561
FXUS65 KCYS 290958
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
358 AM MDT Wed May 29 2024

.KEY MESSAGES...

- Strong to severe thunderstorms mainly along and east of I-25
  into the Nebraska Panhandle Wednesday afternoon with large
  hail and strong winds.

- Active weather pattern will continue through the weekend with
  daily chances for showers and thunderstorms.

- Summertime temperatures will be on the horizon next week.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 352 AM MDT Wed May 29 2024

Latest GOES Nighttime Microphysics RGB shows localized low clouds
and fog developing over the South Laramie Range early this morning
with southerly low-level moisture advection ongoing ahead of the
upper level trough approaching our area. Decided to issue a Dense
Fog Advisory through 7 AM MDT this morning as localized dense fog
could impact the morning commute along the I-80 corridor between
Cheyenne and Laramie.

Main forecast concern Wednesday remains the potential for strong to
severe thunderstorms during the afternoon and evening east of I-25
into the NE panhandle. Steep mid-level lapse rates along with
continuing southerly moisture advection east of the lee trough are
supporting of MLCAPE values around 1500 J/kg overlapped with 0-6 km
shear values 30-35 kt per the latest RAP. SPC continues to show a
Slight Risk for severe thunderstorms with their latest Day 1
Outlook. Biggest uncertainties right now remain the placement of the
dryline in eastern WY as that will likely impact how quickly these
storms turn severe Wednesday afternoon once they move off the higher
terrain of the Laramie Range. NAMNest shows a sharp moisture
boundary with nearly a 20F degree dew point spread, while the HRRR
is much more diffuse with this gradient as well as being positioned
closer to the NE/WY border. Latest HREF guidance shows a 25th-75th
percentile spread in SBCAPE values over 500 J/kg around the NE/WY
border. Will need to continue to monitor forecast and observational
trends throughout the day as these details will better refine
location and timing of the strongest storms today. As of right now,
CI in the higher terrain looks to occur around midday with storms
strengthening later in the afternoon as they cross the moisture
boundary with large hail and strong winds possible through the early
evening. Hi-res guidance suggests upscale growth across the NE
panhandle that will likely transition the main hazard threat to
strong winds and heavy rainfall before storms move east of the CWA
border around 8 PM. Farther west, additional showers and storms may
develop closer to the cold front moving across Carbon Co with
forecast soundings showing dry low-level and LCLs extend well above
the freezing level supportive of dry microburst gusty outflow winds
even with lighter showers/returns on radar.

After high temperatures climb into the upper-70s to mid-80s
Wednesday, Thursday will be cooler, but still near average for late
May. Post-frontal upslope flow Thursday with moisture pooling up
against the Front Range will support widely scattered thunderstorms
in the afternoon despite the better environmental setup well to the
east across the central plains. Daytime PoPs will be limited to the
vicinity of the South Laramie Range during the afternoon ~20%, but
looks to increase and expand into much of the NE panhandle as a
secondary cold front associated with the same upper level trough now
lifting into the north-central CONUS brings increasing moisture
convergence into the CWA.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 352 AM MDT Wed May 29 2024

The pattern of mild to warm temperatures and near daily chances for
thunderstorms will continue through the weekend, but a warming and
drying trend is expected for the first part of next week.

On Friday, a broad trough will be located to our north, and another
piece of 500-mb vorticity will rotate through the area. As a result,
Friday looks like the coolest day of the forecast period across the
area with a sharp north to south gradient in 700-mb temperatures
across the area from about 2 to 3C near US20 to 6 to 8C along I-80.
Meanwhile, a surface high sliding down the east side of the Rockies
Thursday night will push the dryline back west against the Laramie
range and return low-level southerly to southeasterly flow over much
of the High Plains. With some subtle added lift from the passing
vorticity maximum, look for another round of thunderstorms to
develop. While storms may just be isolated to widely scattered, they
may have a slightly greater potential to become strong to severe on
the Wyoming side. Veering wind profiles and more low-level moisture
will be present. Forecast soundings show higher instability and
sufficient wind shear over southeast Wyoming. However, a potent
inversion around 700-mb will be stronger further north and east,
which will limit the severe potential in the northern/eastern
portions of the forecast area.

The upper level trough quickly skirts to the east into the weekend,
with a more quasi-zonal flow pattern setting up after this. The axis
of a subtle ridge moves through the area on Saturday with another
weak vort-max on its leading edge. Expect temperatures to climb back
above normal as 700-mb temperatures return to around +10C. The
dryline position is expected to be a little further east on Saturday
compared to Friday, but its too early to say exactly where this will
set up. While the vort-max aloft should help kick off scattered
showers and thunderstorms, this will mainly be a virga and wind
threat west of the dryline. However, once these storms reach a more
moisture rich environment with southerly low-level flow, strong to
severe convection could be on the table once again. There is still
uncertainty regarding whether this will occur in our forecast area
or to the east. Sunday looks similar, just a touch warmer and a
touch drier as the ensemble mean dryline position edges further to
the east. Thunderstorm activity should be a little bit more limited
with drier air working into the area, but we`ll have to watch the
exact dryline position as areas to the east of this will once again
have a greater strong to severe storm potential. Right now, it looks
like this area will be to our north/east, but it`s very close. Low
level westerly downslope winds should support enhanced warming over
the high plains, with highs expected in the mid 80s. Some of our
hotter spots (Torrington/Scottsbluff, etc.) will also have a chance
at the first 90F day of the year. Probabilities for a high over 90F
are around 60% for Scottsbluff, 50% for Chadron, and 40% for Sidney
at this time.

While 700-mb temperatures may drop briefly behind a weak shortwave
Sunday night into early Monday, they should rebound quickly.
Ensemble mean values over KCYS climb tack to +15C by 00z Monday.
Look at another warm to hot day. Temperatures aloft, around 500-mb,
will warm faster than the low levels as the more potent upper-level
ridge tries to work in across the area. The warmth aloft will limit
instability and thunderstorm potential on Monday, and possibly into
Tuesday as well. Almost all ensemble members keep most of the area
dry on Monday, and most continue this through Tuesday and Wednesday.
There is good consensus around another upper level shortwave pushing
across the northern Rockies in the Tuesday/Wednesday timeframe, but
the chance for precipitation with this is fairly low. However, this
introduces some uncertainty in temperatures, with a more southerly
track leading to a stronger shot of cool air, and a more northerly
track leading to the hot temperatures continuing.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1136 PM MDT Tue May 28 2024

Thunderstorm activity has ended across the area, but a modest
low-level jet is kicking up over the plains that will bring
occasionally gusty southerly winds. This flow in a fairly moist
environment may allow for some low clouds and/or fog to form
near KCYS or KSNY. The probability is too low to add as a
prevailing group, but there is about a 30% probability of IFR at
KCYS and 15% at KSNY between now and 12-13z.

Another round of thunderstorms is expected Wednesday afternoon.
this is expected to start in WY around 20z and move into the NE
panhandle around 21-22z. Most of this activity should be
concluded by around 02z. The highest confidence in t-storm
impacts is at KSNY, but have VCTS in for most other terminals.
Storms will have the potential to produce gusty winds, brief
MVFR/IFR with heavy rainfall, and hail.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Dense Fog Advisory until 7 AM MDT this morning for WYZ116.
NE...None.

&&

$$

SHORT TERM...MB
LONG TERM...MN
AVIATION...MN