502
FXUS65 KRIW 241736
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1136 AM MDT Fri May 24 2024

.KEY MESSAGES...

- Mainly dry and milder today.

- A half and half 3 day weekend on tap: Saturday unsettled,
  Sunday less wet but breezy to windy; nice on Monday.

- Warmer for midweek. More uncertainty for chances of convection
  for the middle and end of next week.

&&

.DISCUSSION...
Issued at 309 AM MDT Fri May 24 2024

The storm that brought the variety of weather over the past 24 hours
is finally exiting stage east into the Dakotas. Shortwave ridging
will build across the area today and bring a decent end to the
workweek and entrance to holiday weekend with sunshine and somewhat
below normal but comfortable temperatures. There will still be a bit
of a breeze but nothing substantial. There is a small chance of a
shower or thunderstorm over the northwestern mountains, but the
chance is less than 1 in 5.

One fact about this part of the country is that May is typically our
wettest month on average. And Memorial Day weekend, for some reason,
is usually rather wet and sometimes chilly. And this will be the
case again, at least for part of the weekend.   A weak shortwave
will approach southern Wyoming Friday night and will have enough jet
energy for a few showers and thunderstorms, stretching across
roughly the southern third of the state. The chance of showers is at
most 2 in 5 across the far south, tapering down further north.

A Pacific cold front and trough will then begin crossing the state
on Saturday. At this point, this looks to be the wettest day of the
weekend with all areas having a chance of showers and thunderstorms.
There will be a decent amount of cloud cover. This should limit
instability a bit. There is some CAPE (generally less than 500 J/KG)
and lifted indices top out at minus 2. So, most thunderstorms will
be of the garden variety and not severe type. The most numerous
activity this day will be found in areas West of the Divide
(anywhere from a 1 in 2 to a 4 in 5 chance of showers or storms.
This chance tapers to around 2 in 5 at the most East of the Divide.
And we still have to talk about the S word, snow.. Temperatures are
not as cold with this system though, bottoming out around minus 1 to
minus 2 for the most part, so this should stay confined to the
mountains.

Sunday is starting to look more optimistic. Most guidance is now
showing this system as an open wave and not a closed low, and
progression will be faster as a result. So, areas West of the Divide
should dry out for Sunday. East of the Divide will still have a
chance for showers, but what we have in the forecast is probably
the worst case scenario. POPs will be highest across Johnson County
and the Bighorns (greater than 1 in 2), tapering to less than 1 in 5
in the Wind River Basin. The culprit for this is a shortwave
brushing by the north and some jet energy diving in from Montana.
The main thing will be some gusty wind that will develop in many
areas. This will especially be the case in central Wyoming, where
mid level convergence and the right front quadrant of the jet
enhancing downward momentum. The 700 millibar winds do not look
sufficient for high wind though.

These showers should end Sunday evening. Ridging should then build
in for Monday and bring a nice end to the 3 day weekend with
sunshine and near normal temperatures. Mainly dry conditions should
last into Tuesday as well with warmer temperatures as flow turns
southwesterly. Guidance diverges more starting Wednesday with some
models wanting to bring some shortwaves across the area with chances
of convection and other keeping activity to the north of the state.
We split the middle for now, keeping lower end POPs across northern
Wyoming with mainly dry conditions further south.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1134 AM MDT Fri May 24 2024

Relatively quiet weather expected through the first 18 hours of the
forecast period. Westerly winds gusting 20 to 25kts will be possible
at most terminals through the afternoon, diminishing by sunset.
Otherwise, VFR conditions prevail at all terminals with only FEW to
SCT mid- and high-clouds through the evening hours. Cloud cover
begins to increase overnight, especially west of the Divide as the
next shortwave pushes into the region. Models indicate two separate
waves of moisture with this system. The first, looks to impact KRKS
overnight, between 09Z and 14Z Saturday. Have kept VFR for now, but
occasional MVFR conditions are certainly possible (30% chance)
during heavier showers. There is also a 20% chance of showers
reaching KLND, KRIW, and KCPR with this wave, but have left any
shower mentions out of the TAF due to low confidence. With the
second wave, hi- res models are struggling to resolve exact
timing, but for now, no impacts are expected before the end of
the TAF period.

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...
None.

&&

$$

DISCUSSION...Hattings
AVIATION...Hensley