883
FXUS65 KRIW 231914
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
114 PM MDT Mon Sep 23 2024

.KEY MESSAGES...

- Slightly above normal temperatures Monday and Tuesday with
  partly cloudy skies becoming mostly sunny along with some
  gusty north winds east of the divide

- High pressure strengthens Tuesday night into Wednesday,
  bringing well above normal afternoon temperatures and gusty
  southwest wind for elevated fire weather conditions

- Similar situation creates elevated fire weather conditions on
  Thursday with gusty west winds and partly cloudy skies across
  northern Wyoming

- Dry and very warm conditions continue Friday and Saturday,
  with the next chance (20-30 percent) of precipitation Sunday
  afternoon and evening into Monday as a disturbance moves
  through the northern Rockies

&&

.DISCUSSION...
Issued at 113 PM MDT Mon Sep 23 2024

Dry northwest flow continues over western and central Wyoming today
as a low pressure system moves out of southern Canada into the
northern Plains.  High-level clouds have increased today with a
northerly component, and some mid-level wave clouds have also
developed in response to the increase in moisture.  One or two high-
resolution models are showing a 5-10% chance of showers over the
east-central Bighorn Mountains late this afternoon and early
evening, but feel that the better chance is well to the northeast.
Near the surface across north-central WY, winds are generally from
the NW to N and are gusty, especially around Buffalo.  In response
the the "boundary", gusty west-southwest winds continue today from
southwest WY into central WY, though central WY should see some
north winds this evening. With 700 mb temps around +6 to +8 C today
and tomorrow, max temps will be about 3-6 degrees above normal in
the 70s to around 80.  Upper-level high pressure is expected to move
from Oregon to the northern Intermountain West overnight, and then
over northern Utah Tuesday afternoon.  Skies will thus clear from
west to east on Tuesday, and the surface pressure gradient will
weaken to bring relatively light winds for an amazing fall day
across the state.

Wednesday will see the ridge axis move over WY during the morning
and then eastward through the day.  A lee-side low is expected to
develop over eastern Montana, strengthening the gradient across the
forecast area, and increasing southwest winds during the afternoon.
Skies will still remain mostly clear, but surface temperatures will
increase in response to 700mb temps also increasing to +12 to +15 C.
 Expected max temps will be from the high 70s to around 90, which is
14-16 degrees above normal for Sept. 25.  Models are showing drier
air moving in Wed afternoon as well with min RH values dropping to
around 15 percent, to create elevated fire weather conditions during
the afternoon and early evening.  The shortwave trough over the
PacNW then moves into Montana on Thursday with no precipitation
expected across northern WY.  Dry and very warm air continues over
the forecast area as winds shift from southwest to west and remain
gusty.  Again, elevated fire weather conditions are expected.  As
the trough moves eastward across MT, a weak surface cool front then
shifts the winds to the north across north-central and northeast WY
Thursday afternoon.

Weak mid-level southwest flow will return to Wyoming Friday and
Saturday as the pressure gradient weakens.  Temperatures will cool
slightly to the mid 70s to mid 80s, but still remain 10-12 degrees
above normal for late September. Skies should remain mostly clear
until late Saturday night as a shortwave trough moves from the west
coast toward the MT/WY area. Winds and clouds will increase Sunday
morning with the advance of a strong cold front and some showers and
thunderstorms are possible (25 percent) through the day.  The north-
south uncertainty with this wave though is likely to bring a better
chance of storminess to northern WY, will considerable cooling
behind the system. Ahead of the front in central WY, gusty
southwest winds and dry conditions will again elevated fire
weather concerns. Hopefully a better picture will develop over
the coming days, as well as some autumn precipitation to keep
drought categories from worsening.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1153 AM MDT Mon Sep 23 2024

Patchy high cirrus clouds are moving southward across the
region, but no low or middle atmosphere clouds are expected
over the next 24 hours. A subtle front originating in Montana
will drift southward east of the Continental Divide, causing a
northerly component to the wind this afternoon and evening;
however, no wind gusts over 20 kts are forecast for TAF sites.
KBYG will be more favorably positioned for 25 to 35 kt gusts
this afternoon, which can already be seen. Winds decrease
overnight, and will remain light on Tuesday.

&&

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

&&

$$

DISCUSSION...McDonald
AVIATION...VandenBoogart