Area Forecast Discussion
Issued by NWS Cheyenne, WY

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116
FXUS65 KCYS 181210
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
610 AM MDT Tue Jun 18 2024

.KEY MESSAGES...

- Strong winds gusting 50 to 60 MPH in the wind prone area of SE
  Wyoming will gradually wane through the morning hours today.

- Much cooler temperatures are expected across the area Tuesday
  and Wednesday followed by another warm up beginning on
  Thursday.

- An active pattern late this week into the weekend with strong
  to severe thunderstorms possible on Thursday.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 330 AM MDT Tue Jun 18 2024

The weather across our area is flashing back to spring (or forward
to fall) as an unseasonably potent upper level shortwave swings
through the area. The cold front passed through the area last
evening, and can be currently seen on radar and satellite imagery
moving across northeastern Colorado. Behind the front, strong winds
have spilled across the area, advecting in much cooler temperatures.
A few gusts of 58+ MPH have been observed so far, with the strongest
reaching 63 MPH at KCYS airport. The forcing for strong winds should
be peaking right around the current hour, with gradual easing
expected through 12z. Expect breezy to windy conditions to continue
through the day today, but the risk for high winds should be
concluded after about 15z when the current High Wind Warnings
expire.

This system is bringing unseasonably cool air across the
area today. 700-mb temperatures below 0C are working into our
western and northern counties under the axis of the trough.
These values are largely below the 10th percentile of
climatology for 700- mb temperatures, and approaching the 1st
percentile in parts of western Wyoming. As a result, expect high
temperatures to run about 10-15F below average this afternoon.
While much drier air is working into the lower atmosphere behind
the cold front, models show a narrow area of decent moisture
remaining in the 400 to 600-mb layer mostly confined to
locations south and east of a Laramie to Wheatland line and west
of the WY/NE border. After a brief clearing this morning,
expect more mid-level cloud cover to return to that area this
afternoon. There may be just enough moisture to kick off a few
showers in the vicinity of the Laramie range, but any
appreciable precipitation reaching the ground is unlikely.

A surface high sliding down the eastern side of the Rockies behind
this trough will settle into the central/northern Plains tonight.
Once the center moves east of our area, the pressure gradient will
start to reverse and turn winds easterly to southeasterly over the
High Plains of our forecast area. This will usher in solid moisture
advection and push the dryline well to the west, as far as Carbon
county by late Wednesday. At least for the morning, the increased
moisture will also result in increasing clouds and possibly some fog
along the I-80 corridor, which will prevent temperatures from really
tanking. However, Wednesday will still get off to a chilly start for
most of the area with lows expected in the 40s. Model guidance shows
the strong moisture advection holding cloud cover in place well into
the day Wednesday, which will then hold down high temperatures
during the afternoon. While areas west of the Laramie range should
be a bit warmer than Tuesday, areas to the east should see the
coolest temps of the week on Wednesday. Some areas, especially the I-
80 corridor from Cheyenne to Sidney, could see highs as much as 15-
20F below average for this time of year.

Meanwhile, the upper level trough will stall in its eastward
progression and start to lift northward on Wednesday as the
extremely potent ridge over the eastern CONUS nudges westward and re-
strengthens. The northward expansion of the ridge will result in
southerly 700-mb flow pushing back into our area. The result will be
a region of decent isentropic lift developing in a pocket of
elevated instability over the southeastern portion of the forecast
area. Aside from any activity that goes up over the Laramie
range/dryline, this isentropic lift looks like the primary mechanism
driving shower and storm activity for Wednesday. Severe weather
looks fairly unlikely at this time due to the elevated nature of
convection, but model scenarios showing greater elevated instability
(such as the RAP) would perhaps be enough to produce isolated large
hail Wednesday evening.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Issued at 330 AM MDT Tue Jun 18 2024


Headed into Thursday, an upper level trough across the western CONUS
will continue to dig towards the Great Basin with southerly moisture
advection farther east across much of the Front Range. This looks to
set the stage for the potential of strong to severe thunderstorms
east of the Laramie Range into the NE panhandle with strong winds
and large hail. 700mb WAA advection Thursday morning across the NE
panhandle will place a notable capping inversion with low-level
clouds that could help keep storms discrete by limiting convection
in the afternoon, as long as it is not strong enough to inhibit
convection altogether. The placement of the upper level jet from
central WY through the north-central CONUS should support more
widespread lift across the area, especially east-central WY, while
also enhancing wind profiles across the CWA. With the position of
the lee trough near the CO/WY border, southeasterly surface flow
will aid in additional hodograph curvature that would be favorable
for right-moving supercells under 45 kt of bulk shear and ~1500 J/kg
of MLCAPE. Additionally, latest NAEFS guidance places climatological
99th percentile PW values into portions of the NE panhandle
approaching 1.25", especially north of the warm front. While mean
flow will support quicker storm motions, Bunkers right-mover motion
of 10-15 kt could lead to excessive heavy rainfall in spots assuming
the cap can be broken across the warm sector allowing for the
development of deep moist convection.

While latest CSU ML Severe Weather probabilities highlight Thursday
with the greatest potential, this guidance also suggests an active
period continuing through the extended. Friday could pose as another
active severe weather day, however the upper level trough is
expected to breakdown leading to weaker flow aloft while the larger
scale upper level low remains well off to the north near the
Canadian border. As shear profiles look to decrease, overall
parameter space still suggests strong storms are possible with the
shortwave passage.

Headed into the weekend, temperatures are expected to climb above
normal with 500mb height rises across the Intermountain-West.
Afternoon temperatures will climb into the mid-90s east of the
Laramie Range (upper-80s west), especially Sunday. However, it does
not appear that the upper level ridge will amplify enough to keep
precipitation out of the forecast as isolated to scattered storms
will be possible Saturday (20-25%) and Sunday (15-20%). Based on
forecast soundings, Sunday`s storms will likely be high-based with
dry low-levels likely leading to gusty outflow winds.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 604 AM MDT Tue Jun 18 2024

Strong west to northwest winds through this morning after the
cold front passage last night. Gusts mostly around 25-35 kt are
expected after KCYS and KCDR gusted around 50 kt overnight.
Latest satellite imagery shows lingering low clouds near KCDR
and KAIA that may continue MVFR ceilings over the next hour or
two, otherwise VFR conditions are expected with mountain wave
clouds through the day along with light showers possible near
KCYS and KLAR late this afternoon. Winds will begin to turn
northeasterly this evening as the surface high pressure slides
into the central US.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until 9 AM MDT this morning for WYZ106-110-
     116-117.
NE...None.

&&

$$

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