127
FXUS65 KRIW 272320
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
520 PM MDT Fri Jun 27 2025

.KEY MESSAGES...

- Elevated to critical fire weather continues across much of the
  area through this evening. A Red Flag Warning is in effect
  today for portions of Natrona County from noon to 9PM local.

- Isolated to scattered shower and thunderstorm potential
  exists across northern Yellowstone National Park this
  afternoon. A shower or thunderstorm could produce gusty winds
  around 50 mph.

- Elevated fire weather remain likely on Saturday. Isolated
  shower and thunderstorm potential (1 out of 4 chance) exists
  across Sweetwater, Natrona, and Fremont Counties late Saturday
  afternoon and evening.

- Afternoon showers and thunderstorms are possible Sunday and
  again Tuesday through next Friday across portions of Wyoming,
  but the chance is generally around 1 out of 5 at any
  particular location.

&&

.UPDATE...
Issued at 1212 PM MDT Fri Jun 27 2025

The forecast largely remains on track with a weak shortwave trekking
across the region today with Wyoming in the right exit region of the
upper-level jet over Montana. This will support isolated convection
across the northern portions of Yellowstone National Park this
afternoon. Showers or thunderstorms could produce gusts around
50 mph. Elevated to critical fire weather conditions this
afternoon with breezy to windy conditions, aided by the
placement of the upper- level jet and deep daytime mixing,
combined with low relative humidity values across the Wind
Corridor, the Green River and Wind River Basins, and the eastern
Bighorn Basin. A Red Flag Warning remains in effect now (noon)
through 9pm this evening for portions of Natrona County (Zone
280), where fuels are critical.

The main changes made this forecast package are for Saturday
(tomorrow). Elevated to critical fire weather conditions are still
expected for the Wind Corridor and the Green River and Bighorn
Basins. However, another weak shortwave, with increased PVA ahead of
it, is forecast to trek across Wyoming late Saturday afternoon. This
weak support aloft combined with increased mid-level moisture could
allow for isolated (around 25% chance) high based thunderstorms
mainly across Sweetwater, Natrona, and Fremont Counties. Given we
are still expecting dry low-levels, anything that does develop will
largely remain as virga. That being said, with dew point depressions
of 50-60F, gusty winds with any virga and high based storms could be
50-60 mph. Red Flag Conditions are NOT met at this time with wind
gusts below 25mph across portions of Natrona County/Zone 280.
However, since conditions will be critical Saturday afternoon into
the evening across the aforementioned area, gusts with dry
convection should be monitored.

&&

.DISCUSSION...
Issued at 250 AM MDT Fri Jun 27 2025

All in all, there is little change in the reasoning of the
forecast this morning when compared to yesterday.

The main concern of the day is fire weather, both of the elevated
and critical variety. The set up looks the same, with a weak
shortwave moving across the area and a jet streak across Montana,
putting the area in the right front quadrant, enhancing downward
forcing. This will mix some mid level wind to the surface, bring
breezy conditions to much of the area. If this was August, we would
likely have fairly widespread Red Flag Warnings. However, most
locations don`t have critical fuels yet. Regardless, fire
weather conditions will be elevated to critical across the CWA.
Natrona County still looks good for the Red Flag Warning, so we
continued it pretty much as is. We did given some thought to the
other critical zone, 282, aka Washakie and Hot Springs County.
However, humidity will be a bit higher here and wind will not be
as gusty. A spot or two may get Red Flag conditions, but most
shouldn`t so we won`t issue. Critical conditions are likely in
southern Wyoming and portions of central Wyoming where humidity
will fall toward 10 percent as well. As for convection, a few
thunderstorms are possible along the northern border but the
chance is less than 1 out of 5. Any shower or thunderstorm would
be high based though, and with very high dewpoint depressions,
any shower or thunderstorm could have a wind gust of 50 mph.

The next question was if we should extend the Red Flag Warnings to
Saturday. For now, we held off. Most guidance is showing
dewpoints running 5 to 10 degrees above those on Friday. In
addition, the jet will shift a bit further north and wind should
be a bit lighter, making conditions more borderline. If trends
favor warnings, we can issue this afternoon or tomorrow morning.
Another shortwave moving through will bring a somewhat higher
chance of showers and storms across the north. However, the
chance will again be less than 1 out of 4.

A weak cold front then moves across the area Sunday. Moisture is
limited, but it could again bring isolated storms, with the best
chance in eastern portions of the area. It will knock temperatures
down a few degrees east of the Divide and raise dewpoints a bit
more, so the chance of Fire Weather headlines looks unlikely at
this point (less than a 1 out of 3 chance).

Ridging then builds back northward over the area for early next
week. Monday at this point looks like a nice day with near normal
temperatures and little chance of storms. Chances may increase
Tuesday and especially Wednesday as deeper moisture may try to
return to the area. Exact details are difficult to hash out
this far out, but PoPs have increased on these days.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 519 PM MDT Fri Jun 27 2025

VFR conditions and lighter winds prevail overnight. Diurnally
driven winds increase Saturday late morning to early afternoon.
Gusts of at least 15 to 25 knots are expected across most areas
Saturday afternoon. Some high-based showers are possible around
KCPR, KLND, KRIW, and KRKS after 21z. The biggest threat with these
would be gusty outflow winds. The latest HRRR model has gusts 50 to
60 knots around KCPR with these showers.

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

&&

.FIRE WEATHER...
Issued at 1212 PM MDT Fri Jun 27 2025

Elevated to critical fire weather conditions continue into this
evening across the Wind River and Green River Basins, the eastern
Bighorn Basin, and the Wind Corridor with low relative humidity
values ranging from 8-15% and gusty winds 15-35 mph. A Red Flag
Warning is in effect from noon to 9PM local today for Zone 280,
where fuels are dried and conditions are met.

Similar low relative humidity values are forecast again on Saturday
across portions of the Wind River and Green River Basins and the
Wind Corridor. However, wind gusts over 25 mph are expected to be
isolated across the area, especially across Zone 280, so no
highlights have been issued at this time. Nonetheless, elevated to
critical fire weather conditions will exist Saturday afternoon.
There are isolated dry shower and thunderstorm chances (around 25%)
across these areas, especially Zone 280, late Saturday afternoon
that will be capable of producing 50-60 mph gusts.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 9 PM MDT this evening for WYZ280.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...Wittmann
FIRE WEATHER...Gerhardt