513
FXUS65 KRIW 161822
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1222 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 /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1112 AM MDT Mon Sep 16 2024

VFR conditions are expected to prevail across mostly all terminals
through the TAF period. Winds will gradually increase for all sites
during the afternoon today. Gusts of 15 to 25 knots will be frequent
through the afternoon and evening. A shortwave approaches the area
from the southwest, which will increase chances for showers and
thunderstorms mainly for KJAC/KRKS/KBPI/KPNA/KCPR this
afternoon and evening. However, overall confidence in any
showers or thunderstorms impacting terminals remains low. The
main concern today will be strong gusty outflows created by any
nearby developing showers and storms. Erratic outflows exceeding
40 knots are possible this afternoon and evening as convection
increases. Winds are expected to remain breezy through much of
the overnight into the early morning hours Tuesday. This looks
to be the case especially at KLND/KRKS/KCPR with KRIW possibly
seeing a light breeze linger into Tuesday morning. A cold front
will move into the state Tuesday bringing with it gusty winds
and chances for showers and thunderstorms. Shower and
thunderstorm groups have already been put into the TAFS for
KRKS/KJAC for the end of the TAF period. Confidence in the
timing of the fronts arrival still remains low at this time. Due
to this uncertainty, the remaining TAF sites will likely see
PROB30 groups be added in for the next TAF issuance.

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 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...Dziewaltowski
FIRE WEATHER...LaVoie