263
FXUS65 KCYS 171710
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
Issued by National Weather Service Riverton WY
1110 AM 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 THURSDAY/...
Issued at 345 AM MDT Wed Jul 17 2024

Latest GOES WV imagery shows with an upper level ridge across
the western CONUS with a large scale upper low across the Great
Lakes region. More locally, scattered clouds have begun to
develop just north of the analyzed surface low in northeast CO
with isolated early morning showers. Latest observations show
generally weak easterly winds across the NE panhandle with upper
50s to low 60s dew points working into the CWA. However,
westerly winds across the South Laramie Range foothills could
support addition shower development early this morning with
additional convergence just east of Cheyenne.

The active summertime convective pattern will continue once
again today with showers and storms developing along the higher
terrain late this morning. As a shortwave rounds the top of the
ridge, better upper level lift will support more numerous
coverage of storms today, with a few strong to severe storms.
Rather marginal mid-level flow will keep bulk shear values
around 30 kt leading to more multicellular storm mode that could
briefly become severe with strong winds and hail. However,
rather moist profiles with PW values near 0.9" will be
supportive of heavy rain producing storms, especially along the
South Laramie Range Foothills across the I-80 corridor through
Kimball where CAMs have been most consistent with storm
coverage. Latest 3-hr FFG for the southern NE panhandle ranges
from 1.5-2". Storms will continue into the early evening before
slowly dissipating and moving southeast out of the CWA.

Headed into Thursday, the ridge axis over the western CONUS
slightly shifts eastward keeping the best moisture along the
eastern CWA border and extending into central NE. As another
weak shortwave passes overtop of the ridge, afternoon storms
will be possible (20-30%). Coverage will be much less than
today, but a few stronger storms could still develop near areas
of enhanced convergence from remnant outflow boundaries from
previous night`s storms in an area of 1500-2000 J/kg MLCAPE per
the latest RAP with HREF 25th percentile values still around
1000 J/kg. These will be in addition to storms developing over
the higher terrain in southeast WY with daytime heating.

&&

.LONG TERM /THURSDAY NIGHT 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...MB
LONG TERM...AM
AVIATION...TW