Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 250531
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1131 PM MDT Thu Sep 24 2020

.SHORT TERM...This afternoon through Friday night
Issued at 1125 AM MDT Thu Sep 24 2020

Dry and unseasonably warm conditions will continue across the
forecast area today, as a weak cold front approaches the area. This
will tighten the pressure gradient and increase the winds this
afternoon. Most locations will be affected by these winds before the
noon hour and certainly by 1pm. This will lead to critical fire
weather conditions across much of the forecast area. Expect RH
values of 10 to 14 percent, southwest winds with gusts of 30 to 40
mph and afternoon temperatures 15 to 20 degrees above normal today.

The cold front will reach northern portions by this evening.
Isolated rain/virga showers will be possible along the front,
mainly over the northern mountains, including Yellowstone. The
front will become stalled along southern portions of the Bighorn
Basin and Johnson County by Friday morning. The front will wash
out through the morning hours, as the region resets for another
cold front. Despite this, temperatures Friday will be about 10
degrees cooler. Another reason for the cooler temperatures will be
the zonal flow (and thus, lower heights) that will be in place,
following today`s cold front. An 80-100kt jet will be over
northern WY/southern MT Friday, which could lead to strong winds
over northern portions of the Absarokas and near Clark. The
impacts near Clark will likely occur Friday night. The proximity
of the jet will also keep the surface pressure gradient tight over
the forecast area, especially central and southern portions. Red
Flag Warnings that are currently in place for portions of Natrona
and Sweetwater counties may need to be extended into Friday. The
main concern is if humidity values will drop below critical levels
Friday afternoon. Either way, fire weather conditions will be
elevated in these areas.

Models are indicating an increasing chance for precipitation over
northwest portions by Friday evening, continuing through the rest
of the night. This will be due to the Pacific moisture being
advected into the area via the jet. The second cold front will
begin to enter Johnson County late Friday night/early Saturday
morning. The front will likely move through the area through the
day Saturday. This will discussed further in the next section.

.LONG TERM...Sunday through Wednesday night
Issued at 1125 AM MDT Thu Sep 24 2020

On Saturday, a weak cold front will brush the north. Models are
in better agreement with keeping precip across the north, though
the ECMWF is still a bit stronger. Jet still appears to be good
with the right front quad entering the area along with the front.
We expect the majority of the rain and mountain snow in the
northwest, with some isolated activity in the northern Bighorn
Basin and Bighorn Mountains. South of the front, winds will
remain strong through the day Saturday. Luckily, the cooler temps
will keep humidities from getting too low. Unless things dry out
a bit more, we will only be looking at elevated fire weather
conditions.

Sunday the upper trough will exit east, while rapidly digging
southward into the Central Plains. This looks to be far enough east
that we will not see much activity from it, however a backdoor cold
front could bring a shot of colder temps and possibly stronger winds
to central Wyoming. Sunday night through Wednesday, strong
ridging builds into the western US which will keep our flow out of
the north- northwest. This should allow our temperatures to
return closer to normal. The ECMWF is hinting at a shortwave
coming in from the north on Wednesday night into Thursday, but
other models keep the ridging dominant. We will keep PoPs out of
the eastern areas, until models indicate better agreement.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1122 PM MDT Thu Sep 24 2020

VFR visibilities will continue across all terminal sites through
the TAF period. Light and variable winds will persist overnight
tonight and persist through the early daytime hours. The wind will
begin to pick up across all sites tomorrow after 18Z. Winds will
be the gustiest at KRKS and KCPR where winds could gust to 40
knots. Winds will gust 25 to 35 knots at all other sites with the
exception of KWRL where winds will be relatively the lightest.
Winds will slowly begin to diminish between 01Z to 03Z and remain
light and variable with the exception of sites KRKS and KCPR where
winds will still be gusty.

&&

.FIRE WEATHER...
Issued at 1125 AM MDT Thu Sep 24 2020

A combination of a gusty winds and low humidity will bring
critical fire weather to the entire area this afternoon, except
the far northwest. Most areas will have wind gusts of at least 25
to 30 mph. Wind prone locations will have gusts of 35 to 40 mph.
This will lead mixing and smoke dispersal to be good to
excellent. A cold front will move across the area tonight, but
little to no precipitation is expected from it. Conditions will
improve across northern Wyoming on Friday, but elevated to
possibly critical fire weather may linger from zone 279 to zone
280.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 7 PM MDT Friday for WYZ277-279-280-289-
300.

&&

$$

SHORT TERM...LaVoie
LONG TERM...AR
AVIATION...Jones/Vorst
FIRE WEATHER...Hattings


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