519
FXUS65 KRIW 290737
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
137 AM MDT Wed Jul 29 2026

.KEY MESSAGES...

- A bit cooler today with an increased coverage of showers and
  thunderstorms. Some thunderstorms could contain heavy rain.

- Chances of showers and storms drop through the rest of the
  week into the weekend, with a near zero chance Saturday into
  early next week.

- Very hot temperatures return for the weekend, with some all
  time monthly record high temperatures possible on Sunday,
  especially in central and southern Wyoming.

- A gusty wind and low humidity may bring critical fire weather
  from Sunday into early next week.

&&

.DISCUSSION...
Issued at 129 AM MDT Wed Jul 29 2026

Sometimes when forecasting the weather, you feel like you are
trapped in a time warp, with the same concerns day after day. For
much of may last string of shifts, the concern was the monsoon and
hot temperatures. And as I arrive back on shift tonight, the concern
once again is the monsoon and hot temperatures. However, one will be
of somewhat less concern than last week.

And that one thing will be the monsoon. We will have another surge
of monsoonal moisture moving northward across the area today with a
shortwave crossing the area. Radar shows some showers moving
toward the southern border, and these will move northward
through the day. Precipitable water values are rather high
today, over an inch in some locations. This would put portions
of the area over 150 percent of normal (average this time of
year is around 0.60 inches). However, it is less than the 200 to
250 percent of last week. Steering flow is not the strongest
though. A few parts of the area are under a marginal risk of
excessive rainfall, mainly far southern and eastern portions of
the area. So there is some risk of local flooding, but the risk
looks low. Most convection should end after sunset, although a
few showers may linger after midnight. As for temperatures, with
more cloud cover spreading over the area, temperatures should
be cooler than yesterday. We did lower temperatures a few
degrees, with many locations staying in the 80s for highs today,
mainly in the southern half of the County Warning Area.

Then, starting on Thursday, we will begin to move into what is known
as an inverse relationship, where one thing goes up and another goes
down. And in this case, temperatures will be the thing on the way
up, and the chance of convection as well as humidity will be on the
way down. On Thursday, precipitable water values drop around 20 to
30 percent across the area. They will still be somewhat above normal
but only about 120 percent of normal versus over 150 percent. There
will still be some convection around but less then Wednesday, with
most concentrated in and near the mountains. But even here, the
chance is generally 1 out of 3 or less. Temperatures will also begin
to rise again, to generally about 5 degrees above normal which would
put highs in the 90s across most locations below 5500 feet once
again. Precipitable water values continue to drop on Friday as the
core of the moisture continues to get pushed to the east. Any storms
on this day would likely be restricted to the eastern portions
of the County Warning Area. Temperatures should run similar to
Thursday, very warm but not unusual for the latter part of July.

The main concern then shifts to heat, possibly record breaking heat,
for the first weekend of August. Another heat dome will continue to
build northward, bringing 500 millibar heights as high as 5980
meters and 700 millibar temperatures as high as 23 degrees celsius.
This will lead to another very hot weekend. Saturday looks hot, with
at least a 1 in 2 chance of high temperatures of 100 or greater
across elevations under 5500 feet. However, the warmest day looks to
Sunday, when a tightening pressure gradient ahead of an advancing
cold front will bring a gusty, southwesterly downsloping breeze. At
this time, there is at least a 9 in 10 chance of high temperatures
over 100 in locations under 5500 feet, with around an 1 in 2 chance
of temperatures 105 or warmer below 4800 feet. Some locations have
already set new monthly record highs 3 times this year (March, May
and July); and this could occur again for August. For example, it
has never been 100 degrees at the Riverton airport in August, and it
is highly likely this will occur on Sunday afternoon. And, this will
raise another concern, possible critical fire weather. Humidity
will be very low on this day, with most areas East of the
Divide seeing humidity in the single digits. With the gusty
wind, we may need some fire weather highlights for Sunday
afternoon as the front approaches. In addition, this looks like
a dry frontal passage with any convection likely remaining over
Montana.

We may finally have some relief from the heat on Monday after the
frontal passage, especially East of the Divide. The 700 millibar
temperatures fall as much as 6 to 7 degrees celsius, which could
drop temperatures back into the 80s for most locations. The
front may be slow to move West of the Divide so there could be
one more very warm day there before cooler air moves in for
Tuesday. The concern for fire weather may continue here is as
well, with humidity remaining low and a gusty breeze continuing.
Ridging may then begin building northward again, bringing more
above normals temperatures for the latter half of next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1005 PM MDT Tue Jul 28 2026

VFR conditions are expected to prevail across all terminals
through the TAF period. Conditions will remain dry through the
rest of the night, as clouds increase from the south-southwest
with SCT-OVC FL100-150 skies over the southern half of the
forecast area by sunrise Wednesday. Showers will become
possible over southwestern portions of the forecast area after
15Z, but should not impact KBPI/KPNA and KRKS until after 18Z.
PROB30 groups remain in the forecast for all terminals starting
around 20Z for thunderstorms occurring across the area through
the afternoon. The main concerns will be frequent lightning,
gusty outflow winds of 35kt, and brief periods of moderate to
heavy rainfall. Have added additional PROB30 groups for KCOD and
KJAC for continuing showers through the evening until 06Z
Thursday. Otherwise, mountain tops may be obscured at times
in association with the showers and thunderstorms and the
overall increased cloud cover.

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.

&&

$$

DISCUSSION...Hattings
AVIATION...LaVoie/Dziewaltowski