Kirby, Wyoming Weather Forecast Discussion

236
FXUS65 KRIW 102154
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
354 PM MDT Wed Sep 10 2025
.KEY MESSAGES...
- Elevated fire weather conditions today and Thursday, with
  wind gusts up to 35 mph being the main threat.
- Isolated to scattered (15-40% chance) showers and
  thunderstorms are possible this afternoon and evening. Wind
  gusts of 40 to 50 mph will be the main threat.
- Rain chances move into southwestern Wyoming this evening,
  spreading north and east through the night. Rain chances
  continue for the day, Thursday.
- Cooler Friday behind a cold front. Daily rain chances continue
  into the weekend, with late Friday through Saturday being the
  wettest period.
&&
.UPDATE...
Issued at 1228 PM MDT Wed Sep 10 2025
Main change to today`s forecast with this update is precipitation
chances. Have added some low chances (15 to 25 percent) across
Natrona and Johnson Counties this afternoon, ending about 6pm MDT.
Hi-res models are in fair agreement with these shower and storm
chances, though differ in locations; some keep it southeast of
Casper, while others have storms over Casper and north to Johnson
County. Overnight rain chances are still likely, moving into
southwest Wyoming starting this evening, continuing through the
night.
The main weather system pushes through the area Thursday, but mainly
into Friday. Cooler temperatures and rain chances accompany it. For
anyone that happens to be above about 11000 feet, you may catch a
snowflake or two. Models are still keeping the main low north into
Montana, which has rain chances reduced compared to previous runs
yesterday, as well as temperatures very slightly warmer (as
discussed in the full discussion below). Freezing temperatures (32F
or less) are possible across the western basins and valleys starting
Friday morning (15 to 40 percent chances), with increasing chances
through the weekend, peaking Saturday night/Sunday morning (40 to 90
percent chances).
&&
.DISCUSSION...
Issued at 120 AM MDT Tue Sep 9 2025
Synopsis...An upper level low, currently over northern CA this
morning, will be the main weather driver through the rest of the
week. A portion of energy will round the base of this longwave
trough later tonight into Thursday, reaching far western portions of
the state by Thursday afternoon. This will lead to a secondary low
to develop and stretch the trough as it moves over northern MT by
Friday morning and the CA low weakens and fills as it moves over the
Great Basin. This will result in an overall weakening of the system,
with waves of showers and thunderstorms moving over the CWA
Wednesday and Thursday.  The MT low will become more dominant as it
strengthens during the day Friday. This will lead to additional
shortwaves over the Great Basin to move over the area Friday
afternoon into Saturday morning. The chances for a widespread
precipitation/wetting rain event have now gone down considerably.
Discussion...
The south-southwest flow pattern aloft will begin in earnest today,
as the Cowboy State becomes squeezed between an upper level low over
northern CA and a ridge axis over the Great Plains. A much drier
airmass will move over much of the forecast area today, with a
dryline in place along the Johnson/Campbell county line as this
airmass meets Gulf moisture. This drier air will lead to elevated
fire weather conditions, with humidity values ranging between 16 to
25% for most areas this afternoon. South to southwest winds, with
gusts of 25 to 35 mph will develop by midday and highs in the 80s.
Areas east of the Divide will be in the middle 80s to 90 degrees,
making it an unseasonably warm day. Showers and thunderstorms will
be isolated to widely scattered (10-20% chance) across much of the
area, but areas along and north of a Kemmerer-to-Worland line will
have the better chances. Wind gusts of 40 to 50 mph will be the main
threat due to large dewpoint depressions, as all the instability
will be east of the CWA (east side of the dryline). Additional
showers and isolated storms will remain possible through the
overnight hours, with southern portions and the Wind River Basin
having the best chances. Isolated lightning and gusty outflow winds
will be the main impacts.
Similar conditions will return for Thursday, although it will not be
as dry. Winds will be slightly stronger, with more locations gusting
toward 35 mph. This will be due to a leeside trough developing over
eastern WY, which in turn will tighten the sfc pressure gradient.
Elevated fire weather conditions will be in place again, mainly due
to the winds. RH values will be slightly higher in the 20-30% range.
Afternoon highs east of the Divide will be in the 80s, with
locations along the Bighorn and Powder Rivers in the upper 80s to
near 90 degrees once again. Isolated showers will remain possible
across the CWA through the rest of the morning, with increasing
shower and thunderstorm activity over the western mountains and
areas west of the Divide early in the afternoon. These showers and
storms will move northward into the central basins through the
afternoon and end and/or exit the area around 06Z Friday. Wind gusts
up to 50 mph will be the main threat, with a low chance (10%) for
brief heavy rain. This push will be from the shortwave rounding the
base of the longwave trough and lead to the MT upper low. This
shortwave will bring the cooler and drier conditions Friday. Any
convection will be very isolated (less than 10%). Winds will
generally be light (gusts less than 20 mph), except over the Wind
Corridor where gusts of 25 to 30 mph will occur. Afternoon
temperatures will also be 5 to 10 degrees cooler, with highs in the
60s to near 70 degrees west of the Divide and in the 70s to near 80
degrees east of the Divide.
Showers and isolated thunderstorms will be possible Saturday
afternoon and evening, as the trough moves over the region. Western
and northern portions look to have the better chances for a wetting
rain from this activity, but confidence remain low. Isolated showers
and thunderstorms will be possible again Sunday, as the region
remains generally unstable in the wake of the passing trough.
Temperatures will be seasonal and winds relatively light.
&&
.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 352 PM MDT Wed Sep 10 2025
We remain in a fairly active pattern over the next several days.
This means scattered shower and thunderstorm chances for most
terminals through the period. These chances and timing varies from
terminal to terminal. CAMs are struggling to resolve specific
details and timing, so timing has been handled in the TAFs as
closely as possible, but have opted for VCSH and PROB30 groups for
most terminals for lower confidence in timing of shower and
thunderstorm chances. Initial convective development this afternoon
has generally been focused across western WY. KCOD may continue to
see impacts of this activity to start the period. Then, a secondary
wave looks to move through this evening and overnight. Satellite and
radar show these showers and storms initiating in northern Utah and
spreading into far southwest WY to start the period. These will
spread northeastward through the evening and overnight but the
location of showers with this wave is uncertain. This wave also
looks to be generally virga showers or even just mid-level clouds,
especially as it approaches the Continental Divide, given little low-
level moisture available.
Wind is the other aviation concern during the period. Gusts 20 to
30kts to start the period, particularly at KRKS and KCPR will
generally decrease towards sunset with mainly light winds prevailing
through Thursday morning at all terminals. Gusty outflows will
remain possible through the evening and overnight with the shower
and thunderstorm activity. With an approaching low pressure system
to the west, southerly gradient winds increase again Thursday late
morning, most notably again at KCPR and KRKS. Gusts 20 to 30kts
will again be common through the afternoon at those terminals.
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...LaVoie
AVIATION...Hensley

NWS RIW Office Area Forecast Discussion