Oregon Trail - Lander Cutoff, Wyoming Weather Forecast Discussion

330
FXUS65 KRIW 161735
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
1135 AM MDT Tue Sep 16 2025
.KEY MESSAGES...
- A weather system will continue to impact the region through
  Thursday, with focus of showers and storms being along and
  east of the Continental Divide.
- Scattered showers and thunderstorms will be possible (20 to
  50% chances) through the afternoon, with small hail and gusty
  winds 30 to 40 mph being the main threats.
- Precipitation spreads into Natrona and Johnson Counties
  Wednesday morning, with 50% chance of seeing 24-hour
  precipitation totals exceeding 0.5" for the Casper area
  through Thursday morning. Upwards of 1" of rain is possible
  (20% chance) for Casper and Casper Mountain.
- Drier and warmer weather settles in after Thursday, with high
  temperatures 5 to 10 degrees above normal Friday through the
  weekend.
&&
.DISCUSSION...
Issued at 207 AM MDT Tue Sep 16 2025
Morning WV imagery shows the latest H5 trough spinning across
the northern Rockies, generally centered on YNP. With this
system blocked from progressing, the system will produce
widespread showers and thunderstorms across western WY today
through Wednesday evening. Some midlevel enhancement is already
sparking showers and thunderstorms across Sweetwater County,
which is quickly spreading NE into Natrona County. This will be
one of two main focus areas for precip today. The other is
closer to the center of the trough, bringing precip across far
NW WY this afternoon. While the main enhancement remains just to
the north in souther Montana, enough energy will be available to
produce precip for the Absarokas, the Parks, and down the Winds
through this afternoon. Once daytime heating kicks in, showers
will also build along the Bighorns. Precip will likely not fade
off overnight, with chances remaining above 50% for all except
far western WY through sunrise.
As the trough slowly nudges east Wednesday, this puts the
circulation over central WY. Models continue to indicate
widespread rainfall east of the Divide, focusing on Johnson and
Natrona Counties. Latest forecast probabilities for rain
accumulations above a half inch remain 50 to 60 percent along
I-25, and around 35 percent probability of a full inch of rain
for Natrona County.
After Wednesday, the trough nudges just far enough east to
loosen its grip on western Wyoming weather for the remainder of
the week. Temperatures will rebound back into the 70s and low
80s into the upcoming weekend.
&&
.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1134 AM MDT Tue Sep 16 2025
A weather system over Wyoming will bring isolated to scattered
shower and thunderstorm chances to most terminals this TAF period.
The exceptions are KPNA and KBPI where shower/thunderstorm chances
are less than 20%. Shower and thunderstorm chances (around 30%)
increase after 19Z at all other terminals. Confidence in exact
location and timing of thunderstorms through 00Z/02Z remains
uncertain so PROB30 groups reflect the convection chances during
this timeframe. There is a 30% chance in a wave of showers at
KRIW and KLND between 02Z and 09Z. After 02z at KCPR, confidence
increases in prevailing rain and/or thunderstorms through the
overnight hours and into Wednesday. Lower ceilings are expected
to accompany rain showers at KCPR as well, with a 30% chance of
MVFR ceilings.
At all terminals (excluding KCPR), conditions are expected to stay
VFR with any showers and thunderstorms; however, a brief period of
reduced flight conditions cannot be ruled out in a heavy shower or
thunderstorm. Areas that see significant rain may see fog after 10Z
Wednesday but confidence is too low to include in TAF at this time.
Otherwise, winds generally remain around or less than 10kts this TAF
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...Straub
AVIATION...Gerhardt

NWS RIW Office Area Forecast Discussion