306
FXUS65 KRIW 171908
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
108 PM MDT Mon Aug 17 2026

.KEY MESSAGES...

- Chances for showers and storms increase today, and peak on
  Tuesday. The best chance for showers and storms will be across
  northern Wyoming.

- Drier air returns starting Wednesday and lasts through Friday,
  with only isolated convection, mainly in and near the
  mountains.

- Above normal temperatures return by Wednesday and last through
  at least Saturday, with widespread highs in the 90s East of
  the Divide Thursday, Friday, and possibly through the
  weekend.

&&

.UPDATE...
Issued at 107 PM MDT Mon Aug 17 2026

Some light showers, and even a few rumbles of thunder, occurred over
western Wyoming this morning, with those continuing to push east
early this afternoon. Showers and thunderstorms will become more
widespread through the afternoon, mainly across western and northern
Wyoming, but nearly everywhere has at least an isolated (10-15%)
chance. Outflow gusts up to about 45 mph are possible with
convection today. Increasing moisture tonight keeps light showers
chances going.

&&

.DISCUSSION...
Issued at 1246 AM MDT Mon Aug 17 2026

The monsoon continues today and, like last night, the models did
another horrible job with the night time convection. There are
less than last night though with the air mass being drier. But,
what I wouldn`t give for a nice, strong cold front to push
through and vanquish the monsoon until next summer. But, we have
to play the hand we are dealt so here we go.

Moisture will increase across the area today, but not
dramatically. Precipitable water values today look to generally
run anywhere from 100 to 140 percent of normal, not excessively
high. The result will be a somewhat greater coverage of showers
and storms that yesterday. Most guidance is also fairly
consistent in keeping the most numerous storms across the
northern half of the area, with mainly dry conditions persisting
in southern Wyoming. Not saying there won`t be any there, but
the chance is only around 1 out of 10. The best chance looks to
be across the northern border where the chance is around 1 out
of 3. As for hazards, dew point depressions are over 40 degrees
so there could be some strong wind gusts. There could also be
some locally heavy rain but there is steering flow today and
that should help mitigate the flooding threat; it is not zero
but it is low. With somewhat higher thicknesses, temperatures
should be a couple of degrees warmer than yesterday, fairly
close to what is normal for mid August. Some convection lingers
into as well, but placement is uncertain given the poor model
performance with the monsoon.

Tuesday looks like the most active day for this round of the
monsoon. The highest precipitable water values are not the highest,
again topping out around 140 percent of normal. However, it is the
most widespread. As a result, just about all locations will have at
least a 1 in 5 chance of a shower or thunderstorm. Just about
everything else looks similar to today, the main threat would be
strong wind gusts, with a lesser chance of minor flooding with
decent steering flow mitigating that threat somewhat. A few showers
may linger into Tuesday night as well, but probably fewer than on
Monday night. Temperatures should also run fairly similar to Tuesday.

Drier air will then overspread most of the area for Wednesday and
Thursday. Precipitable water values fall back to below normal levels
for most of the area on Wednesday and even into Thursday as the
moisture gets suppressed to the south. The chance of convection on
these days is very small, less than 1 out 10 for all locations.
There is a slightly better chance of a shower or storm on Thursday,
but it would likely be restricted to the mountains. With the
associated ridge building northward, temperatures will be on the
rise as well. The forecasted temperatures may be a few degrees too
cool as well. Most of the lower elevations East of the Divide will
probably see high temperatures into the 90s possibly as early as
Wednesday and almost definitely from Thursday through at least
Saturday. Some of the warmer spots could be in the upper 90s Friday
or Saturday.

The chance of showers and storms also begins to rise on Friday as
monsoonal moisture moves back into the area. Most activity would be
confined to in and near the mountains on Friday though. Saturday may
be a day that needs to be watched. Some guidance shows precipitable
water values climbing to over 150 percent of normal, possibly
approaching 200 percent of normal. There is steering flow, so this
should mitigate the flooding threat somewhat. I would not be
surprised to some sort of Excessive Rainfall Outlook for this day
though. Above normal moisture also looks to persist into Sunday and
early next week, keeping the chance of showers and thunderstorms in
the forecast.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1138 AM MDT Mon Aug 17 2026

VFR conditions should prevail through the period. Showers and
thunderstorms are possible (15 to 40 percent) at most terminals
this afternoon, from about 19z until 03z. Gusty outflow winds to
45 mph are possible near any shower or storm, and brief MVFR
conditions are possible if a storm passes directly over.
Increasing moisture overnight may keep low shower chances over
the area, namely across western and northern Wyoming, so have
included some PROB30 groups to cover the chances for now, as
confidence is fairly low. Otherwise, winds follow typical
diurnal trends, with decreasing speeds this evening, and
increasing clouds overnight associated with the incoming
moisture.

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...Wittmann
DISCUSSION...Hattings
AVIATION...Hattings