227
FXUS65 KRIW 300358
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
958 PM MDT Wed Jul 29 2026

.KEY MESSAGES...

- Isolated showers persist into early Thursday morning across
  mainly the north-half of Wyoming. Isolated thunderstorms are
  possible until sunset Wednesday, with the primary hazard being
  gusty outflow wind of 35 to 50 mph and brief heavy rain.

- The chance of showers and storms steadily drops by Friday,
  leaving the weekend with a near-zero chance of any rain.

- 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 west to southwest wind and low humidity, in
  combination with the much above normal temperatures, could
  produce critical fire weather conditions Sunday.

&&

.UPDATE...
Issued at 1240 PM MDT Wed Jul 29 2026

Current satellite has good cloud cover over the area associated with
the last day of good monsoonal moisture. As a result, showers and
thunderstorms will be possible this afternoon and evening. Gusty
outflow winds up to about 40 mph is the main hazard, but a very
isolated stronger gust is possible. High temperatures today are also
less as a result of the clouds, generally sitting in the 80s for
most of the area. Smoke from the western US increases from the west
tonight, with Thursday being pretty smoky for the area as a plume
moves to the east.

&&

.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 my 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 as
well, with humidity remaining low and a gusty breeze continuing.
Ridging may then begin building northward again, bringing more
above normal temperatures for the latter half of next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1000 PM MDT Wed Jul 29 2026

Mostly cloudy conditions transition to partly cloudy to mostly clear
conditions overnight. Scattered rain showers and very isolated
thunderstorms linger late this evening to past midnight, but
prevailing conditions should remain VFR through the TAF period, with
occasional MVFR conditions (low visibilities and ceilings) under the
stronger showers/storms (such as around KJAC and perhaps
KCOD/KCPR/KLND/KRIW).  Showers will linger across northern WY from
30/06-12Z. Winds are still highly dependent on outflow boundaries
tonight. Some gusts around 30 knots may linger. Overnight winds
should become generally light across the area as outflows dissipate
though.

Thursday starts off with some ever lingering showers across west-
central WY. Weak instability with lingering mid-level moisture will
give rise to afternoon showers and thunderstorms. The coverage
should be less than Wednesday but still widespread across western and
central WY. Most TAF sites (except KWRL and KCPR) have a 15-30%
chance of being impacted from mid- to late-afternoon. Again, brief
heavy rain along with 30-40 kts outflow gusts are the primary
hazards through about 31/03Z. Isolated showers will linger through
midnight as skies begin to clear from west to east.

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...McDonald