657
FXUS65 KRIW 150108
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
708 PM MDT Sat Sep 14 2024

.KEY MESSAGES...

- Elevated to critical fire weather conditions today for much of
  central and southern Wyoming.

- Isolated showers and thunderstorms expected Sunday afternoon. Strong
  outflow wind gusts are the primary concern with little
  rainfall.

- Rain chances increase Sunday evening across western Wyoming.

- A more active pattern sets in early next week, with the best
  chances of rain across western Wyoming. Cooler temperatures
  are likely for most areas next week.

&&

.DISCUSSION...
Issued at 230 PM MDT Sat Sep 14 2024

The warming trend continues today under a dry west to southwest
upper flow. Temperatures will peak a few degrees above normal for
this time of year, generally in the 70s to low 80s across lower
elevations. Skies will remain mostly clear other than some high
clouds across the northern half of the area. The one weather concern
we do have today is fire weather. A relatively weak surface gradient
will set up across eastern Wyoming this afternoon, and combined with
ample mixing will nonetheless lead to winds gusting 20 to 30 mph and
humidity dropping below 20 percent for much of the lower elevations.
A Red Flag Warning is in effect through the afternoon for locations
where the strongest wind and lowest humidity align.

On Sunday the predominant flow becomes increasingly southwesterly as
a low pressure system begins pushing into the Pacific Northwest.
Warmer air will be advected into Wyoming in response, bringing highs
5 to 10 degrees above normal. This will also return increased
Pacific/monsoonal moisture to the area. Given the moisture, there
should be enough heating for weak instability (200-400 J/kg of CAPE)
across the southern half of the area for isolated convection in the
afternoon. CAMS are supportive of this, generally showing isolated
activity pulling north into the evening. Given the still fairly dry
boundary layer, much of the associated rain will have a hard time
reaching the ground. Accordingly, virga showers will present a gusty
outflow wind threat, especially across southern Wyoming during the
afternoon.

Shower chances will increase slightly Sunday night (<40%) across
western Wyoming as the low continues to drop south, bringing another
surge of moisture to the area. Most locations will remain dry
through Monday morning. The system finally begins to move more
easterly Monday afternoon, with leading shortwaves helping to kick
off additional showers across western areas. Precipitation chances
will then spread eastward through Tuesday as the low approaches
Wyoming. However, the forecast track of the low still takes it on a
more north/northeasterly approach centered over Idaho, which would
again keep rain focused on western Wyoming. The more widespread
impact may be cooler temperatures as a cold front moves east,
as well as strong winds ahead of the front.

An active pattern appears favorable to persist Wednesday through the
end of the week as another system arrives. Though solutions with
this feature remain uncertain, cooler than normal temperatures with
continued rain chances are likely for the entire area into next
weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 454 PM MDT Sat Sep 14 2024

Besides high cirrus clouds, mostly clear skies this evening through
tonight. Current breezy evening winds will quickly decrease
around sunset.

Models remain consistent regarding the drier air in place tonight
keeping patchy fog out of the KJAC area for Sunday morning. Sunday
afternoon will feature a slight increase in breezy winds for sites
west of the Continental Divide. Scattered afternoon showers and a
few weak thunderstorms bring a 20% chance of brief strong and
erratic winds gusts for KJAC, KBPI, KPNA, and KRKS.

&&

.FIRE WEATHER...
Issued at 230 PM MDT Sat Sep 14 2024

Elevated to critical fire weather conditions will persist through
the afternoon in response to a weak pressure gradient and ample
mixing. A Red Flag Warning is in effect for the few zones where
criteria should be met. Another round of fire weather highlights was
considered for central Wyoming on Sunday where humidity should drop
below 15 percent. However, the outflow-driven wind threat does not
appear as widespread across these areas which should limit fire
weather concerns there.

&&

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

&&

$$

DISCUSSION...Myers
AVIATION...VandenBoogart
FIRE WEATHER...Myers