146
FXUS65 KRIW 191721
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1121 AM MDT Sun May 19 2024

.KEY MESSAGES...

- One more day of elevated to critical fire weather conditions
  is expected for central and southern Wyoming. Wind gusts of
  30 to 40 mph will be widespread.

- Wetter and cooler conditions are expected Monday and Tuesday. Areas
  along and east of the Continental Divide will see the most
  moisture.

- Mountain snow is expected with this system, especially for the eastern
  Winds, Absarokas, and Bighorns. Winter driving conditions are
  expected over the passes.

- Warmer but still unsettled conditions expected Wednesday
  through the end of the week.

&&

.DISCUSSION...
Issued at 442 AM MDT Sun May 19 2024

Midlevel flow has become more cyclonically curved over the area this
morning as the leading wave from a trough approaches Wyoming. This
will keep light shower chances going across the area today, most
favorably in two locations. One is across the Yellowstone area,
which will see increased jet-enhanced lift. The other is across far
southern areas which will benefit from a little Pacific moisture
pushing in there. However, the majority of the area will see
one last warm, dry, and windy day ahead of an approaching cold
front. By midday a surface pressure gradient will once again
strengthen as a surface low develops to our east. Wind gusts of
30 to 40 mph will be fairly widespread. With humidity values
again dropping below 15 percent, elevated to critical fire
weather conditions are expected through the afternoon for most
of central and southern Wyoming.

The front will push south this evening and overnight as the trough
drops south over Idaho. Shower activity will first pick up across
the southern half of the CWA through the early morning hours as
reinforcing moisture and favorable jet dynamics arrive. By early
afternoon precipitation chances will increase across the rest of the
area as the trough begins to push in to western Wyoming. Widespread
showers will continue Monday night as the associated low moves
eastward, centering itself over central Wyoming by midday Tuesday.
This will result in a favorable period of upslope enhancement east
of the Divide through Tuesday afternoon. Precipitation should begin
decreasing Tuesday evening as the system quickly departs and a drier
westerly flow takes over.

Looking at QPF, there continues to be notable model spread on
amounts for this event. Given the low placement, this will certainly
favor areas east of the Divide. The greatest amounts are expected
from the Wind River Basin through the Powder River Basin, as well as
far southern portions of the Bighorn Basin. NBM probabilities
currently give these areas a 40 to 60 percent chance of seeing at
least an inch, with most of that falling in the Monday evening
through Tuesday afternoon timeframe. The current forecast is on the
higher end of those probabilities, with the highest amounts around
1.5 inches for the non-mountain areas. Other areas across the CWA
will see less through the event, generally below half an inch.

With snow levels dropping to around 7,000 feet, this will be a
decent spring storm for the eastern Winds, Absarokas, and
Bighorns. Forecast amounts are currently peaking well above a
foot; we considered a Winter Storm Watch, but have held off in
part due to the continued QPF uncertainty. This may end up being
more of an Advisory event given that the heaviest snow will
fall during the long daylight hours, likely mitigating impacts
on mountain highways somewhat. Regardless, we expect winter
driving conditions in the mountains, including on South Pass,
Togwotee Pass, Powder River Pass, and Granite Pass.

Further ahead, guidance continues to favor an active pattern
lingering through the week. It appears there should be at least a
brief period of transitory ridging resulting in drier and warmer
conditions on Wednesday. Solutions then quickly move in the next
system for Thursday, though there is notable uncertainty on the
placement and strength of this. As it stands now, ensemble cluster
analysis shows anomalously low 500mb heights through next
weekend. The forecast generally reflects this with daily
precipitation chances and temperatures around or below normal.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1113 AM MDT Sun May 19 2024

VFR conditions continue today with SCT midlevel CU bubbling up
this afternoon. Main issue today is gusty winds, with plenty of
southwest gusts of 25 to 30kts through this evening. Winds
decrease overnight, and should stay light into the day Monday.
Some shower activity will begin to develop for the southern half
of the state overnight, and gradually shift northward during
the day Monday. Cigs with this rain will lower to near MVFR
levels during the afternoon Monday.

&&

.FIRE WEATHER...
Issued at 442 AM MDT Sun May 19 2024

One more day of elevated to critical fire weather conditions are
expected ahead of an approaching trough and associated cold front.
Widespread wind gusts of 30 to 40 mph will ramp up by early
afternoon in response to a strengthening pressure gradient. The
lowest humidity will be across southern and central Wyoming,
dropping below 15 percent with peak mixing this afternoon.
Otherwise, there will be some isolated light showers in the area,
with the best chances across far northern and far southern Wyoming.
Fire weather concerns decrease after sunset. Cooler and wetter
conditions are expected for Monday and Tuesday.

&&

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

&&

$$

DISCUSSION...Myers
AVIATION...Straub
FIRE WEATHER...Myers