203
FXUS65 KRIW 170415
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1015 PM MDT Mon Sep 16 2024

.KEY MESSAGES...

- Widely scattered showers and a few thunderstorms will occur
  this afternoon, as a storm system over CA moves closer to the
  region.

- A cold front pushes through Tuesday with more scattered rain
  showers and isolated thunderstorms. Severe wind gusts are
  possible across eastern portions of the CWA.

- Cooler temperatures and continued unsettled weather through
  the rest of the week and into the weekend with another
  system.

&&

.DISCUSSION...
Issued at 1212 PM MDT Mon Sep 16 2024

The closed upper low has moved onshore over northern CA this morning
and will gradually make its way over the Great Basin tonight. This
will result in a diffluent southerly flow over the Cowboy State
today. Elevated to near-critical fire weather conditions will
continue again today as winds increase across southern portions of
the CWA, pushing into central portions through the afternoon. RH
values will not be as low as Sunday, but near-critical values will
occur across areas east of the Divide. Showers and thunderstorms
will be split to western portions and eastern portions
(Johnson/Natrona counties) this afternoon and evening. Additional
showers and thunderstorms could move over Sweetwater County at
varying times this afternoon and evening, but are expected to be
short-lived. An isolated shower could also occur over the central
basins, but confidence is low. The main impact from all this
activity will be strong outflow boundaries once again. Dewpoint
depressions of 40-50 degrees remain in place, so wind gusts over 50
mph will be likely. An isolated stronger gust of 60+ mph cannot be
ruled out either. Isolated showers will remain possible over far
western portions through the night tonight.

The closed low will continue its eastward progression through the
night tonight, as previously mentioned above, reaching the
ID/UT border by 18Z Tuesday. Precipitation chances will increase
across the west as a result, with an additional area of showers
and thunderstorms developing over the Wind Corridor (the
eastern half of Sweetwater County to Natrona County) as a result
of the left front quadrant moving of the area. This activity
will quickly spread into Johnson County by early afternoon, with
surrounding areas (Wind River and Bighorn Basins, Bighorn
Mountains) being impacted as well. The associated cold front
will reach western portions between 12Z and 18Z Tuesday. There
are now indications of a leeside low developing over Sheridan
County/southern MT between 20Z and 00Z Wednesday, which will
also coincide with any severe potential for the thunderstorms
across eastern portions. A lack of moisture looks to be the main
culprit for this. For now, it looks like most of this potential
will be pushed out of the CWA and in UNR and CYS areas. 700mb
temperatures will begin to drop Tuesday morning, averaging
around 2C through the rest of the day, leading to snow levels
around 9000 ft once again. The sfc pressure gradient will
tighten irt to the upper low as well, resulting in southwest
winds gusting 35-50 mph from the eastern half of Sweetwater
County to Johnson County, including the Wind River Basin. Cooler
temperatures will occur Tuesday night, as the cold front exits
to the east and the upper low tracks into MT. Snow levels over
northwestern mountains could drop to 7500 ft late in the
afternoon into Tuesday night, due to a pocket of sub-0C air from
the core of the upper low. Overall, this pattern is similar
that just occurred last Wednesday/Thursday except for the low
becoming negatively-tilted. West-southwest winds in the wake of
the storm will lead to downsloping winds gusting 50-60 mph off
the east slopes of the Wind River, Absaroka and Bighorn
Mountains Tuesday night into Wednesday morning. The winds,
though not as strong, will spread across southern
Fremont/Natrona counties through the overnight hours, with gusts
of 50-55 mph impacting these areas including the Casper area.

As this system exits Tuesday night into Wednesday, another system is
soon to follow. This one pushes further southward, over southern NV
on Thursday. Models have come into better agreement on placement and
speed, as it slowly traverses the Four Corners Friday and clipping
southern portions of the Cowboy State Saturday as it exits over the
Central Plains. Winds will remain gusty Wednesday in the wake of
Tuesday`s storm and looks to be the windiest day through the rest of
the week. Temperatures will remain seasonal through the rest of the
week as well.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1015 PM MDT Mon Sep 16 2024

The large upper-low will move to the northeast into MT through the
next 24 hours. As it cyclonically rotates it will bring waves of
rain showers and thunderstorms through the area Tuesday morning into
Tuesday evening. PROB30 groups are in the forecast for the time
period of highest likelihood for prevailing TSRA and MVFR/IFR
conditions. A strong cold front will sweep through Tuesday
morning/afternoon from west to east, with strong wind ahead of and
behind the front. A wind shift from southerly to westerly will occur
once the front passes through.

Much drier air will surge into the area behind the cold front across
the southern half of the area, so this will shut off shower activity
for most terminals. Terminals that will be closer to the center of
the low will have lingering rain showers into Tuesday evening, which
includes KBPI/KPNA/KJAC/KCOD. A gap flow situation is possible (30-
40%) at KCOD Tuesday evening into Tuesday night as the center of the
low will be situated just north of KCOD in MT. Westerly flow through
the Shoshone Canyon will funnel the already strong wind toward KCOD,
which may result in very strong wind Tuesday night.

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

&&

.FIRE WEATHER...
Issued at 1210 PM MDT Mon Sep 16 2024

Elevated to near-critical fire weather conditions continue today
with southerly gusts up to 25-30 mph across the southern half of
the forecast area. RH values will be slightly higher than
Sunday, but will still range from the upper teens to lower 20s
with isolated locations reaching critical thresholds. Showers
and thunderstorms will be widespread across the area this
afternoon and evening, with strong outflow gusts being the main
threat. Isolated showers will remain possible over western
portions tonight. Showers and thunderstorms will increase across
the area late Tuesday morning into early afternoon with a chance
for wetting rain, as a cold front moves across the area.
Western portions will have the best chances. Strong winds will
occur Tuesday night into Wednesday across zones 276, 280, 288,
and 289.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...LaVoie
AVIATION...Rowe
FIRE WEATHER...Lowe