551
FXUS65 KRIW 141122
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
522 AM MDT Mon Jul 14 2025

.KEY MESSAGES...

- Isolated to widely scattered showers and thunderstorms across
  northern Wyoming today, some with strong wind gusts.

- A more widespread chance of showers and thunderstorms for
  Tuesday and Wednesday, with a few strong storms possible.

- Elevated fire weather likely today and tomorrow, and again
  late in the 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 /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 520 AM MDT Mon Jul 14 2025

VFR flight conditions are anticipated a majority of this TAF period
at all terminals. At KCOD, there is a chance (30%) of thunderstorms
between 20Z Monday and 02Z Tuesday and this is reflected in a
PROB30 group. Confidence in afternoon thunderstorms at all other
east of the Divide terminals and KJAC is around 10%-20% so
there is no mention in TAF at this time. Most
showers/thunderstorms will be virga and will be capable of
strong downdrafts and gusty outflows around 35-40kts. If a heavy
downpour directly impacts a terminal, a very brief period of
MVFR visibilities cannot be ruled out.

Otherwise expect breezy winds at all terminals with gusts around 20-
25kts at KJAC, KBPI, KPNA, 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 is too low (around 20%) so communicated
potential with a FEW030 at this time.

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.

&&

$$

DISCUSSION...Hattings
AVIATION...Gerhardt
FIRE WEATHER...Hattings