713
FXUS65 KRIW 150803
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
203 AM MDT Sun Sep 15 2024

.KEY MESSAGES...

- Mainly dry today, with a breezy afternoon. Isolated (30% or
  less) showers and thunderstorms develop for western Wyoming
  this afternoon. Little rain expected, but strong outflow winds
  are possible.

- A weather system pushes into the area Monday, with rain and mountain
  snow chances pushing east of the Divide late Monday through
  Tuesday.

- A cold front moves through Tuesday, with cool temperatures and
  unsettled weather expected for the remainder of the week. &&

.DISCUSSION...
Issued at 202 AM MDT Sun Sep 15 2024

The next weather system is approaching from the Pacific Northwest
today, but main impacts won`t be until Monday and Tuesday. For
today, flow shifts more southwest as the approaching trough moves in.
This will continue to allow some warmer air into Wyoming, so highs
today will climb up to 10 degrees warmer compared to yesterday. Some
moisture ahead of the main system will spark some isolated (30% or
less) showers and thunderstorms this afternoon west of the
Continental Divide, and mainly for southwest Wyoming. Not expecting
much, if any, rain to be falling with any of these showers. As tends
to be the case, wind is the main hazard. Recent model runs are
showing the potential for 50 mph outflow gusts, which dewpoint
depressions would also support. Outside any storm outflows,
afternoon winds increase, with gusts 15 to 25 mph for much of the
area.

Fire weather concerns are more limited today. Winds don`t look to be
strong enough where relative humidities (RH) are critical, and RHs
don`t get low enough where the strong winds will be. That being
said, there could be some isolated areas of critical fire weather,
especially where storm outflows occur. Will not being issuing any
fire weather highlights, as favorable conditions do not look
widespread enough. HREF also supports this, with less than 40%
chance of critical fire weather conditions today; it does show some
isolated, 60% to 70% chances, in the Bighorn Basin.

Convection chances decrease tonight, but high-resolution models are
showing some lingering over western Wyoming through the next
morning. Confidence is not high in anything widespread, but have
added some low POPs into the forecast to account for this outcome.
Chances pick up again late Monday morning as the sun brings
instability to the area. Again, the focus will be west of the
Divide. Monday also brings another breezy afternoon, especially for
southern portions of the area as a southerly flow occurs ahead of
the upper-low.

Speaking of, the upper-low tracks eastward through Monday, with the
center reaching the Utah/Idaho border Tuesday morning. This brings
precipitation chances east of the Divide late Monday and through the
day Tuesday. A cold front also passes through Tuesday, bringing much
cooler temperatures to the area. With 700mb temperatures dropping to
around 0 to 2 degrees Celsius, snow levels around 9000 feet are
expected. The low track over far northwest Wyoming through Tuesday
and into central/eastern Montana Wednesday. This supports the best
precipitation chances across western Wyoming, though everywhere will
have a shot to see rain/high mountain snow.

As this system exits Wednesday, another system is soon to follow.
This one pushes more southward, into the Nevada area on Thursday.
There is some model disagreement on exact placement. This low tracks
eastward through Saturday. Highs at the end of this coming workweek
range from around 60 to around 70. With this next system,
temperatures look to fall a bit more for the weekend. Long range
models are hinting at yet another low to move through around next
Sunday, though this far out, would not put much confidence into
that.

&&

.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.

&&

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

&&

$$

DISCUSSION...Wittmann
AVIATION...Rowe