140
FXUS65 KRIW 150437
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1037 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 /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1037 PM MDT Sat Sep 14 2024

VFR conditions will prevail through the TAF period. Southerly flow
ahead of an approaching weather system will increase Sunday morning
into the afternoon. This southerly flow will bring moisture into the
area, which will result in isolated to widely scattered rain showers
and thunderstorms Sunday afternoon/evening. These showers and storms
will mainly be confined to western portions of the area, with
possible (30%) impacts at KBPI/KPNA/KJAC from mid-afternoon into the
evening. For now have included PROB30 groups with -TSRA to account
for this convective activity. Brief reductions in VIS to MVFR will
be possible (30%) with convective activity. Other terminals will
only have virga showers with strong downdraft/outflow wind possible
(40%).

Wind will increase Sunday afternoon as the pressure gradient
tightens and the direction will predominantly be from the south.
Unless remnant outflows impact terminals, wind will subside
around 00- 01Z Sunday evening.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.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...Rowe
FIRE WEATHER...Myers