623
FXUS65 KRIW 021920
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
120 PM MDT Tue Sep 2 2025

.KEY MESSAGES...

- Another very warm day today (4-10 degrees above normal) with
  isolated afternoon showers and thunderstorms in the Absarokas,
  Wind Rivers, and Bighorns, with a slight chance of showers
  moving into central WY as well.

- A cool front pushes through north-central WY early Wednesday
  morning to bring cooler temperatures and breezy north winds to
  Johnson and Natrona Counties. Otherwise, continued warm and
  dry across southwest WY.

- A cold front moves through the area from the north on
  Thursday, primarily impacting areas east of the divide,
  bringing strong north winds and cooler temperatures.

- Chances of showers and thunderstorms increase over western
  Wyoming late Friday and spread across the rest of the area
  this weekend. The best chances appear to be across northwest
  WY on Saturday, and north-central WY on Sunday.

&&

.UPDATE...
Issued at 120 PM MDT Tue Sep 2 2025

Weak convection has already started this afternoon over the
central Bighorns and the southeast Absarokas. Have updated the
forecast for probability of precipitation from this afternoon
into the early evening hours, especially over the Bighorns,
Absarokas, and Wind Rivers as well as areas to the southeast
(downwind) where the FV3-based CAMs indicate light showers and
isolated thunderstorms may continue through 8 PM. Given the
relatively dry boundary layer and weak BL winds, gusty outflow
up to 30-35 mph could occur from the showers and storms,
especially in Johnson and Natrona Counties.

The first cool front will push southward through Johnson into
Natrona County Wednesday morning, bringing gusty north winds
during the day as well as some low- to mid-level clouds.
General northerly flow will develop across the forecast area on
Wednesday, with breezy/gusty flow east of the divide. Models are
indicating a very low chance of showers with this
front/boundary, though the Bighorns are the most likely area for
light precipitation.

The second and stronger cold front is still anticipated Thursday
with stronger and gustier north winds east of the divide.
Northern Johnson County has a pretty good chance of 40-50 mph
wind gusts during the day, and the north-northeast winds will
push through the entire forecast area by late Thursday
afternoon, with the least impact over far southwest WY.

&&

.DISCUSSION...
Issued at 135 AM MDT Tue Sep 2 2025

Things are largely quiet on as we head into the post Labor Day
period at the humble Riverton weather abode. Radar does have
some echoes across northern Wyoming. However, with the large dew
point depressions, this is likely just virga or at the most a
couple of sprinkles. All in all, today looks fairly similar to
yesterday, with above normal temperatures and mainly dry
conditions. There will be a few showers and storms around, but
these should largely be confined to the northern mountains and
even here the chance is only around 1 out of 4. Any showers
should end fairly quickly after sunset. The main concern today
will be elevated fire weather. Humidity remains rather low,
falling to under 15 percent in some locations. However, wind
will remain light to moderate, so critical fire weather is not
anticipated.

The first of two cold fronts will approach the area late tonight.
This one looks like more of a glancing blow, with the coldest air
remaining well to the east of the area. It will bring temperatures
down around 10 degrees in Johnson County, but effects elsewhere
look negligible, with only a few degree drop in temperatures.
This frontal passage also looks largely dry, with only a 1 in 10
chance of a shower or thunderstorm. It will bring breezy
conditions to Johnson County, but maximum gusts should only be
at most 30 mph.

The second front approaches the area Thursday, and this one looks to
be the more potent of the two. Once again though, moisture is rather
limited so the chance of convection again, looks small, at most 1
out of 5 with most areas remaining rain free. The main impact looks
to be wind. This is especially the case in the favored northwest
flow / cold advection regions, mainly the northern Bighorn Basin and
especially northern Johnson County. The NBM ensemble guidance
gives a greater than 3 in 5 chance of wind gusts past 50 mph on
Thursday afternoon, and a greater than 3 out of 5 chance of wind
gusts past 40 mph in the northern Big Horn Basin. As for high
wind, chances look less than out of a 1 at this point. The 700
millibar wind is around 40 knots, and I would like to see 50
knots there. Also, the best jet forcing is further east in the
Dakotas. However, it does look like a windy afternoon for much
of northern and even central Wyoming. This front will bring
cooler air, but timing at this point shows most of central
Wyoming having one more warm day before the front arrives. As
for fire weather concerns, humidity looks too high for critical
fire weather at this point but elevated fire weather looks like
a good possibility.

Friday will be a much cooler day, definitely feeling more early
autumn like with areas like Buffalo possibly remaining in the 60s.
On this day, ridging will push eastward and moisture will increase
from the west. The chance of thunderstorms will increase in the
west, but it looks isolated on this day. Coverage of showers and
storms than increases over the weekend as moisture spreads eastward,
but the best coverage of storms looks to be mainly over western
Wyoming.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1107 AM MDT Tue Sep 2 2025

VFR flight conditions prevail at all terminals this TAF period with
northwest flow aloft. There is a low chance (30%) of a shower or
thunderstorm, with accompanying gusts around 20kts, to move
southeast towards/over KCPR between 00Z-02Z. This potential is
reflected in a PROB30 group. Any other showers or thunderstorms
should be confined to the mountains.

Otherwise, expect skies to become increasingly hazy this TAF period
from western wildfires. Winds are expected to remain around or less
than 10 kts.

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...McDonald
DISCUSSION...Hattings
AVIATION...Gerhardt