849
FXUS65 KCYS 101107
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
507 AM MDT Mon Jun 10 2024

.KEY MESSAGES...

- Dense fog with visibilities as low as 1/8 mile will impact the
  southern Laramie Range and adjacent foothills (including the
  I-80 Summit) through early this morning. A Dense Fog Advisory
  remains in effect until 9 AM MDT.

- Numerous showers and thunderstorms are expected along and east
  of the Laramie Range after 18z today. Strong to severe storms
  are possible with the potential for hail, strong winds, and
  perhaps an isolated tornado with any quasi-discrete storms.

- Mainly dry and rather warm conditions will prevail for
  Wednesday and Thursday, with increasing chances for showers
  and thunderstorms for Friday and Saturday, along with slightly
  cooler temperatures. Sunday will see another warming trend
  with decreasing precipitation chances.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 420 AM MDT Mon Jun 10 2024

An area of extremely dense fog persists early this AM across the
southern Laramie Range and adjacent foothills w/ numerous webcam
and highway observations indicating visibilities only around 250
to 500 feet at several sensors along I-80 between CYS and LAR. A
Dense Fog Advisory was issued earlier this AM, which will remain
in effect until 9 AM MDT as T/Td spreads remain unusually low w/
dew points in the middle/upper 50s. Otherwise, light rain shower
activity or perhaps just a few sprinkles can be expected for our
western zones through mid-morning.

The focus for the afternoon & evening hours shifts to the threat
for strong to severe thunderstorms w/ much of the high plains in
a SLGT risk from SPC. South/southeast low-level flow is expected
to maintain robust boundary layer moisture profiles with surface
dew points in the 50s (perhaps locally 60+ F) in the pre-frontal
air mass along and east of the Laramie Range. CI should occur as
early as 18z over the central/southern Laramie Range w/ enhanced
low-level convergence associated w/the Chugwater Cyclone. Should
see convection rapidly increase in coverage to the north along &
east of I-25 between 18z-21z along the dryline & cold front. The
overall evolution is expected to feature rapid upscale growth to
a forward-propagating MCS due to merging cold pools as well as a
favorable wind profile with strong low-level directional shear &
deep unidirectional flow aloft. Would prefer to see stronger mid
level flow for a more substantial severe weather threat, but 30+
knots of 0-6 km shear should support organized clusters. Threats
should include both severe hail/wind given steep the robust low-
level moisture and steep lapse rates supporting MLCAPEs of 1500-
2000 J/kg. CAMs are in excellent agreement showing a quite large
linear complex impacting areas along/east of a line from Lusk to
the Colorado state line and beyond, mainly between 18z & 00z. We
may see several additional rounds of convection across far south
east Wyoming into the southern Nebraska Panhandle w/ the passage
of a train of H5 vorticity maximums through the evening and into
the overnight hours. The intensity of this activity remains very
uncertain at this time, and will most likely depend on how those
earlier storms evolve.

Aside from the convective threat, it will be quite warm today w/
most of the area seeing daytime highs in the 80s, near 90 across
the Platte River Valley including EAN/TOR/BFF. Expect to see 700
millibar temperatures peak near 12-15 deg C this afternoon, then
dropping 8-10 deg C during the evening and overnight w/ the cold
frontal passage. Substantial WAA will occur early Tuesday, ahead
of a building broad/flat upper-level ridge. That said, we should
not see much of a change in daytime highs between Mon and Tue.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 420 AM MDT Mon Jun 10 2024

Tuesday night/Wednesday...Even warmer as the flow aloft turns
westerly, and with 700 mb temperatures near 17 Celsius, maximum
temperatures will be in the mid 80s to mid 90s. Warmer temperatures
aloft will increase the convective inhibition, and with less low and
mid level moisture around, it looks like it will be dry.

Thursday...Similar synoptic pattern as on Wednesday, though low
level winds will become east, and with an increase in low and mid
level moisture, we expect to see isolated late day showers and
thunderstorms. Should be a bit cooler than Wednesday with upslope
winds and more cloud cover.

Friday...Ridging aloft, over our forecast area, begins to break down
as the next shortwave trough aloft approaches, and with surface lee
trough convergence and an increase in atmospheric moisture, we
expect scattered late day showers and thunderstorms. Slightly cooler
with an increase in cloud cover and precipitation coverage.

Saturday...Looks like we will likely see a shortwave trough aloft
passage in the afternoon, helping to spark scattered late day
showers and thunderstorms. Maximum temperatures look similar to
those of Friday.

Sunday...The flow aloft backs to southwest, inducing the development
of another surface lee trough, and with less atmospheric moisture
and 700 mb temperatures near 18 Celsius, maximum temperatures will
increase into the 80s at most locations. With less moisture aloft,
only isolated to widely scattered late day showers and thunderstorms
are expected, mainly along and east of Interstate 25.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1051 PM MDT Sun Jun 9 2024

KCYS will trend towards IFR/LIFR between 9Z to 13Z Monday
morning. After that, expect VFR until we see an increase in VCSH
and VCTS during the Monday afternoon timeframe. All other
terminals are anticipated to remain at VFR through the forecast
period. Wind gusts on Monday will ramp up 20-35 knots at times.
Please see individual TAFs for further information.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Dense Fog Advisory until 9 AM MDT this morning for WYZ116-117.
NE...None.

&&

$$

SHORT TERM...CLH
LONG TERM...RUBIN
AVIATION...BW