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327
FXUS65 KCYS 251007
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
407 AM MDT Sat May 25 2024

.KEY MESSAGES...

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- Mountain snow showers will accumulate in the Snowy and Sierra
  Madre tonight and early Sunday, with total possible
  accumulations of 2 to 6 inches above elevations of 9500 feet.
  Scattered rain showers and an isolated strong to potentially
  severe thunderstorm can`t be ruled out east of the Laramie
  Range and Nebraska Panhandle today.

- Strong winds are possible in the wind prone and gap areas of
  southeast Wyoming early Sunday morning through the mid-
  afternoon. Please see the latest High Wind Watch Statement
  for further details.

- A much warmer, milder week ahead as an upper-level ridge
  overtakes the region.
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&&

.SHORT TERM /THROUGH MONDAY/...
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Issued at 330 AM MDT Sat May 25 2024

Overnight cloud cover and isolated rain showers are slowly
moving through the area as of 9Z this morning. Temperatures
remain elevated thanks in part to the cloud cover, with several
areas seeing the upper 40s to near 60 degrees while looking at
surface obs east of the Laramie Range. A shortwave trough over
the Four Corners region of the desert Southwest currently has a
piece of upper level energy shearing off ahead of it, which is
creating enough of a disturbance aloft over our cwa. This will
continue to occur through the morning hours, and isolated rain
showers in the lower terrain will be the result. Temperatures in
the higher terrain of the Snowy and Sierra Madre Mountains will
hover in the 30s early this morning through the rest of today,
so we will continue to see the threat for snow showers.
Hopefully this doesn`t dampen the holiday weekend spirit for
those that may be recreating in elevations above 9500 feet for
those mountain zones. Total snowfall accumulations of 2 to 6
inches are anticipated between this evening and midday Sunday
for elevations above 9500 feet.

The aforementioned upper level shortwave trough will propagate
upstream, with its axis being positioned nearly over our cwa by
0Z this evening. As this occurs; diffluent flow aloft, weak
instability, sufficient wind shear, and PWATs should allow for
scattered rain showers and thunderstorms to develop. The best
atmospheric dynamics will reside east of the Laramie Range, so
we can expect an isolated strong thunderstorm to develop, should
model guidance for CAMs of 0Z/6Z prove true. The forcing
mechanism is favored to be a semi-stationary frontal boundary
associated with the shortwave trough aloft. PWATs of 0.5 inch to
0.75 inches and dew point readings in the middle 40s are
modeled. The atmospheric lapse rates will take their time to
increase to values above 6.5 degrees C/km. Surface CAPE looks
to be on the weak side, with values barely eclipsing 500 J/kg
between 18Z and 0Z today. EBWD for shear values of 45-60 knots
will quickly scoot to the east along with the convective rain
showers. Our highest confidence resides in the NE Panhandle for
stronger, more organized thunderstorm propagation. SPC Day 1 has
a sliver of a Marginal Risk in the western Nebraska Panhandle,
so that would be the area for stronger wind gusts and hail up to
the size of an inch in diameter. The best dynamics reside
further to the east, so our window of convection will be short-
lived as it evolves and increases in coverage for the Central
Plains. Lingering rain showers are modeled to persist through
the early morning hours of Sunday before winding down.

Once the shortwave trough ejects to the east by Sunday morning,
there will be a period of subsidence behind it. This will be our
window for the stronger winds aloft at 700mb to mix down in the
gap and wind prone corridors of southeast WY. While the signal
is not as strong as 24 hours ago, have still gone with a High
Wind Watch due to the potential of wind gusts reaching 55-60mph
for the usual areas of southeast WY through the afternoon.
Another shortwave disturbance aloft will quickly move from west
to east across the northern Plains on Sunday afternoon. The
majority of the energy will remain to our north, but there will
be enough daytime heating and atmospheric instability for a few
isolated rain showers and thunderstorms in our northern forecast
zones through the afternoon hours.

The holiday forecast looks to be mostly dry and warm. Upper
level ridging will begin to propagate toward our cwa on Monday.
Northwest flow aloft is anticipated on the backside of an upper
level shortwave progged to impact the Great Lakes. Daytime highs
in the 60s and 70s are forecast, with mild temperatures even
making it into the higher terrain of the mountain zones. Winds
will be slightly elevated, in the 15-25mph range at times during
the afternoon. A few cumulus clouds may bring a low chance of
sprinkles towards the evening hours for the I-80 corridor.
Overall, a tranquil weather pattern for the holiday should
prevail.
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&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
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Issued at 330 AM MDT Sat May 25 2024

Upper ridge lays across western Wyoming Monday evening with
clear skies and dry weather. This ridge moves east through
Tuesday afternoon with showers developing over our western
mountains.

Diurnal showers and storms Wednesday into Thursday developing
during the afternoon and early evening and dying off overnight.
Next cold front expected Thursday as a low tracks across
Montana. Timing of frontal passage looks to be THursday
evening...which should limit convection and thunderstorm
severity. Guidance trending towards cooler temperatures for
Friday as GFS 700mb temperatures fall to the single digits above
0C. After temperatures in the 80s Thursday...Friday`s highs look
to be in the 60s and 70s.
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&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1003 PM MDT Fri May 24 2024

Westerly flow aloft will become southwest overnight, while a
weather disturbance and increased moisture bring scattered
showers and thunderstorms Saturday afternoon and evening.

Wyoming TAFS...Ceilings will lower from 11000 to 15000 feet
overnight, to 4000 to 8000 feet on Saturday. Showers will be in
the vicinity of the terminals from 15Z to 00Z. Winds will gust
to 25 knots from 15Z to 00Z.

Nebraska TAFS...Mostly clear skies will become broken clouds
around 12000 feet after sunrise, then lower to 6000 to
10000 feet Saturday evening. Showers will be in the vicinity of
the terminals after 00Z, except at Chadron.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
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WY...High Wind Watch from late tonight through Sunday afternoon for
     WYZ106-110-116-117.
NE...None.
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&&

$$

SHORT TERM...BW
LONG TERM...GCC
AVIATION...RUBIN