568
FXUS65 KRIW 051828
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1228 PM MDT Sat Jul 5 2025

.KEY MESSAGES...

- Showers and thunderstorms this afternoon and evening, with a
  chance for severe storms, especially in the Bighorn Basin, the
  Bighorn Mountains, and Johnson County. Hail and strong winds
  are the main hazards.

- Drier weather returns Sunday, with warmer conditions and
  increasing chances for elevated fire weather from Monday to
  Wednesday.

&&

.UPDATE...
Issued at 1226 PM MDT Sat Jul 5 2025

Current radar has some showers and weak storms over far northwest
Wyoming. A few storms are beginning to develop further
east/southeast as instability increases with the afternoon sun.

Latest hi-res model trends keep a similar set-up to the previous
morning discussion (see below discussion). The HRRR has some 50 to
60 mph outflows in the Bighorn Basin and east over Johnson County
between 2pm and 9pm. RAP mesoanalysis also keeps CAPE and shear
values indicative of 1" or greater hail for any stronger storms.

As the convective activity winds down late evening, a cold front
pushes southwards into the Bighorn Basin and Johnson/Natrona
Counties. This may bring northerly gusts around 30 mph, with some
models indicating gusts 40 to 45 mph. These winds should decrease
within a few hours, after the front passes.

&&

.DISCUSSION...
Issued at 133 AM MDT Sat Jul 5 2025

Showers continue to end across the CWA early this morning and are
expected to dissipate no later than sunrise. Clouds will scatter out
through this time with only high clouds over far western portions.
This would leave mostly clear skies for the rest of the area, which
could lead to the possibility for fog (especially lower lying areas
along the rivers and creeks). However, any fog that would formed
would be short-lived due to the approaching system.

A shortwave trough will move across the northern half of the CWA
today, providing sufficient lift and upper level divergence for
showers and thunderstorms. This activity will mainly occur east of
the Divide, but will extend over the Wind River Mountains and the
northern half of the Upper Green River Basin. Low level moisture
will remain in place this morning, with dewpoints in the 40s to
middle 50s for most areas today. Low level lapse rates of 7 to 9
C/km will be prevalent as well, along with lifted indices of minus 4
to minus 6. Mixed layer CAPE of 500 to 1500 J/kg and bulk shear of
30 to 40 kt will all combine for another day for strong to
severe thunderstorms. The Bighorn Basin will have the best
chance for these storms moves through the area between 19Z and
00Z. This line will quickly push eastward over the Bighorns and
northern Johnson County by 22Z and exit the CWA after 01Z. Wind
gusts of 60 to 70 mph is expected again with this activity
(similar to what occurred Friday), but large hail (1 inch or
higher) cannot be ruled out. Additional showers and
thunderstorms will develop from the Wind River Basin to the
Upper Green River Basin (as previously mentioned) as the tail
end of the shortwave moves over the area. These storms are not
expected to be as severe, but wind gusts up to 60 mph could
still occur along with small hail. No showers or thunderstorms
are expected over Sweetwater County or southern Lincoln County,
due to a lack of instability. Convection will exit the area by
sunset with only isolated showers remaining possible through
06Z, mainly over Natrona County. A weak cold front from the
shortwave will drop southward over areas east of the Divide
during this time, quickly reaching the Divide and into the Wind
River Basin by 09Z. This will only result in the obvious wind
shift and slightly dropping temperatures a few degrees Sunday
morning.

The passage of this shortwave will mark the end of this latest
active weather pattern, as drier air moves into the region. Flat
ridging will be in place Sunday, with the front washing out through
the day and keeping daytime temperatures similar to today. The focus
will shift to fire weather, as an upper level ridge builds over the
region from a strengthening high center over the Four Corners.
Winds look to remain light through Wednesday, however 90 degree
temperatures will return by Tuesday for areas east of the
Divide. Locations on the east side of the Bighorn Basin, such as
Greybull and Worland, could possibly reach 100 degrees by
Wednesday.

The next chance for precipitation looks to occur Thursday,as a
remnant low moves over the northern side of the Four Corners high.
This would lower temperatures a bit an d cause the high to retreat
further south. There is a chance this low comes in phase with trough
that will pass over southern Canada. This could lead to more
widespread precipitation chances, but will have to see how to
forecast evolves over the coming days.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1056 AM MDT Sat Jul 5 2025

Showers and thunderstorms are already beginning to build over
far northwest WY, and will expand across northern and central WY
this afternoon. Best chances still look to be KCOD/KWRL, so have
added tempo groups for those sites, as confidence is higher
there. Showers will build around KJAC/KLND/KRIW/KCPR later this
afternoon, but chances at these sites are lower. Shower activity
will be moving east quickly this evening, so most precip should
be done by 00Z. Skies remain mostly clear overnight with lighter
winds.

&&

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

&&

$$

UPDATE...Wittmann
DISCUSSION...LaVoie
AVIATION...Straub