906
FXUS65 KRIW 230921
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
321 AM MDT Thu May 23 2024

.KEY MESSAGES...

- Snow continues for the mountains today, with the highest
  amounts in the Bighorns.

- Strong wind is likely this afternoon, with a potential for a
  frost or freeze in some areas tonight.

- Saturday and Sunday still look unsettled and cool, although
  details remain in flux.

&&

.DISCUSSION...
Issued at 319 AM MDT Thu May 23 2024

Winter has returned to portions of the area this morning, but this
is not an unusual occurrence in western and central Wyoming in May.
Decent snow has already fallen in the western mountains, with Grand
Targhee already showing over 6 inches of snow around 2 am. Snow has
also occurred in the lower elevations. Webcams show accumulation is
only on the grass though, the roads look largely wet or maybe
having a some slush.

The low pressure area that is causing this is now seen rotating over
eastern Idaho. It will move across northern Wyoming today and shift
the heaviest precipitation East of the Divide today. Reasoning today
remains similar to yesterday, The heaviest snow will fall in the
Bighorn range. The heaviest will be in the northern Bighorns,
especially north of Granite Pass. The question is the difference in
accumulation vs impacts. Areas north of Granite pass do have a
greater than 1 in 2 chance of over a foot of snow, which is warning
criteria. However, areas south the chance is much less, with only a
1 in 10 chance at Powder River Pass and no chance further south. In
addition, the heaviest snow is expected between 9 am and 3 pm. In
other words, during the time of peak solar insolation. And, we are now
less than one month from the summer solstice. As a result, a lot of
the snow may have trouble sticking to roads during this time unless
it is very heavy. As a result, impacts will be less. This, plus the
fact that over 85 percent of the zone has less than a 1 in 2 chance
of warning amounts, we will stick with the advisory. Some moisture
will also wrap back into western Wyoming. However, here as well,
accumulation on paved surfaces will be limited. We will maintain the
winter highlights and Special Weather Statements as is for now.

Elsewhere, precipitation will be mainly rain. There could some brief
mixing with snow at times, but no impacts. Another concern will be
wind. This will be the favored area. Mainly from Rock Springs to
Casper earlier in the day and afternoon, and then across Johnson
County in the afternoon after the low passes and wind turns
northwest. There are a few 50 knot wind barbs at 700 millibars,
However, these occur rather briefly. For now, we will issue Special
Weather Statements to cover for this. There could be a stray gust to
60 mph in the favored areas, like Wyoming Boulevard or the Buffalo
airport, but it does not look widespread. There is a greater than 1
in 2 chance of wind gusts to 50 mph though.

All precipitation should end this evening as the low. And then we
have one more concern. That is for the potential of a frost or a
freeze in areas where that is a concern. This is mainly the Bighorn
Basin at this time. It is borderline but in some locations there is
a greater than 1 in 2 chance of temperatures falling to 32 or below.
It does not look like a hard freeze though. Clearing looks to be
fairly certain. The question is if wind can decrease enough. Still
questions on this. We will issue a freeze watch for now and let the
day shift decide if they want to upgrade or not.

Friday at this point looks quiet with transitory ridging across the
area. The quiet weather won`t last though. The third and last in the
series of lows will move onshore in the Pacific northwest for Friday
night. A weak shortwave ahead of it could bring a few showers later
Friday night across southern Wyoming. The models continue to
struggle with the details of this system. Saturday looks fairly
unsettled with a scattering of showers and storms. Models then
diverge starting Sunday, some guidance moves the low away quickly,
drying things out. Others linger the moisture and unsettled weather
into Sunday. For Sunday, we tapered chances from east to west.
Uncertainty is still high on Sunday though. And even on Saturday,
details on placement and precipitation amounts are still rather
uncertain.

Ridging should then bring dry weather for Memorial Day and Tuesday.
The next system may try to approach on Wednesday with the next
chance of showers, but details this far out are always
questionable.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1028 PM MDT Wed May 22 2024

The next weather system continues to approach Wyoming tonight with
moderate to heavy precipitation impacting the western part of the
state. Rain has turned over to snow at KJAC and should remain IFR
through 07-08Z as the area of precip moves northeast. A mid-level
cold front will move west to east overnight, bringing snow and rain
to KBPI and KPNA between 06-12Z/Thu.  A low chance of showers is
possible at KRKS around sunrise too, not enough to mention in the
TAF though. Behind the cold front, surface winds will increase from
the west and gust 30-40 kts at most sites through the day Thursday.
As the storm center moves across the middle of the state, wrap-
around precipitation with SW then NW winds will bring showers to
KJAC during the middle of the day.  And then as the storm center
moves to eastern WY in the afternoon, a surface cold front will push
southward into the Bighorn Basin to bring MVFR/IFR conditions to
KCOD and KWRL with gusty north winds and rain turning to snow.  The
cold front will push into central WY around 00Z/Fri for 1-2 hours of
gusty north winds at KRIW/KLND/KCPR and a 30 percent chance of light
showers. Some east winds then push into KRKS just before 06Z/Fri.
The mountains will be mostly obscured for the TAF period with
moderate snow in the west-central mountains tonight, and moderate
snow over the Bighorns from early morning through sunset Thu.

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...
Freeze Watch from late tonight through Friday morning for
WYZ003>006.

Winter Weather Advisory until 6 PM MDT this evening for WYZ008-
009.

Winter Weather Advisory until 4 PM MDT this afternoon for
WYZ012.

&&

$$

DISCUSSION...Hattings
AVIATION...McDonald