792
FXUS65 KRIW 300443
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1043 PM MDT Sun Sep 29 2024

.KEY MESSAGES...

- Red Flag conditions today across much of the area. Red Flag
conditions continue across the southwest tomorrow.

- Cold frontal passage Tonight, with gusty north winds across
Johnson County and the Bighorn Basin.

- Isolated frost/freeze potential Tuesday morning across some
central Basins with widespread below freezing temperatures across
the western valleys.

- Dry and warm and dry through the rest of the week.

&&

.DISCUSSION...
Issued at 230 PM MDT Sun Sep 29 2024

The sky remains mostly cloudless again today, though there are a few
more speckled Cu across the west today than we`ve seen in past days.
These clouds are ahead of a trough that will push through overnight.
Not only have we seen a few more clouds than previously expected,
but we also are seeing increased winds as the pressure gradient
tightens. Gusts 25 to 30 mph are already common across much of the
area and will continue through the afternoon. These gusty westerly
to southwesterly winds, coupled with RHs between 10 to 15 percent
have led to Red Flag Warning conditions across much of the region.
Higher elevations and across the northwest have RHs that are just
outside of critical criteria, and thus, those locations remain out
of the Warnings. Everyone will be getting the wind, however.
Temperatures also remain much above normal with near record warm
temperatures expected for many locations. Several locations may also
be seeing their latest 90 degree day on record!

A cold front (associated with the aforementioned trough) will push
through areas east of the Divide overnight. The front looks to reach
the northern Bighorn Basin shortly before midnight tonight, then
barrel through the rest of the area east of the Divide through the
night. Gusty northerly winds will come with the front. This will be
a strong, but dry, front, and favored areas in the Bighorn Basin and
northern Johnson County could see gusty near 40 to 45 mph with the
frontal passage. The gusty northerly winds will persist through
Monday morning, decreasing through the afternoon. Across areas west
of the Divide, since they won`t be touched by the cold front,
westerly winds continue through Monday afternoon. With critical RHs
continuing as well, the Red Flag Warnings across southwest WY
continue through Wednesday evening. Behind the cold front, tomorrow
will be significantly colder than today across the whole region.
West of the Divide, high temps will be 15 to 20 degrees cooler than
today, while east of the Divide, temps will be 25 to 30 degrees
colder than today. This will bring temperatures back to more normal
values for this time of year (that`s how much warmer than normal we
have been!!)

While I`m on the topic of colder temperatures, I do want to mention
the possibility of frost/freeze across portions of the region.
Tuesday morning will be the coldest morning with clear skies and
light winds post-cold front, so this is the day of concern for the
frost/freeze potential. Widespread below freezing temperatures will
occur across the west of the Divide valleys, however, as those areas
have already had several hard freezes to date, the growing season is
over and we will not be issuing any frost or freeze products for
those locations. East of the Divide, and across Sweetwater County,
however, the growing season is still going, and thus frosts and
freezes are still a concern. Currently, confidence in a widespread
frost or freeze event is low (20 to 30% chance), however, isolated
patches of near freezing temperatures are expected across the Wind
River Basin and Natrona County. Lower valleys in the Bighorn Basin
(such as northwest of Thermopolis and around Worland) could also see
some isolated freezing temps briefly Tuesday morning. Because this
looks to be a isolated event and is marginal even then, will hold
off on any Watch/Warning products. Best confidence (though still
only 30% chance) would be for lower elevations of Natrona County,
particularly west of Casper.

High temperatures go back on a upward trend on Tuesday, though still
comfortable. By Wednesday, we are back up into the 80s across much
of the area east of the Divide with mid 70s across lower elevations
west of the Divide in westerly flow. A shortwave will start to
approach from the northwest on Wednesday, which will increase
gradient winds. Thus, could see additional fire weather highlights
needed for Wednesday across portions of the region. Behind this
shortwave, Thursday will again be cooler (upper 60s to low 70s).
Ridging rebuilds over the Four Corners region late in the week, and
by Saturday, we see the return of the 80s. Certainly not feeling
like October! With persistent ridging, there are also no chances for
precipitation in the next 7 days.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1043 PM MDT Sun Sep 29 2024

A dry cold front tracks southward to start the TAF period, moving
from northern Wyoming southward. KCOD is the first to be impacted at
the very start of the period. Gusty winds 20 to 30 knots accompany
the initial passage of the front. These winds decrease within a
couple hours of frontal passage. The front moves through other sites
through the TAF period, shifting winds to a northerly/northeasterly
direction, excluding western-most sites (KBPI, KJAC, and KPNA).
Winds will be strongest for more northern sites, with lesser gusty
wind impacts the further south the front travels. KJAC and KRIW
could see a few gusts up to 20 knots Monday afternoon, but not
expecting it to be frequent, so have left out of the TAFs. A few
high clouds may accompany the front, but the sky should otherwise
remain clear to mostly clear.

The front also pushes the smoke from the Pack Trail fire southward.
This should move the thicker smoke out of the vicinity of KWRL near
the start of the period. However, the smoke begins to filter into
central Wyoming, including KCPR, KLND, and KRIW Monday morning.
Smoke should increase towards the afternoon as instability allows
greater smoke production. These central sites could see some
visibility reductions with the smoke, especially KRIW due to its
proximity to the fire. Have kept any mention out of the TAFs for now
due to lower confidence at this range (main smoke impacts look to
begin after 21Z/Mon). Will need to re-evaluate for future issuances.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on smoke, icing, and turbulence
forecasts.

&&

.FIRE WEATHER...
Issued at 230 PM MDT Sun Sep 29 2024

Red Flag Warnings remain in place through 1900 this evening for most
east of the Divide zones. Temperatures remain very warm in the upper
80s to near 90 this afternoon and RHs are less than 12% in many
areas. Ahead of an approaching cold front, winds will continue to be
from the west to southwest gusting 25 to 35 mph. Across the
northwest and higher terrain, RHs are marginal, but should stay
above 15%, and thus not critical. Across the southwest, Red Flag
Warnings also remain in effect through the rest of today with
westerly winds gusting 25 to 35 mph through the afternoon along with
critical RHs and warm temperatures. These zones will also see Red
Flag Conditions Monday afternoon as the gusty west winds persist and
RHs again drop to critical levels. Thus, Red Flag Warnings for zones
277, 278, 279, and 289 will continue through Monday afternoon.

A cold front will impact east of the Divide zones overnight, with a
rapid wind shift to the north with the passage of the front. The
front should reach the northern Bighorn Basin and northern Johnson
County around midnight tonight, and reach the Wind River Basin and
Natrona County by 0400 Monday. Gusty northerly winds will persist
through the morning, becoming light by late afternoon. RHs should
stay above critical thresholds Monday east of the Divide, and thus
no fire weather highlights are in effect across those areas after
this evening. Behind the cold front Monday will also be 20 to 30
degrees colder than Today across the region.

Tuesday and Wednesday will see the return of a warming trend. RHs
will once again meet critical thresholds as well. Elevated to near
critical fire weather conditions will thus persist through the rest
of the week. Wednesday will see the strongest winds, as a shortwave
trough passes through, and thus, the most likely chance for critical
fire weather will be Wednesday. No precipitation is expected for at
least the next 7 Days.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 7 PM MDT Monday for WYZ277>279-289.

&&

$$

DISCUSSION...Hensley
AVIATION...Wittmann
FIRE WEATHER...Hensley