897
FXUS65 KRIW 291911
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
111 PM MDT Sun Jun 29 2025

.KEY MESSAGES...

- Elevated fire weather continues across southern Wyoming this
  afternoon.

- Isolated to scattered thunderstorms this afternoon and early
  evening mainly in Johnson, Natrona, and Sweetwater Counties.
  Thunderstorms could become strong to severe with gusty winds
  around 60 mph and marginally severe hail, mainly in eastern
  Johnson and Natrona Counties.

- Mostly dry and warm Monday and Tuesday with shower and
  thunderstorm coverage increasing Wednesday and continuing
  through the end of the week.

&&

.UPDATE...
Issued at 110 PM MDT Sun Jun 29 2025

19Z surface observations show the frontal boundary roughly along the
Divide. East of the Divide, dewpoints in the 40s and 50s are common
with north to northeasterly winds. Whereas dewpoints west of the
Divide are commonly in the 20s and 30s with west to southwesterly
winds. Because of these low dewpoints west of the Divide and gusty
winds, relative humidity values in the teens are expected so
elevated to critical fire weather conditions continue this afternoon
across southern Wyoming.

The forecast largely remains on track for afternoon convection
across portions of the area. The only change made this forecast
package is increasing shower and thunderstorm chances across Johnson
and Natrona Counties. Morning cloud cover across Johnson and Natrona
Counties has dissipated, allowing for the atmosphere to begin
destabilizing. As of 19Z, the SPC mesoanalysis indicates around 1000
J/kg of MUCAPE, around 40kts of effective bulk shear, and lifted
indices around -3 across Johnson and Natrona Counties. This amount
of instability as well as these counties being in the right exit
region of an approaching upper-level jet supports isolated to
scattered showers and thunderstorms (30-40% chance) across the
aforementioned areas this afternoon. A Marginal Risk (level 1 of 5)
of severe weather continues across portions of far eastern Johnson
and Natrona Counties. The threat of stronger thunderstorm a bit
farther to the west in these counties cannot be ruled out,
however, as storms trek off the Bighorns and into the eastern
parts of these counties, they will be able to tap into greater
instability. The primary threats with any severe storms will be
gusty winds around 60mph and 1 inch hail.

Shower and thunderstorm potential (up to 40% chance) across southern
Sweetwater county exists as convection initiates over the Uinta
Mountains and treks east. Because of the dry low-levels with
dewpoint depressions between 40-50F, gusty winds will be possible
with any showers or thunderstorms. All convection chances diminish
across Sweetwater, Natrona, and Johnson Counties just after sunset
this evening.

&&

.DISCUSSION...
Issued at 303 AM MDT Sun Jun 29 2025

As of 2 am, the last of the showers have ended and things are quiet
right now. Like yesterday, we have two concerns; one has
improved somewhat and one is about the same. The one concern that
has improved at bit is fire weather. We will not have any Red
Flag Warnings today. Flow has turned to the north or northeast
east of the Divide due to a weak frontal boundary and that has
brought somewhat cooler temperatures and higher humidity. As a
result, critical fire weather is not expected here, although
local elevated fire weather is still possible. Elevated to
critical fire weather will continue in southern Wyoming, but
fuels are not critical here.

The concern that is the same is the chance of convection. We do have
an area of Marginal risk largely along and east of Interstate
25. The best chance will be in eastern portions of the area,
close to the frontal boundary and in the area with somewhat
higher dewpoints. Instability parameters are decent, especially
in the Marginal risk area with over 1000 J/kg of CAPE and
lifted indices as low as minus 3. Lapse rates are decent as
well, with some shear (close to 40 kt). There will also be an
approaching jet streak to enhance lift somewhat. For now, have
kept storm chances largely from the Bighorn Range eastward. The
main risk from any thunderstorm would be gusty wind and/or
small hail, although there could be a few storms that have
marginally severe hail. This looks like an earlier show, with
most storms over shortly after sunset. Coverage does not look
very big though, at most 1 out of 3 at any particular location.
Otherwise, expect a seasonable day across the area.

Most areas should have a nice summer day to end the month of June on
Monday with lots of sunshine, warm but not hot temperatures and,
thankfully, lighter wind. I can`t completely rule out a shower or
thunderstorm, but the chance is less than 1 out of 10 so we kept the
forecast dry for now. The chance of thunderstorms increases again on
Tuesday as moisture begins to move in from the south, but this would
initially be of the mid level variety. Storms on this day would
likely stay near the mountains with only around a 1 in 6 chance
at any location. Moisture return has slowed somewhat in the
latest model runs for Wednesday, so we did trim PoPs somewhat.
A shortwave and much deeper moisture moving in now makes
Thursday a more likely time for the arrival of more widespread
showers and storms. This could especially be the case on
Thursday and Friday, as some guidance shows some precipitable
water values possibly climbing as high as 200 percent of normal
(for example, around 1.2 inches at our office, normal is closer
to 0.60 inches). A lot will depend of the progress of an upper
level low now off the California coast and move eventually open
up and move northeastward. These are notoriously fickle and hard
to time and track, especially this far out. However, we have
the potential for an active period of showers and storms later
next week. As for temperatures, look for above normal temps through
around Wednesday, with cooler temperatures later next week as
cloud cover increases from the increased moisture and possible
showers/thunderstorms.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1126 AM MDT Sun Jun 29 2025

VFR conditions will prevail at terminals through the period.
Convection will be much more limited today, with the best chances at
KCPR. Gusty outflow winds are possible with this activity. There is
a lower chance of showers or storms near KRKS as convection builds
off of the Uinta Mountains. While direct terminal impacts are
unlikely, there will be potential for outflow winds approaching KRKS
after 23Z. Convection will end after sunset leaving mostly clear
skies through Monday morning. Several terminals will continue to see
a breeze overnight as a cold front pushes south.

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.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...Myers