
of inspection, it does not appear we will need any high wind
headlines. Fortunately, temperatures will be much warmer in
comparison to a previous weather system earlier this month that
resulted in widespread wind gusts near or above 60mph for
several hours. We will see several rounds of scattered to
numerous rain showers and thunderstorms from Thursday night into
Friday behind a cold front passage. The net result will be
cool, cloudy and rainy conditions through Friday afternoon.
Areas of highest focus for moderate to heavier rainfall totals
will be across the Nebraska Panhandle. This will assist with the
spring green-up across the area as several locations are
anticipated to see a range of 0.1 to 0.5+ inches of rainfall
from Thursday afternoon to Friday afternoon. The surface low
will slowly propagate to the east by Friday evening, further
away from our cwa. However, the break in between precipitation
will be short-lived as the next weather disturbance will be on
its way by this weekend. Stay tuned for updates on that in the
long-term discussion.

&&

.LONG TERM /THURSDAY 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...
LONG TERM...
AVIATION...WFORIW