000
FXUS65 KCYS 251746
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1146 AM MDT Thu Apr 25 2024

.KEY MESSAGES...

- Scattered to numerous showers and thunderstorms are expected
  across southeast Wyoming and the western Nebraska Panhandle
  today and tonight. A few storms may be strong to locally
  severe, especially east of I-25. The primary hazards will be
  large hail and damaging wind gusts, although an isolated
  tornado or two will be possible. The potential will exist for
  cold-core funnel clouds as well.

- An extremely active weather pattern persists through the
  weekend with widespread stratiform rain w/ embedded thunder
  likely for most areas from Friday through Sunday.

- Accumulating snow is expected at elevations above 8000 feet,
  with the potential for 12+ inches in the Snowy and Sierra
  Madre Ranges from Friday through Sunday. The I-80 Summit
  between Laramie and Cheyenne will be very close to the main
  transition zone between rain and snow. Travel impacts remain
  possible, mainly from Friday night through Saturday.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 402 AM MDT Thu Apr 25 2024

A highly complex & extremely active mid & upper level pattern is
expected to continue to evolve across the western and central US
over the next 24 to 48 hours, giving way to chances for multiple
hazards including severe convective storms, significant snowfall
accumulations above 7500-8000 feet elevation, as well as chances
for locally heavy rainfall for the lower elevations. The overall
upper-level pattern will remain characterized by expansive long-
wave troughing encompassing much of the western & central CONUS,
with two distinct pieces of energy pivoting across the 4 corners
region before ejecting northeastward across the southern/central
high plains. Overall, models are in excellent agreement with the
overall evolution of the pattern. However, there are a number of
notable complexities with potential outcomes highly sensitive to
the precise timing of disturbances and resulting interactions of
boundaries which eventually take place. Nonetheless, it is quite
likely that the majority of the CWA will see some sort of impact
from these systems.

The first notable short-wave disturbance will lift to the north-
east across the 4 corners this afternoon, contributing to strong
lee cyclogenesis over eastern Colorado with a sub-990-mb surface
low expected to develop by 21z. The resulting southeasterly low-
level flow should provide a sufficient fetch of moisture to help
maintain appreciable surface dew points in the lower 50s, mainly
across the western Nebraska Panhandle. The western extent of the
deeper moisture remains highly uncertain with a sharp cut-off in
the spread of HREF members along/southwest of a BFF-IBM line for
progged dew points indicating uncertainty regarding the eastward
progression of the dry line. The 10th %ile of the HREF ensembles
are in the mid 40s to the east of this line, but quickly drop to
the mid/upper 20s to the west. This may play a key role in storm
severity today w/ better MLCAPEs (1000-1500 J/kg) possibly being
focused more across our far eastern zones. Even so, there should
be good instability today as thermal profiles rapidly cool later
this afternoon as the mid-level cold pool approaches. Scattered/
numerous showers and thunderstorms should develop across western
areas by early afternoon, with a slightly delayed initiation for
the high plains closer to 21-00z given substantial inhibition w/
the initially warm, capped air mass aloft. High-res models would
suggest initiation occurring in the vicinity of the dryline near
the WY/NE border by 00z w/ numerous clusters developing through-
out the evening. Vertical shear will be more than sufficient for
organized, potentially rotating storms capable of large hail and
perhaps locally damaging wind gusts. The last several iterations
of the HRRR and NAMNest have suggested a few narrow UH tracks as
well, suggesting potential for a low-topped supercell or two and
the risk for a weak tornado or two. As thermal profiles cool, we
would not be surprised to see at least several cold-core funnels
develop in the early evening w/ numerous storm mergers likely to
be taking place during this time.

Dry-slotting on the back side of the rapidly maturing cyclone is
expected to limit convective coverage across western zones after
03z, with the primary focus shifting toward stratiform rain and/
or embedded convection over the western NE Panhandle through mid
day Friday. Brief short-wave dirty ridging should develop as the
first low departs. A weak, slow-moving disturbance will uncercut
this ridge over central Wyoming, resulting in a burst of notable
700-mb warm air advection across Carbon & Albany counties during
the afternoon and evening hours on Friday. As such, rain & high-
elevation snow should quickly re-develop as the next low deepens
and pivots northeast across the 4 corners from Friday night thru
Saturday. Yet again, lee cyclogenesis will result in substantial
wrap-around moisture in the TROWAL/deformation axis on the back-
side of a rapidly deepening cyclone over central KS. As a result
we should see deep/moist northeasterly low-level upslope flow as
well as favorable dynamics supporting widespread stratiform rain
and embedded convection over much of the CWA. PWATs of 0.5+ inch
will be near the 90th percentile of climatology, so would expect
some pretty impressive rainfall totals through Sunday. Localized
areas could see over 1.5 inches of rain over the next few days!

