164
FXUS65 KRIW 231745
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1145 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 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, with the heaviest snow
falling in the Bighorns range and more specifically from Granite
Pass northward. 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 of the Pass have a much lower chance, 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 occur in favored areas, mainly from Rock
Springs to Casper this morning into the afternoon, and then
across Johnson County in the afternoon after the low passes and
the wind turns northwest. There are a few 50 knot wind barbs at
700 millibars in this area, however these occur rather briefly
late this afternoon into the evening. 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 moves over
the Black Hills and into the Dakotas. Then we have one more
concern for tonight. That is for the potential of a frost or a
freeze. This looks to be mainly the Bighorn Basin at this time.
It is borderline, but some locations here 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 fast enough
to allow for temperatures to plummet to freezing or below.
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 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 /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1145 AM MDT Thu May 23 2024

The last wave of the upper low pressure system will push through
today. Showers will mostly be confined to the north (KJAC, KCOD,
KWRL terminals), though there is a chance (20%) that KPNA, KRIW, and
KCPR terminals could see some brief rain showers this afternoon and
evening with best chances (50%) at KRIW. MVFR conditions are
expected with the showers at the aforementioned terminals. KJAC will
see IFR conditions with snow showers through the first few hours of
the period, with conditions improving to MVFR and prevailing as such
through the rest of the afternoon. All terminals should see
prevailing VFR conditions and cloud cover beginning to scatter out
by 06Z tonight.

Winds will be gusty through much of the afternoon at most terminals
this afternoon. A northerly push will continue to move through the
Bighorn Basin and impact KRIW and eventually KCPR terminals around
00Z-02Z. This northerly push will be associated with a cold front
and accompanying rain showers. Winds will decrease overnight (by
06Z) and increases in winds Friday morning will be limited to the
favored locations (KPNA and KCPR).

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...Hensley