423
FXUS65 KRIW 141750
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1150 AM MDT Sat Sep 14 2024

.KEY MESSAGES...

- Warmer and dry today. A breezy afternoon for much of the area brings
  elevated to critical fire weather conditions.

- Warmer Sunday. An approaching weather system brings isolated showers
  and thunderstorms in the afternoon across western Wyoming.

- Eastward spreading rain/mountain snow chances Sunday night
  through Tuesday. A cold front brings cooler temperatures
  Tuesday.

- Active weather for the end of the workweek, favoring cool and
  wet weather.

&&

.DISCUSSION...
Issued at 112 AM MDT Sat Sep 14 2024

With an approaching upper low still a couple days out and high
pressure over the southern United States, Wyoming sits in the middle
of this synoptic set-up through the weekend. Today, upper flow is
southwesterly, allowing some warmer air to makes its way in. Highs
peak 5 to 10 degrees warmer today compared to yesterday. This means
low to middle 70s west of the Continental Divide, with middle 70s to
low 80s east. These values are near, to a bit above, average for mid-
September.

The main weather story for today is fire weather concerns. Given the
southwest flow and decent pressure gradient, winds begin to increase
late morning. The least windy locations look to be the central
basins, like the Wind River and Bighorn Basins. Elsewhere, gusts 15
to 25 mph occur. The windiest locations are those favored in
southwest flow, such as the Wind Corridor from roughly Rock Springs
to Casper. Models are also indicating some windier locations for
southwest Wyoming, like in Lincoln County, extending a bit into the
Green River Basin. For these windier locations mentioned, gusts of
25 to 35 mph are likely (60%-90%). Where these windier locations
are, relative humidities (RH) will also be to critical levels,
generally 10 to 15 percent. The 00Z HREF also highlights the Wind
Corridor and southern Lincoln County with 60% to near 100% chances
of Red Flag Conditions today. Will continue the Red Flag Warnings
for this afternoon and evening, with the addition for likely (70%)
Red Flag conditions in southern Lincoln County. The slight lack in
confidence for this area comes from two factors. The first is if
critical RHs are going to be widespread enough. The other is if
winds of 25 mph or more are going to occur for a prolonged period of
time. Current thinking is these conditions will occur, but it may
end up being a marginal day.

Sunday, flow turns more southwest to south ahead of an approaching
upper-level low moving into the Pacific Northwest. Winds should look
similar to the previous day, with emphasis over the Wind Corridor.
Afternoon RHs bring another day of elevated fire weather conditions.
Will hold off on any highlights for now as winds are looking a bit
too low, and RHs improve very slightly across southern Wyoming. Most
of the day remains dry, though some leading moisture could spark a
shower or thunderstorm over the west, though chances are about 20%
or less through much of the afternoon. Late Sunday evening through
Monday morning, better moisture filters in. Current models keep
precipitation chances along and west of the Divide through Monday
morning as the upper low takes more of a due south direction.
Chances spread east of the Divide Monday evening as the low begins
tracking eastward. The low then tracks northeast through Tuesday,
the center of it clipping northwest Wyoming. This should produce the
best precipitation chances over western and especially northwest
Wyoming. This being said, everywhere looks to have at least a 20%
chance of showers or thunderstorms, those 20% chances being more for
eastern Sweetwater and southeastern Natrona Counties - locations
furthest from the low`s center. Cooler temperatures also arrive
Tuesday as the low`s cold front passes through. Given 700mb
temperatures around 0 to 4 degrees Celsius, snow levels as low as
8500 feet could occur, but should generally be 9000 feet or higher.
Low elevation highs ranging from the 50s to middle 70s occur for the
middle and end of the workweek.

Past this system moving out Wednesday, long range models diverge a
little bit on the details, but show another system moving through.
This low travels further south, moving through or just south of
Wyoming in the Thursday to Saturday timeframe. In other words,
continued active weather for the second half of the week as well,
with a favoring for cooler and wetter weather.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1150 AM MDT Sat Sep 14 2024

Besides high cirrus clouds, mostly clear skies are forecast this
afternoon through tonight. Several locations will see typical
afternoon breezes develop, with winds quickly decreasing just
before sunset.

The drier air in place tonight should keep patchy fog out of
the KJAC area for Sunday morning. Sunday afternoon will feature
a slight increase in winds for sites west of the Continental
Divide. Scattered showers and a few weak thunderstorms could
bring (20% chance of) brief strong and erratic winds gusts for
KJAC, KBPI, KPNA, and KRKS.

&&

.FIRE WEATHER...
Issued at 112 AM MDT Sat Sep 14 2024

Elevated to critical fire weather conditions this afternoon and
evening. A Red Flag Warning is in effect for fire zones 277, 280,
289, and 300 from noon until 7pm MDT today. RHs 10 to 15 percent
occur for these zones, as well as most lower elevation locations
east of the Divide and across zone 279. Afternoon winds gusting 15
to 25 mph will be common. For the zones with a Red Flag Warning,
favored southwest wind brings gusts more in the 25 to 35 mph range.

Breezy and dry again Sunday, with elevated fire weather conditions.
Some showers and thunderstorms begin moving into western Wyoming in
the afternoon.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 7 PM MDT this evening for WYZ277-280-
289-300.

&&

$$

DISCUSSION...Wittmann
AVIATION...VandenBoogart
FIRE WEATHER...Wittmann