129
FXUS65 KRIW 141800
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1200 PM MDT Mon Jul 14 2025

.KEY MESSAGES...

- Isolated to widely scattered showers and thunderstorms across
  northern Wyoming today, some producing strong wind gusts. The
  best chance for thunderstorms is from 3PM to 8PM MDT.

- Elevated fire weather conditions across southwest Wyoming
  Monday given above normal temperatures, minimum relative
  humidity around 10 percent, and westerly wind 15 to 25 mph.

- A better chance for showers and thunderstorms Tuesday. Areas
  east of the Continental Divide could see strong to possibly
  severe storms and the potential for brief heavy rain.

- Cooler Wednesday, especially east of the Continental Divide,
  behind a Tuesday evening cold front.

&&

.UPDATE...
Issued at 1200 PM MDT Mon Jul 14 2025

A shortwave seen over central Wyoming in water vapor imagery midday
Monday tracks east through Monday afternoon. Dry and breezy
conditions along with above normal temperatures across southwest
Wyoming leads to elevated fire weather concerns until 8 PM
Monday. An area of better moisture over northern Wyoming along
the northern periphery of the shortwave will aid convective
development Monday afternoon. Storms form first over the
Absaroka Range and eventually the Bighorn Range before moving
east into the basins and plains. Bulk shear values favor
stronger storms across Johnson County where SPC has a marginal
risk of severe thunderstorms. Temperature-dew point spreads
range from 40-50F across northern Wyoming Monday afternoon and
evening, so gusty outflow wind would be the primary hazard.
Convection shows a downturn in coverage and intensity between 8
and 10PM Monday. However, a second shortwave dropping south from
Canada toward the Northern Rockies should channel moisture over
the north into Tuesday morning. This allows for lingering light
showers across this region overnight.

The northern stream shortwave drops into the Northern Rockies
Tuesday and helps push a cold front south into the state. The
front impacts areas east of the Continental Divide Tuesday,
providing a northerly wind, cooler air, and increased surface
moisture. Meanwhile, increasing west-southwest flow aloft
transports a batch of moisture across the state by afternoon.
This sets the stage for scattered showers and thunderstorms,
less numerous across the far south, Tuesday afternoon into the
evening. Storms west of the Continental Divide earlier Tuesday
afternoon will remain high-based, with inverted-V soundings
suggesting gusty outflow wind as the primary hazard. East of the
Divide, the increased moist, upslope low-level flow favors a
better chance of stronger thunderstorms by mid-afternoon. Bulk
shear, a 500mb 50kt speed maximum, and surface dew points 40-50F
all point to the potential for strong to potentially severe
thunderstorms east of the Divide. SPC has a marginal day 2 risk
for a chunk of this region. Additionally, high precipitable
water values of 0.8 to 1.0 inches promotes the idea of heavy
rain potential, which is noted in this region within the day 2
WPC excessive rainfall outlook. All-in-all, Tuesday is shaping
up to be more active than the recent week.

&&

.DISCUSSION...
Issued at 159 AM MDT Mon Jul 14 2025

The weekend was of the nice, summer variety. One warm day and one
hot one with light to moderate wind and lots of sunshine for most
locations. Things do turn more active today though, and this will
last for the next three days. And they fall into two camps, fire
weather and thunderstorms.

As for today, we will have both concerns. We do have a small area of
showers and embedded thunder moving into Yellowstone Park but this
should be gone by morning. A cold front will be dropping through
Montana today but will remain north of the state today so it will be
another hot day, possibly a bit hotter with some southwest flow
ahead of it. A few of the warmer spots, like Greybull and Worland
may make a run at 100 degrees today and even Casper could get close.
In addition, it will become breezy ahead of the front, And this
brings us to the fire weather concerns. Humidity will be plenty low,
falling to around 10 percent in some locations. Wind remains
marginal though. Latest guidance actually has it a bit lighter than
last time at this time. So, we have made the decision not to issue
and Red Flag Warnings, since any critical fire weather would be
brief. Elevated fire weather looks like though for roughly the
southern half of the area though. And this brings us to the
thunderstorm aspect. We have a marginal risk across the northeastern
half of Johnson County, where the atmosphere will be a bit more
moist and could bring some hail. The main threat by far today will
be strong wind gusts. We have a very good set for strong to possibly
high convective winds. A cold frontal boundary and shortwave
approaching to increase instability. Mid level moisture will be
increasing as well. Lower levels remain very dry as well very large
dew point depressions, over 60 degrees in some cases. Model
soundings are showing some very large inverted V signatures as well.
So, any thunderstorm, shower or even cumulus cloud could produce a
wind gust past 50 mph when it the updraft collapses. This would be
largely across northern Wyoming this afternoon and evening. Coverage
looks small though, with any location having at most 1 out of 4. But
where they occur even innocent looking showers could have a strong
wind gusts. All in all, a good set up for one of our "Little Green
Blob" days.

