208
FXUS65 KRIW 162011
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
211 PM MDT Tue Jul 16 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms, mainly over the west-
  central mountains and adjacent lower elevations, continue
  until sunset Tuesday and produce brief heavy rain and gusty
  outflow wind of 35 to 45 mph.

- A building ridge of high pressure over the Great Basin will
  keep temperatures at or slightly above seasonal averages
  through the weekend. Daily diurnally-driven convection
  continues.

&&

.DISCUSSION...
Issued at 211 PM MDT Tue Jul 16 2024

Scattered showers and thunderstorms have ignited across west-central
and southwest Wyoming early Tuesday afternoon, the result of surface
dew points in the mid 40s to around 50 and a plume of mid-level
moisture draped across mainly the southern half of the forecast
area. While gusty outflow wind around 40 mph remains a primary
hazard, with precipitable water values hovering around 0.75 inches
brief heavy rain and small hail are also likely with the strongest
storms. Initial convective development has been over and downstream
of the Wind River Range and southern Absaroka Mountains in
northwest flow aloft. These areas and the high deserts of southwest
Wyoming see the best chance (30-40 percent) of convection through
early evening. Water vapor imagery shows drier air aloft from
Yellowstone, across the Bighorn Basin, to the southern Bighorn
Range. Convection will be isolated at best over the higher
terrain within this swath of drier air. Any lingering convection
fades between 9 and 11 PM leaving a clearing sky. A surface
high pushing south through the Northern Plains has already
shifted winds to the north in Johnson County and the northern
Bighorn Basin Tuesday afternoon. Winds will continue to shift to
the north and northeast through the evening, reaching areas
west of the Continental Divide. The highest speeds of 15 to 25
mph will be found in northern Johnson County Tuesday afternoon,
with 10 to 15 mph speeds elsewhere. Western wildfire smoke
drifts into the state riding western winds aloft.

Other than smoky conditions, Wednesday dawns under a mostly clear
sky. Convection begins to develop over the central and northern
mountains late Wednesday morning. The bulk of the convection
Wednesday afternoon remains over the northern two-thirds of the
forecast area. A building ridge of high pressure over Utah and
Arizona begins to amplify and nudges into southwest Wyoming where
convection will be isolated. Mean layer winds track convection
to the east-southeast 10 to 15 mph Wednesday afternoon. The
better precipitable water values of 0.80 inches are located
along and east of the Continental Divide. The potential for
stronger storms is across eastern portions of Natrona and
Johnson Counties, where morning easterly low-level flow boosts
surface dew points and bulk shear becomes more favorable late in
the day. Rising heights and warming mid-level temperatures lead
to above normal daytime highs Wednesday. Other than convective
outflows, surface winds remain generally less than 10 mph.
Convection persists a bit longer Wednesday evening before
winding down.

The ridge remains in place over Utah/Arizona and amplifies, with
the axis extending into the Northern Rockies Thursday and
Friday. The ridge retrogrades and takes hold over Nevada for
the weekend. This westward shift places Wyoming within northerly
flow aloft. A weak shortwave topping the ridge Thursday slides
south and clips the forecast area Friday. This leads to an
uptick in precipitation chances from Thursday to Friday. The
overall trend through the weekend is for precipitation chances
to decrease slightly as the main southerly surge of moisture
shifts to our west. Nonetheless, lingering mid-level moisture
within the ridge remains sufficient for diurnal convection.
Temperatures remain at or slightly warmer than seasonal average
highs, which typically range from 82 to 92 degrees.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1130 AM MDT Tue Jul 16 2024

An axis of upper-level high pressure remains just to the west
of Wyoming today, keeping warm, yet somewhat moist and unstable
air over the area. Mostly clear skies during the morning are
giving way to convective development by noon, with thunderstorms
starting over the eastern slopes of the Wind Rivers and Wyoming
range. Convection will focus today over west-central WY into the
Wind River Basin and southern Absarokas, and extend down to
Sweetwater County. Not as certain at KJAC but have included
vicinity showers in case some storms move around that airspace.
Winds are generally light, though thunderstorm outflows could
exceed 30 kts. As such, VFR conditions generally expected, with
brief MVFR is storms directly impact terminals. Showers are
expected to linger in the early evening hours, especially at
KPNA and KLND around the Wind Rivers. Winds will decrease
overnight, though skies should remain partly cloudy for some
areas. A couple of forecast models are indicating an area of
light showers early Wednesday morning over northwest WY,
possibly bringing some rain to KCOD around sunrise.

The high pressure axis shifts slightly eastward on Wednesday
which should bring showers and thunderstorms to areas along and
east of the divide. SW WY should remain mostly dry, though can`t
rule out an isolated storm or two. WNW winds will continue over
SW WY, while east of the divide will see NE to E winds Wed
afternoon, with stronger ENE winds pushing southwestward into
KRKS Wed evening.

Finally, smoke concentrations are increasing over the area due
to regional fires, but not yet enough to reduce visibilities at
the TAF sites. Will continue to monitor and adjust forecasts as
necessary.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on smoke, icing, and turbulence
forecasts.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...CNJ
AVIATION...McDonald