743
FXUS65 KRIW 121121
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
521 AM MDT Wed Mar 12 2025

.KEY MESSAGES...

- Another dry and mild day expected today.

- Snow spreads into western Wyoming early Thursday morning,
  spreading East of the Divide later Thursday night and Friday.

- Strong wind is likely later tonight and Thursday morning, and
  again Friday morning in portions of the area.

&&

.DISCUSSION...
Issued at 225 AM MDT Wed Mar 12 2025

After a stretch of dry and mild weather, the knock down, drag out
battle between Winter and Spring that March is will commence. We
have a few events on the card, with some uncertainty now regarding
the possible main event for early next week. And given some
contrasts in the weather, we will split the discussion into West and
East of the Divide.

West of the Divide... No real concerns today as we will have one
more dry and mild day, although it will be a bit breezier in some
locations. The stretch of quiet weather will come to an end later
tonight but especially Thursday as a Pacific system approaches the
area. Some lighter snow will likely break out after midnight but the
bulk of the snow holds off until after sunrise Thursday. Periods of
snow then continue through the day and into night before tapering
off to showers before sunrise Friday. At this point, the steadiest
and heaviest snow looks to fall later in the afternoon and into the
evening when the cold front moves through and the area gets a bit of
right rear quadrant jet dynamics to enhance lift somewhat. Overall
snow amounts look moderate in the mountains, so we issued advisories
in areas where there is a greater than 1 in 2 chance of 6 inches or
more over the period according to the NBM ensemble. As for the
valleys, southwest flow ahead of the front will keep the area fairly
mild during the day Thursday with 700 millibar temperatures from
minus 3 to minus 4 which would put snow levels around 6500 to 7000
feet. In addition, the strong mid March sun would likely keep most
roads mainly wet through the day. Snow levels will fall to the
valley floors with the frontal passage. However, the chance of 3
inches or more in the valleys is less than 1 in 10 through the
period so no highlights here.

Snow showers will continue into Friday, but any accumulation here
looks to remain on the light side. Another trough will approach the
area Saturday with some more snow showers, but amounts look to
remain sub advisory at this time. Another shortwave then approaches
for Sunday, again with light accumulations of snow possible.
Another, potentially stronger system may then approach Monday or
Tuesday with a chance of more substantial snow. However, models
agreement is poor right now so confidence in any solution remains
rather low.

East of the Divide...Much like western Wyoming, we have few concerns
today as it will be another mild and dry day across the region.
Concerns start to creep in tonight though. The concern is for strong
wind. With the approaching Pacific system, the pressure gradient
will tighten up and southwest flow will increase. This will bring
gusty to strong wind in the favored areas, mainly from Muddy Gap
through Casper. This looks like a situation similar to Monday night,
except it would be largely restricted to central Wyoming.
Probabilistic guidance is showing a greater than 1 in 2 chance of
wind gusts past 50 mph in the favored areas on the south side of
Casper. This appears likely with the 700 millibar winds increasing
to around 50 knots after around 3 am Wednesday night. This does look
like a brief occurrence though, as the 700 millibar winds weaken
after 9 am. Wind will decrease somewhat following that, but it will
remain rather blustery through the day. For now, we will not issue
high wind watches since the chance of high wind (gusts to 65 mph on
south side of Casper) is less than 1 out of 6. There could be
isolated gusts to around 65 mph, but it shouldn`t be widespread. The
day crew can reevaluate this if needed. The rest of Thursday looks
like another mild and dry day. Southwest flow should downslope
enough to keep the area dry until around midnight Thursday night.

The main time of concern here for snow is from midnight Thursday
night through around noon on Friday. At this time, the cold
front will swing across the area and turn flow northerly across
the area. Precipitation will break out across northern Wyoming
after midnight and spread southeastward through late Thursday
night. There could be some rain at the beginning of the
precipitation before everything changes to snow by sunrise. The
heaviest snow looks to follow the left front quadrant of a 60
knot jet streak that will move west to east across the area.
With the northerly flow, the highest amounts of snow would be in
the favored upslope locations, mainly the Cody Foothills, the
Lander Foothills and toward Casper. This is a fast moving
system though, with most of the snow falling in around a 6 hour
period for most locations, this being generally 5 am to 11 am.
Impacts are up in the air though. The main time of concern
would likely be from around 5 am to 9 am when there could be
some accumulation on the roads. After that, the high March sun
angle should improve conditions. The other concern is wind,
mainly in the northwesterly flow / cold advection areas like the
northern Bighorn Basin, Johnson County and the Wind River
Basin. This looks especially possible around Buffalo, where
there is a 1 in 2 chance of wind gusts past 50 mph from around 5
am to 12 pm on Friday. This could lead to some blowing and
drifting snow for a time, leading to difficult travel
conditions. As for snowfall amounts, this storm could be
dependent on elevation. The best chance for 2 inches or more
(about a 2 out of 3 chance) looks to be in higher elevations
between Riverton and Casper, especially above 6000 feet.
Elsewhere, the chance is less than 1 out of 3. As for
highlights, we will hold off for now but take another look
tomorrow night to see if they are needed.

The heaviest snow should end by noon with only a few leftover
showers. Saturday at this point looks like a dry but chilly day with
below normal temperatures across the region. The approach of another
Pacific system  may bring another period of strong to potentially
high wind to the southwestern Wind Corridor later Sunday into Sunday
night. There continues to be the potential for a more potent system
early next week but guidance really diverges and details are
impossible to determine this far out.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 520 AM MDT Wed Mar 12 2025

VFR conditions are expected through most of the period with one more
day of dry weather. Wind will increase beginning late this morning
as the next system approaches, with southwest gusts of 15 to near 30
knots through the afternoon for many terminals. Wind will subside
after sunset, but pick up again late in the period at KCPR. Ceilings
will begin to lower across western Wyoming 04-09Z Thursday.
Precipitation will move into the KJAC vicinity around 09Z Thursday,
though MVFR/IFR impacts are most likely to begin after 12Z.

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 from 6 AM Thursday to 6 AM MDT Friday
for WYZ001-012-024.

Winter Weather Advisory from 6 AM Thursday to noon MDT Friday
for WYZ014-015.

&&

$$

DISCUSSION...Hattings
AVIATION...Myers