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465
FXUS65 KCYS 051002
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
402 AM MDT Sun May 5 2024

.KEY MESSAGES...

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- Warmer today with strong southerly winds and the chance (~30%)
  for showers and thunderstorms to develop near the NE/WY
  border late this afternoon.

- Accumulating snowfall expected starting late tonight in the
  Sierra Madre and Snowy Ranges, specifically at elevations over
  8000 feet.

- Strong winds return to southeast Wyoming and western Nebraska
  Monday with a prolonged period of elevated winds continuing
  through midweek. Wind gusts of 70-80 mph are possible for
  wind-prone locations.
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&&

.SHORT TERM /THROUGH TUESDAY/...
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Issued at 330 AM MDT Sun May 5 2024

Latest GOES WV imagery shows a large scale upper level low
churning across the NV/OR border and will continue to move
towards the Intermountain-West today. Strong southerly flow
ahead of this system will see moisture advection into the NE
panhandle with CAMs showing dew points in the mid-40s. With a
dryline setting up near the WY/NE border, will need to monitor
this afternoon for enhanced areas of convergence along with
increasing lift approaching from the west that could be
supportive of shower development late this afternoon. Could
even see a few thunderstorms develop with latest RAP soundings
analyzing a few hundred J/kg of MLCAPE present and strong enough
wind shear to support organized cells. Otherwise, expecting a
warm day across southeast WY and western Ne with afternoon highs
in the upper-60s to 70s.

As far as headlines go, went ahead with upgrading to a High Wind
Warning across much of western Carbon Co today with strong 50+ kt
southerly 700mb flow moving in aloft ahead of the large scale upper
level low. Steep low-level lapse rates will support vertical
momentum transport during the afternoon and early evening leading to
occasional gusts around 60 mph near Baggs, Rawlins, and Muddy Gap.
Stronger flow will continue to move east this evening with a frontal
passage bringing ample lift across areas west of the Laramie Range
early Monday morning. Higher elevations above 8000 feet could see
accumulating snowfall, mainly in the Sierra Madre and Snowy Ranges.
Went ahead with issuing a Winter Weather Advisory for the Sierra
Madres beginning late tonight as 6 to 12 inches of snowfall will be
possible by Monday afternoon. Farther east across the NE panhandle
Sunday night, a tight east-west mslp gradient develops ahead of
the lee troughing with the approaching system. This will help
setup a strong nocturnal LLJ over the NE panhandle with the
strongest winds likely across the Pine Ridge, possibly
downsloping into the Chadron area. Therefore, decided to add a
new High Wind Watch for Dawes Co for Sunday night.

Jumping ahead to Monday, strong winds still appear likely across
much of southeast WY, possibly expanding in the NE panhandle during
the afternoon in the wake of the strong negatively tilted upper
level low with a near 980mb surface low developing in western
SD. NAEFS continues to show climatological maximum 700mb winds
across much of southeast WY and continues to trend upward. This
is also highlighted by in-house random forest guidance with
probabilities of high winds climbing to over 90% with the 18z
and 00z model runs. Additionally, this ML guidance is now
suggesting 70-80 mph wind gusts will be possible for wind-prone
locations along Interstate 80 near Arlington and Interstate 25
near Bordeaux. Therefore, decided to issue High Wind Watches for
much of southeast WY beginning Monday morning. Strongest winds
look to occur midday Monday with the strongest flow aloft and
steep low-level lapse rates supporting vertical momentum
transport and this is also highlighted with the highest NBM
probabilities of exceeding 55 mph gusts on Monday vs Tuesday.
However, wind-prone locations will likely begin early Monday
morning with strong flow running underneath a mountain top
inversion per forecast soundings. Wind headlines very well may
need to be expanded to cover portions of the NE panhandle Monday
afternoon, but will continue to assess the latest model trends.
In addition to winds on Monday, precipitation will be most
likely across east-central WY and the northern NE panhandle,
closer to the surface low, however isolated showers could move
across areas farther south.

Chances for precipitation return to south-central WY, mainly
southern Carbon Co, Tuesday with lift associated with the secondary
vort max approaching from the northwest. Additionally, strong winds
look to continue for wind-prone locations, but uncertainty remains
with how widespread high winds will be across adjacent foothills and
plains throughout Tuesday. Upcoming forecast updates can look to
refine the High Wind Watch timing for various zones across the CWA,
specifically with the endtime.
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&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
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Issued at 330 AM MDT Sun May 5 2024

Tuesday night/Wednesday...Another shortwave trough aloft pivots
around the primary upper trough producing scattered showers for
our forecast area, with the least coverage across far southeast
Wyoming and the southern Nebraska panhandle due to downslope
winds. Continued windy due to low level pressure gradients and
winds, with 700 mb temperatures near -2 Celsius yielding high
temperatures from the upper 40s to upper 50s.

Thursday...Continued cool with 700 mb temperatures near -4 Celsius
yielding maximum temperatures from the upper 40s to upper 50s. Low
and mid level moisture looks plentiful enough for scattered to
numerous showers and isolated thunderstorms.

Friday...Although dynamics look rather limited, it appears there
will be enough low and mid level moisture for scattered showers and
isolated thunderstorms. 700 mb temperatures rise slightly from
Thursday, thus temperatures will be slightly warmer.

Saturday...The upper trough continues to spin across the Great Basin
states sending another shortwave trough aloft over our counties and
will combine with remaining low and mid level moisture to produce
isolated to widely scattered late day showers and thunderstorms.
Temperatures will show a warming trend as 700 mb temperatures rise
to near 4 Celsius, yielding maximum temperatures mostly in the
60s.
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&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1135 PM MDT Sat May 4 2024

A potent weather disturbance will move into eastern Utah Sunday
evening, sending an associated cold front into southeast Wyoming
at that time.

Wyoming TAFS...At Rawlins, cloudiness will increase with
ceilings lowering from 10000 feet around midday Sunday to
5000 feet Sunday evening, with periods of showers developing
after 02Z, reducing visibilities to 4 to 6 miles. Winds will
gust to 45 knots after 15Z.

At Laramie and Cheyenne, cloudiness will increase with ceilings
lowering to 10000 to 15000 feet after 16Z. Winds will gust to
35 knots after 16Z.

Nebraska TAFS...Cloudiness will increase with ceilings lowering
to 5000 to 10000 feet Sunday afternoon and evening. Low
level wind shear occurs until 19Z at Scottsbluff, and after
03Z at Alliance, Scottsbluff and Sidney. Winds will gust to
50 knots.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
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WY...High Wind Watch from Monday morning through Tuesday evening
     for WYZ101-103>105-107>109-115-117>119.
     High Wind Warning from 9 AM this morning to 9 PM MDT this
     evening for WYZ104-109-111.
     High Wind Watch from late tonight through Wednesday morning
     for WYZ106-110-116.
     Winter Weather Advisory from 11 PM this evening to 6 PM MDT
     Monday for WYZ112.
NE...High Wind Watch from this evening through late tonight for
     NEZ002.
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&&

$$

SHORT TERM...MB
LONG TERM...RUBIN
AVIATION...RUBIN