084
FXUS65 KRIW 162150
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
350 PM MDT Fri May 16 2025

.KEY MESSAGES...

- A 20% chance of showers and 10% chance of isolated
  thunderstorms persists across northern WY through the
  afternoon, diminishing quickly after sunset.

- A better chance of showers and storms Saturday (30 to 50%),
  with strong storms possible over southwestern Wyoming.

- Turning much cooler Sunday and Monday with rain and mountain
  snow across much of the area.

&&

.UPDATE...
Issued at 235 PM MDT Fri May 16 2025

The forecast remains on track for the next 24 to 48 hours. The HRRR,
along with several other CAMs have been tracking well with weak
convection developing this afternoon. Convection has already begun
to develop along the Cody Foothills, but as expected is very weak,
generally just virga. By 1600-1900L there will be slightly better
instability, though still <100 J/kg SB CAPE. Thus, during that time
frame, there could be an isolated stronger storm with gusty winds to
30 to 40 mph and up to pea size hail being the primary threats. Best
chances for these stronger storms (10 to 20%) will be along the Cody
Foothills and eastern Bighorn Basin, as well as Yellowstone NP. Any
lingering convection will taper off and end quickly after sunset and
things should remain fairly quiet overnight.

Saturday morning, models remain on track with timing showers and
thunderstorms pushing into southwest WY then spreading east through
the day. Isolated stronger storms will be possible (30% to 40%
chance) across southwest WY, with gusty winds being the main threat
but small hail could also be a concern. SPC continues with a
marginal risk for Severe for that area as well, which aligns well
with local thoughts and model guidance.

&&

.DISCUSSION...
Issued at 317 AM MDT Fri May 16 2025

Active weather will continue as we head through the next several
days. We do have signs summer is coming though. The first sign
is the predawn light when I go out to prepare the weather
balloon for launch. This is around 4:45 am. Another sign is
twilight still in the sky when I drive to work around 9:45 pm
for my favorite shift, the midnight shift. (Yes, I am one of
those weird people who prefers working at night). It will not
feel like summer the next several days though, and some
locations will feel like winter at times, especially early next
week.

Today looks like one of the least active days of the next several.
Shortwave ridging will move across Wyoming and bring a lull in the
persistent showers and storms. There will be a weak trailing
shortwave that could being some showers this afternoon and
evening, but these would be restricted to northern Wyoming and
most of the population centers should stay mainly dry.
Temperatures will also be somewhat warmer, but still only around
5 degrees below normal for most. And the shortwave will bring
gusty wind to Johnson County for another day, but there should
not be 60 mph gusts like yesterday. Ensemble guidance only gives
a 1 in 3 chance of gusts past 40 mph for most at this time.

Things start getting more active on Saturday, as an upper level
low moves onshore in the Pacific Northwest and increases
moisture across the area once again. Saturday should start dry
for most, but coverage of showers and thunderstorms will
increase as the low approaches. And things may get a little
interesting. There will be decent upper level divergence across
the area, and some morning sunshine may destabilize the
atmosphere somewhat. Lifted indices will be negative and CAPE
values will increase to 500 J/kg at times across southwestern
Wyoming. The Storm Prediction Center has put this portion of
Wyoming under a Marginal risk, with the main threat being
strong wind gusts. The most numerous storms will be across
western Wyoming. In areas east of the Divide, southwesterly
flow will raise some high temperatures to around 70 degrees. It
is also likely to keep most areas dry most of the time, with
the average chance of a shower or thunderstorm around 1 out of
4 in this area.

The aforementioned upper level low will then dive southeastward
through the weekend and into Monday, with lee cyclogenesis likely
occurring Sunday night into Monday in eastern Colorado and lifting
into the Plains, with another upper level low developing over the
western states. As a result showers will become more numerous
across the area on Sunday, with another chance of afternoon
thunderstorms, this time with thunder mainly east of the Divide
where the best instability will be. And this will be the day we
may make a transition back into winter for some locations. As
flow turns northerly behind the low, much colder air will move
in. Some model guidance drops 700 millibar temperatures as low
as minus 5C across western Wyoming Sunday night, and this could
lead to snow levels falling as low as 6000 to 6500 feet,
possibly bring a coating of snow to the western valleys. The
colder air then spreads east of the Divide Sunday night and
especially Monday. And this is where the bulk of the
precipitation will be that day. The air won`t be quite as cold
here, with 700 millibar temperatures generally bottoming out at
around minus 7C, which would put snow levels around 5000 feet.
And, given the position of the low and north-northeasterly flow,
the Bighorns would be the most impacted area. Some
probabilities for over 6 inches of snow include around 3 out of
4 in the higher elevations of the Bighorns and Wind River Range,
with about a 3 out of 5 chance in the Tetons and Absarokas
through Monday night. Chances of a foot are generally only 1 out
of 2 at the very highest elevations so if highlights are needed
it would likely be Advisories. This also looks to be a wet
storm, with most locations east of the Divide and the northern
mountains having at least a 1 out of 2 chance of a half an inch
or more of QPF and favored north- northeast upslope locations
like Buffalo, Thermopolis and Lander having a 1 in 2 chance of
an inch of QPF or more. As for flooding, areas northeast of a
Casper to Cody line have a marginal risk of excessive rainfall
on Sunday and areas along and east of Interstate 25 have one for
Monday. The rain will fall over a fairly long period though. In
addition, with the colder air moving in, rain on snow is not
likely as the higher elevations would see mainly snow. We will
watch it though. Monday will be the coldest day of the week,
with many locations having highs in the 40s.

We should begin to dry out as the low moves away on Tuesday,
although some lingering moisture will keep a few showers around,
especially in eastern Wyoming. Flat ridging should then hold
for the rest of the period, bringing more seasonal temperatures.
Another shortwave may bring some showers and storms by midweek,
but timing is all over the place this far out.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 349 PM MDT Fri May 16 2025

VFR flight conditions are expected this TAF period at all
terminals. Low chances (30%) of showers continue at KCOD and
KJAC through 02Z Saturday morning. At KWRL, isolated light rain
cannot be ruled out (30% chance) through 06Z. Shower activity is
isolated so confidence in direct impacts is low at the
aforementioned terminals and this is reflected in the PROB30
groups. Confidence is high (80%) that conditions should remain
VFR with any light rain.

Shower and thunderstorm chances increase again on Saturday after 15Z
at KRKS followed by KJAC, KPNA, KBPI, and then KCOD. There is
uncertainty in timing of convection this far out and this
uncertainty is communicated as a PROB30 group. Confidence in
shower and thunderstorm activity at KRIW, KLND, KWRL, and KCPR
is less than 20% after 20Z so opted to keep out of TAF at this
time.

Otherwise, expect gusty winds to diminish at the west of the
Divide terminals and KCPR by sunset tonight. Winds increase
with gusts around 20kts at KCPR and KBPI around 17Z 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...
None.

&&

$$

UPDATE...Hensley
DISCUSSION...Hattings
AVIATION...Gerhardt