623
FXUS65 KRIW 302143
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
343 PM MDT Thu Jul 30 2026

.KEY MESSAGES...

- Remaining warm with scattered showers and thunderstorms today
  and Friday, although coverage decreases each day. Storms could
  have gusty outflows 30 to 50 mph.

- Very hot this weekend, especially on Sunday with many
  locations over 100 degrees. All-time record temperatures for
  August may be set at some locations.

- Increasing wind on Sunday and into Monday along with low
  humidity brings elevated to critical fire weather each
  afternoon.

&&

.UPDATE...
Issued at 1236 PM MDT Thu Jul 30 2026

No notable changes to the forecast with this update. Considered
possible heat or fire weather highlights for this weekend, but want
to give it another day to see how models trend. Wind, for example,
has lessened a bit Sunday for some locations. In any case,
widespread elevated to critical fire weather conditions are expected
Sunday and into next week. For today, have update precipitation
chances following the latest hi-res model guidance. Most convection
should occur near and around the mountains, quickly ending between
about 5pm and sunset.

&&

.DISCUSSION...
Issued at 132 AM MDT Thu Jul 30 2026

I`ve often talked about how meteorology is a mathematical science.
And one of the most often cited examples are long waves, which
are essentially sine waves. The weather often works that way,
with a pattern reaching a peak and then dropping to a trough,
and the upcoming several days are no exception. The chances of
convection reached the peak of the wave, and will generally only
drop over the next several days. Meanwhile, temperatures will
climb out of the trough today and reach a peak over the weekend,
before falling again early next week. And for fire weather
concerns, they start in the trough of the wave and then rise
toward the end of the weekend and even early next week.

Monsoonal moisture remains over the area right now, and we continue
to have some light to moderate showers ongoing overnight,
and even a stray flash of lightning, especially in northern Wyoming.
This will be the peak of the monsoon and probably the wettest period
for the next several days. Most guidance shows drier air moving in
from the west today, dropping precipitable water values by an
average of 20 percent across the area. This is still above
climatological means, but only around 125 percent of normal versus
the 150 to 175 percent yesterday. Meaning there will be still be
some showers and thunderstorms around. However, most locations will
have a less than 1 in 4 chance. The best chance also shifts East of
the Divide where deeper moisture will remain, especially across
Johnson County where there is as much as a 1 in 2 chance. With more
sunshine, temperatures will be a bit warmer as well, generally about
1 to 4 degrees warmer, quite warm but nothing unusual for late July.
Convection should also be an earlier show, with most done by sunset
and almost all done by midnight.

The progression on the sine wave continues Friday. Precipitable
water drops another 10 to 20 percent on this day, bringing even less
coverage of convection. The main areas of concern on these days
would be the northern mountains as well as Johnson and Natrona
County where there will be a bit more moisture. However, the chance
is even smaller this day, nothing more than 1 out of 4. And, it will
be another very warm day to close out July, with temperatures
similar, to a couple of degrees warmer, than on Thursday. Some
of the warmer spots, like the eastern Bighorn Basin, will get
close to or may reach 100 degrees on this afternoon.

Over the weekend, we reach the peak of the sine wave for
temperatures. A heat dome will build back north over the area. The
500 millibar heights climb as high as 5970 meters with 700 millibar
temperatures as high as 22 to 23 degrees celsius. This will lead to
a very hot first weekend of August with some possible record
breaking temperatures, including a few all time high
temperatures for August, especially on Sunday. As for specifics,
most of the Bighorn Basin and the Wind River Basin below 5500
feet have at least a 1 in 2 chance of highs temperatures at or
over 100 on Saturday. On Sunday, this increases to include most
locations below 6000 feet, and also includes much of Natrona
and Johnson Counties as well as the lower elevations of
Sweetwater County. Elevations below 5300 feet have at least a 1
in 2 chance of high temperatures over 105 on Sunday. This would
be the day some all time record high temperatures for August
could fall. As for highlights, we will hold off for now. We will
likely need at least heat advisories for the weekend. Leaning
against extreme heat warnings though given how low the humidity
will be. This, also with the fact nights are getting longer and
there will be more time for cooling after sunset. And there is
one more concern for Sunday, as this variable climbs up from the
trough of the sine wave, fire weather. A cold front will be
approaching the area from the north, and bringing a gusty
southwest breeze ahead of it. Humidity will be very low,
possibly under 5 percent in some areas in the afternoon. Wind
gusts are borderline in some areas though. All areas will have
at least elevated fire weather though with critical fire weather
likely for some. There is still time to decide on highlights
though. And, with the very dry air over the area, the chance of
any showers this weekend is almost zero.

The cold front will move southward into the area Sunday night and
into Monday. This should bring some relief from the possible record
heat, with some areas seeing as much as a 20 degree drop in high
temperatures compared to Sunday. It will take a bit more time to
reach West of the Divide, so it looks like another hot day on
Monday, but a few degrees cooler. The somewhat cooler temperatures
should last into Tuesday. After that, the ridge may begin to build
back northward, bringing more above normal temperatures. And,
through the middle of next week, precipitable waters look to
remain below normal, so the chance of any convection remains
slim to none. In addition, it still looks breezy so at least
elevated fire weather looks to continue into early next week as
the fire weather sine wave reaches it`s peak.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 341 PM MDT Thu Jul 30 2026

Isolated showers and thunderstorms linger for a few hours this
evening. Conditions should remain VFR through the TAF period,
although may briefly drop to MVFR in the strongest storms. Moisture
values are decreasing, so wet thunderstorms will be transitioning to
dry thunderstorms into tomorrow. PROB30 groups are in place at
terminals with the best chance for storms Friday afternoon, which
includes KBPI, KCOD, KLND, KPNA, and KJAC.

As showers and storms transition to more virga and dry storms gusty
downdrafts and outflows will increase in likelihood and magnitude.
Outside of downdrafts and outflows wind will generally stay 10 knots
and lest through the TAF period. Fog is not expected at KJAC again
Friday morning thanks to the decrease in moisture.

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

&&

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

&&

$$

UPDATE...Wittmann
DISCUSSION...Hattings
AVIATION...Dziewaltowski/Rowe