841
FXUS65 KRIW 281725
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1125 AM MDT Fri Jun 28 2024

.KEY MESSAGES...

- Cooler today. Gusty afternoon winds and dry air bring elevated fire
  weather conditions. A front moves through this afternoon and
  evening, bringing a gusty north wind to the Bighorn Basin and
  Johnson County.

- Elevated fire weather conditions Sunday, with above average temperatures,
  gusty winds, and very dry air.

- A weather system moves through late Sunday into Monday,
  bringing cooler temperatures and the chance of rain, mainly
  for northern Wyoming.

&&

.DISCUSSION...
Issued at 205 AM MDT Fri Jun 28 2024

A bit cooler today, with highs sitting near, to slightly below,
average for the end of June. Highs are forecasted to be in the low
70s to about 80 degrees for the lower elevations. An upper-level low
sits near the Montana/Canada border today, tracking eastward. This
keeps a tight pressure gradient. As a result, afternoon winds
gusting 30 to 40 mph are common across Sweetwater County and
portions of Central Wyoming. Western Wyoming has gusts more in the
15 to 30 mph range. As the trough continues eastward, it drops into
northern Wyoming in the afternoon and evening. This brings a
northward shift in the wind direction for places like the Bighorn
Basin and Johnson County. Gusts to 40 mph are possible as this front-
like feature passes through. This front makes its way down into
Natrona and Fremont Counties in the evening.

For rain chances today, almost everywhere remains dry with the
moisture from yesterday pushed to our east. An isolated shower or
storm is possible over far southeast Sweetwater County as the last
of the moisture exits. A few showers may also form over the
Absarokas or northern Bighorn Mountains, but should be of little
impact if they do form.

The final point about today is elevated fire weather conditions. The
dry air brings afternoon relative humidities (RH) down to the lower
to middle teens across much of Sweetwater, Fremont, and Natrona
Counties. Combined with the gusty wind described above, elevated
fire weather conditions are in place this afternoon. A Rangeland
Fire Danger Statement (RFD) is in place this afternoon and evening
for those mentioned counties.

A ridge begins to build in for Saturday. This has two main impacts.
The first is that the winds will be much lighter. The second is
warming temperatures. No rain is expected at this time, though an
argument could be made for a stray shower in southern Sweetwater
County during the afternoon.

For Sunday, ridging continues, but will be on the way out as a
trough over the Pacific Northwest approaches the area. This
increases the pressure gradient once again, with a gusty west-to-
southwest afternoon wind. With the ridge in place a couple days,
very dry air is also a factor. Afternoon RHs 8% to 15% look to be
common across most of the area. Current models have 700mb
temperatures in the 16 to 19 degree Celsius range, translating to
middle 90s to around 100 degree surface temperatures east of the
Continental Divide and highs in the middle 80s to low 90s west of
the Divide. These temperatures would be around 10 degrees above
average. All of this adds up to an elevated fire weather day.

That aforementioned trough moves through late Sunday. Some scattered
showers and thunderstorms are possible over northern portions of the
area and southeastern Sweetwater County Sunday evening. The trough`s
passage overnight brings cooler temperatures for Monday, (closer to
average) as well as continued rain chances, mainly across the
northern half of the area.

The rest of the week hinges on the development of a high over the
western U.S. Depending on its strength and position will determine how
low pressure systems move through the area. Currently, there is
potential for some low rain chances through the rest of the
workweek, mainly for the northern half of the area. This has low
confidence at this time.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1125 AM MDT Fri Jun 28 2024

All terminals to be VFR through the forecast period. A shortwave
passing north of the state nudges a frontal boundary south through
the Bighorn Basin this afternoon and central Wyoming early this
evening. Gusty westerly wind 15-30kts prevails ahead of this
boundary with similar speeds expected from the north following
passage of the boundary. KJAC will not see the strong northerly
push, although downvalley light northerly flow develops there by
late evening.

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 205 AM MDT Fri Jun 28 2024

Elevated fire weather conditions today. Minimum RHs between 10% and
15% for much of Sweetwater, Fremont, and Natrona Counties. A
Rangeland Fire Danger Statement (RFD) is in effect this afternoon
for those counties. Elsewhere, RHs down to around 20%. Gusty west
winds pick up late morning for the Wind Corridor. Afternoon gusts 30
to 40 mph. A front brings a north wind shift to the Bighorn Basin
and Johnson County this afternoon, with gusts to around 40 mph
possible. This pushes down into Fremont and Natrona Counties this
evening.

Dry conditions continue for Saturday, but winds are much lighter.
Elevated fire weather conditions return for Sunday, with RHs in the
upper single digits to low teens for much of the area. Gusty winds
also return, with high temperatures around 10 degrees above
average.

&&

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

&&

$$

DISCUSSION...Wittmann
AVIATION...CNJ
FIRE WEATHER...Wittmann