659
FXUS65 KCYS 172032
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
232 PM MDT Wed Jul 17 2024

.KEY MESSAGES...

- Increasing moisture today will support strong to briefly
  severe thunderstorms through this evening. Additionally, heavy
  rainfall could lead to localized flash flooding.

- Severe potential late in the week followed by cooler weather
  for the weekend. Warm-up begins to start the week.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 240 PM MDT Wed Jul 17 2024

Stationary front lays along the east slopes of the Laramie Range
from Cheyenne to Douglas to Sheridan...then northwest into
western Montana this afternoon. Still seeing 60 Tds in the
Panhandle and 50 Tds on the southeast Wyoming plains. Radar
showing a good coverage of convection in this moist air to the
east of the front. Have a few reports of large hail with the
strongest storms. Looks like HRRR simulated radar is doing a
pretty good job on the development and track of storms this
afternoon.

RAP mesoanalysis showing SBCAPE still pretty high around 2500
J/KG in the central Panhandle with 2000 J/KG extending just west
of the Wyoming Nebraska state line currently. As the storms move
into the Panhandle...they tap into this instability and go
severe for a time before weakening. The central Panhandle still
has been untapped...so would expect this is where the strongest
storms are going to occur.

Looks like storms continue through 02Z before beginning to
dissipate. This lines up with the mesoanalysis forecast CAPE and
what the HRRR simulated radar is showing as well.

Evening shift will need to keep a close eye on TAFS overnight as
GFS continues to show 850mb humidity increasing across the
southern Panhandle in southerly low level flow. GFS showing
80-90 percent 850mb humidity from Sidney to Alliance and west to
Cheyenne at 12Z Thursday. May be dealing with low stratus and
fog around that time.

Diurnal showers and storms expected once again Thursday
afternoon/evening. Greatest concentration of storms looks to be
along the I-80 corridor from Sidney to Laramie Thursday.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Issued at 345 AM MDT Wed Jul 17 2024

Northwesterly flow will continue across the region for much of the
weekend into early next week. With a stout, 500mb high over the Four
Corners region, monsoonal moisture will continue to advect into the
region, increasing precipitation chances. Friday will have the best
chance for precipitation as an upper-level shortwave traverses
through the ridge and 500mb cyclonic vorticity advection increases.
A 700mb low attempts to develop with this upper-level shortwave with
a surface low trying to organize. The surface low will bring with it
a cold front and strengthened winds at the surface. The approaching
cold front will support lift across the region, leading to the
development of showers and thunderstorms. The NAM currently shows
around 1000-1100 J/kg of MLCAPE across the Panhandle for Friday
afternoon. Forecast soundings suggest an inverted-V profile with
strong mid-level lapse rates around 9 C/km. Relatively weak steering
flow supports slower moving storms and 1.0in PWAT values indicate
the potential for slow moving, heavy rain producing showers.
However, the dry low-levels will likely lessen some of the heavy
rain as a good portion will evaporate before hitting the surface.
Nonetheless, strong to severe thunderstorms cannot be ruled out and
the Storm Prediction Center currently highlights the Panhandle in a
Marginal Risk for Friday.

The active pattern will continue for the weekend as northwesterly,
upper-level flow remains over the region. The 500mb high begins to
retrograde to the west, which will decrease the monsoonal moisture
across the region, evident by PWATs decreasing towards the 0.5-0.8in
mark on Saturday. Northwesterly flow is favorable for precipitation
across the region, as several vorticity maxima will traverse through
the ridge and initiate afternoon showers and thunderstorms both days
this weekend. With lower moisture in the area, these showers are
expected to be more isolated in nature and likely not overly heavy
rain producers. Temperatures will be pleasant this weekend in the
mid-70s to mid-80s area-wide.

Predominantly northerly upper-level flow will develop by Monday as
the upper-level ridge amplifies to the west and a large fetch of
northerly flow develops across the region. With this fetch of
Canadian air, temperatures will be quite nice on Monday, with highs
in the mid-70s to mid-80s area-wide once again. Afternoon showers
and thunderstorm remain possible throughout the early portions of
the week, typical for this time of year. Long range models are in
agreement on increasing temperatures by mid-week, with highs
returning to the mid-80s and mid-90s area-wide by Wednesday.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1108 AM MDT Wed Jul 17 2024

Afternoon showers and thunderstorms are possible at all sites today.
Given the isolated nature of the convection, difficult to determine
exact locations that will be impacted, so VCTS and TEMPO groups are
in most TAFs, with a higher confidence prevailing group at KCYS.
Gusty outflow winds and brief MVFR conditions are the main hazards.
Convection decreases this evening, with lighter winds overnight. Low
clouds, and potential fog, is possible to form at KAIA and KSNY
overnight and into Thursday morning. Confidence is not high at this
time, with SCT MVFR clouds indicated in TAFs.

&&

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

&&

$$

SHORT TERM...GCC
LONG TERM...AM
AVIATION...WFOCYS