771
FXUS65 KRIW 151723
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1123 AM MDT Sun Sep 15 2024

.KEY MESSAGES...

- Mainly dry today, with a breezy afternoon. Isolated (30% or
  less) showers and thunderstorms develop for western Wyoming
  this afternoon. Little rain expected, but strong outflow winds
  are possible.

- A weather system pushes into the area Monday, with rain and mountain
  snow chances pushing east of the Divide late Monday through
  Tuesday.

- A cold front moves through Tuesday, with cool temperatures and
  unsettled weather expected for the remainder of the week.

&&

.DISCUSSION...
Issued at 1042 AM MDT Sun Sep 15 2024

Weather today will be relatively quiet, as a longwave trough digs
along the PACNW. A shortwave is moving northward over Sweetwater
County this morning, with isolated showers and thunderstorms
occurring over the northern half of the county. Isolated (30% or
less) showers and thunderstorms will be mainly confined west of the
Divide, but some of these could cross into the Wind River Basin and
the Cody Foothills. Not expecting much, if any, rain to be falling
with any of this convection this afternoon, as the boundary layer
remains rather dry. Hence, strong outflow winds up to 50 mph will be
the main threat due to the large dewpoint depressions of about 40
degrees. Showers and thunderstorms will remain possible tonight and
become confined to far western portions of the CWA, as a closed low
forms and moves onshore over northern CA. These shortwaves moving in
from the south will also lead to moisture advection, so a wetting
rain could occur in this area tonight. Speaking of dry boundary
layers, fire weather conditions will be limited this afternoon.
Critical relative humidity (RH) values will return once again east of
the Divide, however winds are not expected to reach criteria.

The closed upper low will slowly move over northern CA into the
Great Basin through the day Monday. This will result in a diffluent
southerly flow over the Cowboy State. Elevated to near-critical fire
weather conditions will continue as winds increase across southern
portions, pushing into central portions through the afternoon.
Showers and thunderstorms will occur over southern portions again,
as another shortwave arrives along the nose of the supporting jet of
the upper low. Some of these will move over central portions through
the afternoon as well.

The closed low will continue its eastward progression through the
night Monday night, reaching the ID/UT border by 18Z Tuesday. Most
of the precipitation looks to stay along the eastern periphery of
the low, which would keep it over western portions Monday
night/Tuesday morning. This trend will likely continue through the
day Tuesday, as there is a hint of a dry slow developing east of the
Divide and Sweetwater County. The associated cold front will also
reach western portions Monday night/Tuesday morning and continue
eastward through the day Tuesday. 700mb temperatures will average
around 2C across western portions, 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. There will be a chance for severe
weather Tuesday afternoon over eastern portions of the CWA, as the
low begins to pivot and become negatively-tilted as it moves over
northwestern portions of the state. So far, models are not hinting
at any moisture being in place in these areas likely due to the
possible dry slot. We will certainly be keeping an eye on this for
any changes. 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. Overall,
this pattern is similar that just occurred last Wednesday/Thursday
except for the low becoming negatively-tilted.

As this system exits Tuesday night into Wednesday, another system is
soon to follow. This one pushes more southward, into the Nevada area
on Thursday. There is some model disagreement on exact placement.
This low tracks eastward through Saturday. Highs at the end of this
coming workweek range from around 60 to around 70. With this next
system, temperatures look to fall a bit more for the weekend. Long
range models are hinting at this second low to finally move through
Saturday, with hints of another trough dropping southward from
Canada around the Sunday time frame.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1042 AM MDT Sun Sep 15 2024

VFR conditions to start that period across all TAF sites. Winds will
begin to increase during the early afternoon across mostly all
terminals. The strongest winds will be at KJAC, KBPI, KPNA, and KRKS
with gusts of 20 to 30 knots at times this afternoon. Scattered
showers and thunderstorms are expected to develop this afternoon.
KJAC/KBPI/KPNA have the best chances of seeing a shower or
thunderstorms this afternoon into this evening. Other sites such as
KRKS/KLND/KRIW may see a shower but confidence remains low. MVFR/IFR
conditions are possible at any sites where a storm moves directly
overhead. Strong gusty outflow winds are possible with any nearby
developing storms. Precipitation is expected to dissipate after
sunset with a few lingering showers possible across western
terminals into the early morning Monday. Winds begin to decrease
after sunset as well with all terminals seeing light winds into the
end of the period.

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 1042 AM MDT Sun Sep 15 2024

elevated to near-critical fire weather conditions will be in
place today through Tuesday. Critical RH values will remain in
place across areas east of the Divide this afternoon, while
winds will be slightly below criteria. RH values will decrease
slightly Monday and again Tuesday, as winds begin to increase
much of the Casper dispatch area. A cold front will move across
the area Tuesday, cooling temperatures back down to seasonal
levels.


&&

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

&&

$$

DISCUSSION...LaVoie
AVIATION...Dziewaltowski
FIRE WEATHER...LaVoie