189
FXUS65 KRIW 170744
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
144 AM MDT Tue Sep 17 2024

.KEY MESSAGES...

- Widely scattered rain showers and thunderstorms this morning
  west of the Divide that will spread east in the afternoon.

- A cold front pushes through west to east from late morning
  through mid afternoon with gusty west to north of westerly
  winds behind it, some of which could be high wind warning
  criteria.

- Cooler temperatures for the rest of the week and into the
  weekend with continued precipitation chances throughout as
  well.

&&

.DISCUSSION...
Issued at 122 AM MDT Tue Sep 17 2024

IR currently depicts some dry air intrusion across the CWA ahead
of the main upper level low pushing across Nevada. Any rain
shower activity has dissipated and pushed into Montana out of
the Bighorn Basin. An upper level shortwave will begin to push
out of northern Utah/eastern Idaho by mid morning that will
spark some widely scattered rain showers and isolated
thunderstorms west of the Divide. Another area of note will be
from Sweetwater County up to Natrona with an area of surface
convergence ahead of the cold front. This will all spread north
and east into the afternoon hours becoming more numerous with
daytime heating. Hi-Res models are picking up on leeside
cyclogenesis along the triple point off the Bighorns that will
strengthen these aforementioned thunderstorms along the I-25
corridor by 3-4PM before exiting to the east in a short hour or
two. Main concerns will be with damaging winds as the main
threat with the support a strong mid level jet over 50kts that
will aid in mixing to the surface in this area of Johnson County
east of the interstate. Otherwise, the bulk of the main energy
will be well east of the CWA where more ample CAPE and steeper
lapse rates will be.

The main concern outside of precipitation activity will be with
FROPA and wind shifting out of the west to north of west. Timing
is more aligned with the models improving confidence of pushing
out of Idaho by late morning and progressing east through the
afternoon hours exiting east by 4-5PM. On the backside of the
low, moisture will wrap back into northwestern portions of the
state with the occlusion emphasizing the stronger winds with
CAA. Wind gusts up to 50-60 mph are possible in some areas of
the Cody Foothills (funneling) and into eastern parts of Fremont
County and Natrona County as well. Other areas will see winds
up to 40-50 mph, staying below any high wind thresholds.
Precipitation will be for the northern third of the CWA with
mountain snow likely for elevations of the Winds, Tetons, and
Absarokas over around 8kft as the cold air pushes in behind the
aforementioned cold front. Nothing significant in accumulations
but a decent amount as the first bigger snow of the season.
Lower elevations will see rain showers progress east into the
Bighorn Basin after midnight and into Wednesday morning.

Precipitation will linger in these areas into the afternoon as
winds decrease by late morning and into the afternoon with a
diminishing gradient behind the occlusion. A more stratiform
type event through the afternoon and evening with the cooler
air making its way across the CWA. Afternoon high temperatures
will be in the 50s and 60s for points west of the Divide (and
some even cooler), with 60s to low 70s to the east for the
remainder of the week and into the weekend. Induced upper level
ridging ahead of another deepening low making its way down the
west coast for Thursday the the CWA remaining mostly dry for
most. The EC has the low cutting off well to the south pushing
across the Four Corner Region late week with the bulk of the
greater impacts out of the state. Regardless, continued cooler
temperatures and unsettled weather with daily precipitation
chances to include snow in the higher elevations into the
weekend and the extended forecast.

&&

.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.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from late tonight through Wednesday morning
for WYZ003-019-020.

&&

$$

DISCUSSION...Lowe
AVIATION...Rowe