825
FXUS65 KRIW 031845
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1245 PM MDT Fri May 3 2024

.KEY MESSAGES...

- Snow ends this afternoon, with a clearing sky overnight.
  Patchy fog is possible (20%) for basins and near Casper due to
  recent moisture (snow).

- High pressure Saturday brings warmer temperatures and dry
  weather. Warmer Sunday.

- Gusty south wind Sunday. Gusts 30 to 50 mph for most of the
  area. Strongest winds over southern Sweetwater County, with
  high winds (gusts greater than 58mph) likely (80%). Impacts to
  I-80 and South Pass due to strong crosswinds.

- Precipitation moves in Sunday, with widespread precipitation
  Monday. Snow levels drop to around 6000 feet. Cool and
  unsettled weather pattern through next week.

&&

.DISCUSSION...
Issued at 1238 PM MDT Fri May 3 2024

Snow has been falling across the area this morning as a cold front
passes through. As the sun rises through the afternoon, any snow
that has accumulated on grass should melt quickly. Clouds linger
through the rest of the day, limiting high temperatures today. Many
places peak in the upper 40s today, with the Bighorn Basin seeing a
few places hit 50. These temperatures are roughly 10 degrees below
normal. Any remaining precipitation comes to an end by around 03Z
(9pm).

As the trough moves eastward tonight, transitory ridging builds in
for the weekend. Starting with Saturday, the day brings warm
temperatures and a mainly clear sky (a stark difference from today).
High temperatures for many reach the mid-to-upper 60s, with a few
places hitting 70. A slight breeze develops in the afternoon (10-25
mph gusts), but should otherwise be a good day for any outdoor
activities or chores.

The high pressure shifts eastward for Sunday as a potent upper-level
low swings into the northern California area. This brings a tight
pressure gradient, as well as a 100 knot jet positioned into
southern Sweetwater County. A strong south wind develops after
sunrise, and especially for the afternoon and evening across
Sweetwater County, northward into most of the CWA (exception being
far northwest Wyoming). Where the exit region of the jet is
(southern Sweetwater County), gusts up to 70 mph are possible if the
upper levels are able to fully mix down. Shift of tails highlights
this area in the 90th percentile. Elsewhere, widespread gusts 35 to
50 mph occur. Being a south wind, roads like I-80, South Pass/Red
Canyon, and US-26 will be impacted as strong crosswinds occur.

Precipitation chances move into western Wyoming Sunday morning,
spreading east of the Divide for the afternoon and evening, with
about a 20% chance of thunderstorms. Snow levels begin around 9000
feet Sunday, but drop to roughly 6000 feet through Monday morning.
Therefore, lower elevations are expected to receive mostly rain from
this system. That upper-level low moves into the Utah/Idaho/Wyoming
area Sunday night and into Monday. Winds begin to shift to a more
southwesterly direction, decreasing the gusts some from the day, but
gusty winds continue through the night, especially from Sweetwater
County up through Natrona County.

For Monday, the center of the low is projected to move across
Wyoming, then traveling northeastward near the Montana/North Dakota
border. The GFS and ECMWF vary a bit in placement on this.
Widespread precipitation occurs Monday. The heaviest precipitation
will be where favored upslope occurs. Currently, this would be for
the Bighorn Mountains and Johnson County, but as mentioned, there is
still some uncertainty in exact storm track, which could alter where
the best precipitation totals are favored. Will need to watch
closely the next few days on where models agree on the storm track.

The low is slow to move, as ridge over the eastern half of the
country keeps it mostly in place. This would keep precipitation
chances over the area Tuesday, especially over the northern
half of Wyoming. Currently, models are keeping the influence of
the low through Friday, so a cool, cloudy, and unsettled next
week is in store.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1139 AM MDT Fri May 3 2024

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals.

An upper-level low is rotating through southwest Wyoming late Friday
morning. Cold core and added lift will generate scattered snow
showers and potentially brief MVFR conditions until around 21Z.
In general, terminals remain VFR through the period. Gusty west
to northwest wind 15-25kts becomes widespread by mid-afternoon
on the backside of the upper low. Wind speeds diminish early this
evening and lingering mid-cloud decks give way to a clearing
sky. Mountain tops frequently obscured above 10K feet MSL until
01Z/Saturday.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals.

Moisture axis streaming across central Wyoming coupled with jet
energy will support IFR/MVFR at KCPR until 20Z-21Z Friday.
Improvement is already being noted at KRIW and KLND where the
snow ends and VFR returns between 18Z-20Z Friday. KCOD sees VFR
conditions develop during that same time period. Mid-cloud
decks will clear between 23Z-02Z Friday evening. A clearing sky
and sufficient boundary layer moisture may allow for fog
development at KCPR and KRIW between 10Z-14Z/Saturday. This will
need to be monitored for inclusion with the next routine TAF
issuance. Mountains frequently obscured until 21Z, with tops
obscured until 01Z/Saturday.

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...
Winter Weather Advisory until 3 PM MDT this afternoon for
WYZ015.

&&

$$

DISCUSSION...Wittmann
AVIATION...CNJ