463
FXUS65 KRIW 200759
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
159 AM MDT Fri Sep 20 2024

.KEY MESSAGES...

- Fire weather concerns this afternoon across central WY, with
  gusty wind and low RH. Red Flag Warnings have been issued
  from noon to 7 PM MDT.

- A couple of cold fronts will bring cool temperatures to the
  area from Saturday through Tuesday. These cold fronts look
  quite dry, so precipitation chances/amounts do not look high.

- A dirty ridge will bring back above normal temperatures by the
  middle part of next week.

&&

.DISCUSSION...
Issued at 159 AM MDT Fri Sep 20 2024

A loosely coupled upper-low currently located over central CA and a
trough in western MT will slide eastward today into tomorrow. These
two upper-level features will pass south and north of WY
respectively, which means most of the area will miss out on any
precip chances. As this tandem passes through Friday night into
Saturday a cold front will sweep through the area bringing much
cooler temperatures Saturday into Saturday night. The upper-low to
the south looks to be a bit slower to pass by and may bring a bit of
moisture to extreme southeastern Sweetwater County/Natrona County as
it does so Saturday night. Rainfall amounts would be anywhere from a
trace to a few hundredths of an inch in the aforementioned areas.
Overnight lows Saturday night will be chilly in the mid to upper 30s
for lower elevations east of the Continental Divide. Wouldn`t be
surprised if there was frost in low spots Sunday morning. For those
who still have a garden it doesn`t look like this will be a freeze
event, so impacts should be minimal to vegetation. The most notable
weather impacts from these weather systems will be the elevated to
critical fire weather conditions this afternoon across central WY,
which will be discussed in more detail in the Fire Weather
Discussion below.

A brief ridge moves overhead on Sunday in between weather systems,
which will allow for temperatures to rebound a bit, back to about
seasonal averages for this time of year. Another trough surges down
from the northwest very quickly on Monday. Very light precip
accumulations are possible (30%) with this weather system as it
really won`t have enough time to drop more than a few hundredths
of liquid for any location. By Tuesday there`s a bit of a
disconnect in model guidance with how the pattern evolves. By
that time ensemble model guidance starts to diverge greatly. The
deterministic GFS wants to develop a cut-off upper-low from the
Monday system and have it retrograde northwest through the
middle part of next week, which does seem to align with a few
ensemble members of the GEFS, but the majority do have a ridge
developing behind Monday`s trough. For now the latter seems
to be the more likely (70%) scenario, so expect a substantial
warming trend by the middle part of the week as the ridge builds
overhead.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1010 PM MDT Thu Sep 19 2024

Light winds and mostly clear skies prevail overnight into Friday
morning. During the early afternoon hours on Friday, typical 15
to 25 kt breezes will develop for many TAF locations.

A cold front will push in from the north during the evening and
early overnight, causing winds to shift to northerly and become
gusty (see TAFs for more precise timing). An hour or two of 20
to 30 kt gusts are likely (60% chance) at sites east of the
Continental Divide as the front passes, then decreasing to
lighter northerly flow overnight into Saturday morning.

&&

.FIRE WEATHER...
Issued at 159 AM MDT Fri Sep 20 2024

Today will be the most consequential day in terms of fire weather
concerns in the next 7 days. Increased wind ahead of a strong cold
front and good mixing leading to low RH will result in elevated to
critical fire weather conditions across central WY. As such, have
issued Red Flag Warnings for zones 280, 281, 289, and 300. Zone
281 is more marginal, especially across the northern half of
the zone, but criteria is likely (60%) to be met across the
southern half of the zone. This also aligns better with the Red
Flag Warning in southern Campbell County. The Livingston Draw
Wildfire is located in northeastern Johnson County where wind
will be lower and RH higher, but conditions will still be
favorable for fire growth and spread of that wildfire this
afternoon. A strong cold front will bring an abrupt northerly
wind shift Friday night, with wind gusts of 15 to 25 mph across
the Bighorn Basin and Johnson County down into Natrona County.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon today to 7 PM MDT this evening for
WYZ280-281-289-300.

&&

$$

DISCUSSION...Rowe
AVIATION...VandenBoogart
FIRE WEATHER...Rowe