Grass Creek, Wyoming Weather Forecast Discussion

711
FXUS65 KRIW 201058
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
458 AM MDT Sat Sep 20 2025
.KEY MESSAGES...
- A warm and mainly dry start to the weekend, with any showers
  restricted to southwestern Wyoming.
- Turning much cooler and wetter Sunday night and Monday as a
  cold front moves across the area.
- Mainly dry and warm weather returns for the middle and end of
  next week.
&&
.DISCUSSION...
Issued at 257 AM MDT Sat Sep 20 2025
We are now entering the last summer weekend of 2025, and it will be
a warm one to close it out. The basic reasoning of the forecast
remains the same as the previous couple of days. For most locations,
it will be a dry and warm start to the weekend, with
temperatures averaging 5 to 10 degrees above normal. There will
be a bit more convection around today though. Radar does show a
few showers in northern Utah and these are moving into southern
Lincoln County, but nothing appears to the reaching the ground
at this time. We made few changes to the placement today, with
showers mainly near the Unitas and up to around Afton, and even
here chances are low, only around 1 out of 4 at the most. Deeper
moisture and an approaching shortwave will keep showers and a
few storms going into the night, with coverage expanding to much
of the western mountains by sunrise Sunday. The bulk of the
showers should hold off until Sunday though.
And speaking of Sunday, a cold front will move into western Wyoming
and bring more widespread showers and thunderstorms for this time.
Most guidance has slowed a bit though in the eastern progression of
the front. As a result, most areas east of the Divide should be
largely dry through the daylight hours. And, with increasing
breezes as the pressure gradient tightens ahead of the front,
and better mixing, this could be the warmest day of the weekend
in places like Buffalo and Casper. There could be enough of a
breeze for elevated fire weather as well, although humidity and
wind does not look sufficient (only a 1 out of 5 chance) for
critical fire weather at this time. Although some showers are
possible Sunday evening east of the Divide, the bulk of the
precipitation should hold off until Monday.
We have some of the same concerns for early next week this
morning as we had yesterday. We have a high confidence (greater
than 9 out of 10) that it will be a much cooler day than Sunday
(possibly as much as 30 degrees cooler in northern Johnson
County). We have high confidence in a breezy day, with the
strongest wind in the favored cold advection / northwest post-
frontal locations like Johnson County where some 40 mph wind
gusts are likely. The continued uncertainty is with the
progression of the upper level low behind the front and
placement of the heaviest precipitation. There continues to be a
spread of around 18 hours as to when the low will move away to
the southeast, anywhere from later Monday night to Tuesday
afternoon, depending on what guidance you look at. And there is
little agreement on placement of the heaviest precipitation as
well. Generally, it looks to be in northern Wyoming but exact
locations are still up in the air. And finally, we still have
the likelihood of some snow in the higher elevations of the
northern mountains (generally above 9000 to 9500 feet). This
should not pose many travel concerns, but the highest locations
have at least a 1 in 2 chance of a couple of inches of new
snow.
Following the departure of the low, things look relatively tranquil
as ridging builds back into the area, bringing above normal
temperatures and mainly dry conditions through much of the middle
and end of next week.
&&
.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 435 AM MDT Sat Sep 20 2025
VFR conditions will continue through the TAF period, with SCT-
BKN FL180-250 conditions in place for much of the forecast area
this morning. Weak showers showers will be possible this
afternoon along and west of a KAFO-to-KRKS line. Lower ceilings
080-120 will be possible in this area. Gusts of 15 to 20 kt will
also occur this afternoon at KRKS, KBPI and KPNA as a result
from any outflow from the showers. Clouds will begin to scatter
out after 00Z, with high clouds remaining over areas west of the
Divide through the rest of the period.
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.
&&
$$
DISCUSSION...Hattings
AVIATION...LaVoie/Straub

NWS RIW Office Area Forecast Discussion