872
FXUS65 KRIW 282220
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
420 PM MDT Fri Jun 28 2024

.KEY MESSAGES...

- Slightly below normal temperatures continue for the rest of
  the day and gusty WNW winds persist through sunset, with very
  dry air across central WY. Elevated fire weather conditions
  exists for parts of central and southern WY.

- Isolated thunderstorms are possible this afternoon along the
  WY/CO border, and a cool front will push southward out of MT
  into northern WY this afternoon and reach central WY this
  evening.

- Near normal conditions with mostly to partly cloudy skies are
  expected Saturday

- Sunday will see windy and gusty southwest winds with very warm
  and dry air over much of the area. NW WY will see isolated
  showers and thunderstorms during the late afternoon and evening.

- A broad storm system across the northern Rockies will bring
  much cooler air and a chance of showers and thunderstorms
  Monday and Tuesday.

&&

.DISCUSSION...
Issued at 115 PM MDT Fri Jun 28 2024

As a secondary trough moves through Montana this afternoon, the
mid- and upper-level gradient is strengthened across WY,
bringing breezy/windy and gusty WNW flow to the forecast area.
Temperatures have cooled to below normal though very dry air of
humidities below 15 percent has developed across parts of
central and southern WY. A moisture boundary along the WY/CO
border should allow for isolated thunderstorm development across
far southern Sweetwater County, though most models are
indicating they should stay south of the border with perhaps
some gusty outflow pushing back into WY. As the MT trough moves
eastward, a cool front is expected to push southward into
northern WY along and east of the divide. Breezy and gusty north
winds will move southward during the afternoon, increasing
humidities and reach central WY early in the evening. As the
central/southern WY winds diminish this evening and the pressure
gradient relaxes, the north/northeast winds will then move into
Sweetwater County by midnight. Higher RH values in this air
mass could lead to localized low clouds in the Bighorn Basin and
Johnson County overnight.

Saturday will see high pressure and the ridge axis build over
the area, leading to breezy west winds across southwest WY, and
cooler east/southeast winds east of the divide. Temperatures
will return to near normal for late June for a very nice mid-
summer day. Mostly clear skies in the morning should become
partly cloudy is some areas with clouds moving northward from
eastern UT. Sunday will see strong dry southwest flow return to
the forecast area and a trough deepens over the PacNW and
increases moisture and energy into western MT. As a result,
temperatures will climb sharply to be 10-12 degrees above
normal for 80s and 90s (even near 100) in the lower elevations.
As SW winds increase to 15-25 mph and 25-40 mph gusts with
humidities dropping to below 15 percent, fire weather conditions
will become elevated. YNP is about the only place to not see
these conditions as cloud cover with a slight chance of storms
move over NW WY in the afternoon and evening.

Models are currently showing the first wave of the broad trough
to move strongly into NW WY after midnight early Monday morning,
increasing showers and isolated thunderstorms across western and
northern WY. Temperatures will cool again on Monday as weak
instability bring continued showers and thunderstorms through
the day along with breezy westerly wind. The best chances remain
across the northern half of the forecast area. A secondary wave
in the trough then moves through southern MT early Tuesday,
continuing the northern WY showers into the afternoon as well as
the cooler temperatures. Southern WY however will see dry
westerly flow. Dry zonal flow returns on Wednesday to increase
temperatures and west winds. Thursday and Friday appear to have
another situation of dry, very warm, and windy across SW WY, and
cooler, moist, and north wind breezy east of the divide as a
couple of shortwaves drop down from Canada into the northwest
Plains.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 420 PM MDT Fri Jun 28 2024

VFR prevails at all terminals through 00Z/Sunday. Gusty westerly
surface wind in central Wyoming shifts abruptly to the north between
00Z-02Z/Saturday as a surface boundary pushes through the region.
Gusty northerly surface wind 15-30kts already seen in the Bighorn
Basin will follow the boundary and persist until late evening. The
boundary reaches KRKS between 05Z-06Z/Saturday with gusty 15-25kt
winds arriving from the east-northeast. KJAC, KBPI, and KPNA avoid
this push and instead see typical diurnal winds as the sun sets.
Southwest flow aloft allows for increasing mid-level moisture
Saturday afternoon, but convective chances are less than 10 percent.
Little more than increasing mid/high cloud expected.

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 1245 PM MDT Fri Jun 28 2024

Elevated fire weather conditions continue this afternoon.
Minimum RHs between 10% and 15% for much of Sweetwater, Fremont,
and Natrona Counties, as well as parts of southern Lincoln and
southern Sublette Counties. A Rangeland Fire Danger Statement
(RFD) is in effect this afternoon for those areas. Elsewhere,
RHs down to around 20-25% in the lower elevations and east
slopes of the Wind River Mountains and Wyoming Range. Gusty
west winds have picked up during the late morning for the Wind
Corridor from southwest WY into central WY. Afternoon gusts 30
to 40 mph will continue through 1900 to 2100. A cool front
brings a north wind shift to the Bighorn Basin and Johnson
County this afternoon, with gusts to 30-40 mph possible. This
pushes down into Fremont and Natrona Counties this evening, with
breezy northeast winds even getting into Sweetwater County
around midnight.

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...McDonald
AVIATION...CNJ
FIRE WEATHER...McDonald/Wittmann