High-elevation snow will also be likely, as 700-hpa temperatures
fall to -2 to -4 deg C. A foot or more of accumulation will be a
good possibility for both the Snowy and Sierra Madre ranges with
this second wave, with more significant uncertainty below around
8000 feet elevation and potential impacts over the Interstate 80
Summit between Laramie and Cheyenne. Models continue to trend on
the cooler side for Friday Night & Saturday, validating previous
concerns of cold air damming along the front range favoring low-
tier percentiles of the model spectrum for temperatures. Decided
to populate snow levels with the 25th %ile of the NBM, which may
bring a brief rain/snow mix into Cheyenne but would restrict the
accumulations to the higher terrain to the west. The Summit will
be right near the transitional bubble, with the worst-case being
around 6-8 inches of heavy and wet snow. Ensemble means are near
2-3 inches using a 10:1 ratio. Will continue to hold off for now
on any headlines given lingering uncertainty. Daytime highs will
likely struggle to reach the middle 40s for many areas Saturday.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
Issued at 402 AM MDT Thu Apr 25 2024

We will have an active weather pattern continue for the extended
forecast package. Near average temperatures for the weekend are
forecast, and gradually increasing through the week to be
approximately 5-15 degrees above climatological averages to round out
the last few days of April and into early May. At this time of
inspection, snow showers across the higher elevations will wind down
on Sunday and Monday, with rain showers and thunderstorms
anticipated daily for the entire cwa through the end of next week.

Saturday night into early Sunday, we will have a slow-moving upper
level low slowly propagating to the east-northeast, ultimately
ejecting out toward the Upper Midwest and Great Lakes region by late
Sunday. As this negatively tilted upper level system with an
attendant mid-latitude surface low propagates away from our region,
we will have wrap-around moisture coupled with CAA. 700mb
temperatures ranging from 0C to -5C along with scattered rain
showers in the lower elevations, and accumulation snowfall in the
higher terrain will slowly dissipate from Saturday night to late
Sunday afternoon. Isolated thundershowers are also possible, but sub-
severe. Areas of highest confidence for additional rainfall
accumulations in the lower terrain will be along and east of I-25
toward western Nebraska. Much needed rainfall is anticipated for
these areas.

The water faucet will be turned off for only a brief period as we
transition to a quiet weather setup for approximately 24 hours. A
shortwave trough will arrive from the Pacific Northwest by Monday
evening, bringing a renewed chance for showers and isolated
thunderstorms. This weak disturbance is anticipated to eject
downstream toward the Northern Plains and Great Lakes region
throughout the day on Tuesday, limiting our chances of rainfall from
that.

Tuesday is advertised by model guidance to transition to a more
quasi-zonal flow regime aloft, with weak shortwave disturbances
embedded within the H5 and H7 flow aloft. Diurnal convection is
signaled to be present as mid-levels will adiabatically cool from a
passing shortwave trough to our north, increasing the low-level and
mid-level atmosphere lapse rates. Not expecting severe weather from
this weak threshold of a disturbance, but lingering showers and
thunderstorms could persist overnight into early Wednesday morning.

Wednesday and Thursday will see an amplification of an upper level
ridge, with it originating across the Mexican Plateau, and advecting
warmer are northward across the Intermountain West. Weak shortwave
disturbances are forecast to propagate off a much larger longwave
trough from the Pacific Northwest on Wednesday, bringing another
opportunity for showers and thunderstorms during the peak heating
hours of the day. Thursday will have the aforementioned upper level
ridge axis positioned over the Central Plains. The potent longwave
trough will elongate and intensify across the western CONUS. This
trough is modeled to become negatively tilted by Thursday of next
week, with diffluent flow being coupled with moisture and
instability across the Central Rockies. We will need to pay
attention to this trend as it may be our next opportunity for
stronger, organized convection across the cwa to round out the
week.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1139 AM MDT Thu Apr 25 2024

A prolonged window of scattered showers and thunderstorms is
expected to arrive during the forecast period. VFR conditions
expected before thunderstorms develop and outside of when
showers and thunderstorms move directly over terminals. Expect
localized fluctuations to MVFR/IFR from VIS/CIGs reductions
during the afternoon, evening, and overnight period. Wind gusts
will pick up for most terminals in the 20-35 knot range, with
isolated higher gusts when -SHRA/-TSRA are present at
terminals. Initial rounds of thunderstorms are expected to taper
off in southeast Wyoming between 00Z and 03Z at KLAR and KRWL,
but lasting until near 08Z-10Z at KCYS. The Nebraska panhandle
TAF terminals will likely see showers and thunderstorms through
overnight hours with a brief break possible between 03Z and 05Z.
Please see individual TAFs for further information.

&&

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

&&

$$

SHORT TERM...CLH
LONG TERM...BW
AVIATION...LEG