Thayne, Wyoming Weather Forecast Discussion

488
FXUS65 KRIW 230811
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
211 AM MDT Tue Sep 23 2025
.KEY MESSAGES...
- Lingering showers are expected through mid-day today, mainly
  across Natrona County and Sweetwater County.
- A Freeze Warning is in effect from 5AM to 10AM this morning
  for below freezing temperatures across the western valleys and
  southern Lincoln County near Cokeville. Dense fog is possible
  across the western valleys and Yellowstone through mid-
  morning.
- Dry weather with above normal temperatures are forecast
  Wednesday through much of the weekend.
&&
.DISCUSSION...
Issued at 210 AM MDT Tue Sep 23 2025
The upper-level low that aided in the production of yesterday`s
widespread rain showers and cool temperatures is evident on 08Z
water vapor imagery over southwestern Wyoming. There is ensemble
consensus that this upper-low will trek southeast into Colorado by
mid-day today and take the rain chances with it. As the low makes
its southeastward shift through early morning hours, low-level high
pressure shifts in and cloud cover is forecast to diminish across
the west. The combination of being behind Monday`s cold front and
diminishing cloud cover creates prime radiational cooling
conditions. So, a hard freeze (28F) remains likely (80-90%
confidence) across western valleys this morning. These locations and
Yellowstone could also see fog development this morning given
yesterday`s rain and tonight`s diminishing cloud cover. However, if
clouds take longer to diminish across the west, temperatures may be
a few degrees warmer and fog development would be less likely.
As mentioned above, the upper-low`s southeast shift into Colorado
will bring an end to our precipitation chances by mid-day. However, a
weak vorticity maximum rounding the low could support a very
isolated shower or thunderstorm across far southern Natrona County,
eastern Sweetwater County, and high elevations of Wind River
Mountains and northern Bighorns during the afternoon. The chances of
this occurring are around 15%. Otherwise, with abundant sunshine and
5C-7C 700mb temperatures, today`s highs are forecast to be anywhere
from 5 to 15 degrees warmer compared to Monday across northern and
western portions of the state. Lingering cloud cover across the
southeastern CWA will keep high temperatures comparable to
yesterday, if not a few degrees cooler.
There is good agreement among ensemble and deterministic guidance
members in an upper-level ridge and an associated surface high
dominating the Intermountain West Wednesday through early Friday. A
lack of support aloft and slightly below normal PWATs brings near
zero precipitation chances. Expect a return of slightly above normal
temperatures with mean 700mb temperatures around 10C. In fact, these
700mb temperatures are around to slightly above the 90th percentile
of climatology.
A weak trough along with an associated weak front is forecast to
trek across Wyoming on Friday into Saturday. At the same time, an
upper-level low over southern California will keep moisture cut off
from Wyoming. Thus, this front is currently expected to remain dry
and have very little influence on temperatures Saturday and Sunday.
The movement of this upper-low as we get into late Sunday becomes a
bit more uncertain. There is a general consensus is that the upper-
low will be absorbed into a long wave trough and push north and
east, along with some moisture, across Wyoming early next week.
Considering we are nearly a week out, there are differences in exact
timing and evolution of this potential system.
&&
.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1034 PM MDT Mon Sep 22 2025
Center of showers and a few embedded thunderstorms has begun to
shift east, and should keep most impacts across central WY
tonight. Most cloud cover has eroded from KJAC, and should begin
to clear out from KBPI/KPNA and KCOD between 08Z and 10Z. Main
spin will continue light rain for KLND/KRIW through most of the
overnight, but should be ending by 12Z. Focus for rain remains
KCPR and KRKS overnight, with KRKS lifting by midmorning, while
KCPR holds light rain and low clouds through mid afternoon. The
system will move east during the afternoon, drawing drier in
from the northwest, with all sites likely mostly clear by
00Z/24. Clear skies and L/V winds for all sites Tuesday night.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
Freeze Warning until 10 AM MDT this morning for WYZ013-023-027.
&&
$$
DISCUSSION...Gerhardt
AVIATION...Straub

NWS RIW Office Area Forecast Discussion