Thunderstorms will remain possible on Tuesday, although the main
threats will shift. For one, storms look more widespread across
roughly the northern half of the area. Instability parameters also
looks more impressive, with CAPE as high as 1500 J/Kg and limited
indices to minus 5. The big change will be moisture though. Much of
the area may have precipitable water values approaching 1 inch East
of the Divide and even over. For example, models give around 0.95
inches at Riverton. For reference, the mean this time of year is
around 0.60 inches. This puts Riverton over 150 percent of average
with this amount around the 95th percentile. So, this could lead to
locally heavy rain. There will be some steering flow that will
mitigate this somewhat but local flooding is a possibility. Shear
isn`t the best I`ve seen but there is some so we could have some
tilted updrafts and some larger hail. The main threat of this would
be in Johnson  and Natrona County, where we have a marginal risk
along with low level east to easterly flow to bring upslope and aid
in storm initiation. This will be a cooler day as well, anywhere
from 10 to 25 degrees cooler depending on where the front is. This
is also where the strongest storms would be, wherever the front is
in the afternoon. As for fire weather concerns, these will continue
across the south. However, somewhat cooler temperatures and higher
humidity may mitigate this to a certain degree.

The threat of thunderstorms will continue on Wednesday.
However, models show less of a chance on this day. Most guidance
shows drier air moving in from the west that will drop
precipitable waters about 25 percent across the area. Most areas
East of the Divide will continue to have a chance of a shower
or storm but the greatest chance will be in the eastern
counties. This will also be the coolest day. MOS guidance
continues to have highs in the 50s for Buffalo but I still don`t
buy it since any sort of sun that day would warm things up. It
could should stay in the 60s though. This will be the coolest
day across the area though, with high temperatures East of the
Divide anywhere from 10 to as much as 25 degrees below seasonal
averages. Warm temperatures will continue West of the Divide
though with more elevated fire weather possible for Wednesday.

Flat ridging then controls the weather from Thursday into the
weekend. This will bring a return to near to somewhat above normal
temperatures. Thursday at this point looks largely dry. Following
that, a couple of shortwaves will bring chances of diurnal
convection, but timing of these is still uncertain. With the flat
ridging, breezy conditions will be common and could lead to elevated
to critical fire weather on any of those days.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1200 PM MDT Mon Jul 14 2025

VFR flight conditions are anticipated a majority of this TAF period
at all terminals. Hi-res models continue to indicate shower and
isolated thunderstorm chances across northern WY this afternoon. It
is still looking likely (70% probability) that these will primarily
be virga. KCOD has the most likely chance of seeing rain or TS
impacts at the terminal, and thus have continued with the PROB30 for
the afternoon, with no changes to the original timing. Have also
added a TEMPO for KWRL for gusty outflow winds, as CAMs have
consistently been keying in on outflow wind impacts near that site.
Otherwise expect breezy winds at most terminals with gusts around 20-
25kts at KJAC, KBPI, KPNA, KRKS, KCPR, and KWRL after 19Z through
around sunset.

At KCOD, low ceilings begin to develop with gusty north winds around
10Z Tuesday with the arrival of a cold front. Confidence in MVFR
ceilings has increased slightly from the previous forecast, but
remains too low to include prevailing MVFR, so have continued to
communicated potential with a FEW030 at this time. Gusty north to
northeast winds are expected at most east of the Divide terminals
behind the front with gusts expected to be generally 20 to 25kts
with locally stronger gusts possible particularly at KWRL.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on smoke, icing, and turbulence
forecasts.

&&

.FIRE WEATHER...
Issued at 159 AM MDT Mon Jul 14 2025

A cold front will move toward Wyoming today, bring locally
breezy conditions across central and southern Wyoming. With
relative humidity falling under 15 percent, elevated fire
weather is likely this afternoon. Wind gusts over 25 mph do not
look widespread enough for critical fire weather though.
Elevated fire weather will remain likely in southern Wyoming on
Tuesday, with improve conditions in areas East of the
Continental Divide.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...CNJ
DISCUSSION...Hattings
AVIATION...Hensley
FIRE WEATHER...Hattings