Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 301121
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
521 AM MDT Fri Jul 30 2021

.SHORT TERM...(Today through Saturday)
Issued at 301 AM MDT Fri Jul 30 2021

The H5 ridge is slowly stretching northwestward into southern WY
this morning, with the monsoonal moisture continuing to wrap
around it into the state. Moisture remains well above normal, with
PW values of 1.0 to 1.2 across most of the state today. Minimal
capping noted on latest sounding, so showers should have little
trouble developing starting late this morning. Initial focus will
be along the higher elevations of the Wind River Ranges and the
Salts and Wyomings of SW WY. With the ridge expanding across
southern WY, any showers that do form will not have much midlevel
support to move them around, so some locally heavy rainfall is
possible. Have opted for a Flash Flood Watch for today through
Saturday for areas west and south of the Divide, focusing on the
higher elevations where the threat of runoff will be elevated.

Saturday will continue this pattern with ample monsoonal moisture
in place, triggering showers first along the higher elevations.
The ridge in the vicinity will keep storm motion low, bringing a
continued threat for localized flooding. A push of drier air will
drop in from the north late afternoon, gradually ending the rain
chances for Johnson and Natrona Counties by early evening
Saturday.

.LONG TERM...(Saturday night through Thursday)
Issued at 301 AM MDT Fri Jul 30 2021

The newest model run shows fairly good agreement on the weather
pattern for the long term. The 500mb ridge axis will continue to our
west, with southwest flow bringing in monsoon moisture across
southwest and western Wyoming. Showers and thunderstorms will
diminish early Saturday along and west of the Continental Divide and
then redevelop Sunday afternoon. The drier air in northerly flow
aloft will help keep areas east of the Divide drier on Sunday, with
surface high pressure helping to cap convective activity. It will be
warmest east of the Divide as well, with less cloud cover and warmer
temperatures aloft.

Monday will see a shift to more southwesterly flow aloft, allowing
the monsoon moisture to expand east of the Divide. The best chance
for convection remains west of the Divide Monday afternoon and
evening. However, better coverage can be expected east of the Divide
as the drier air slides east and is replaced by deeper moisture from
the west.

Models are in a little better agreement on the shortwave energy
reaching northwest Wyoming Tuesday afternoon. Tuesday looks to be
drier across the west, as the better moisture begins to track
eastward across the forecast area. Overall, the ridge shows some
weakening Tuesday into Wednesday as the aforementioned shortwave
energy transits the pattern. This generates a more zonal pattern
Wednesday into Thursday with less widespread convection and
gradually warming temperatures.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 519 AM MDT Fri Jul 30 2021

Scattered showers and thunderstorms already bubbling up around
the state, and will become more numerous by this afternoon.
Downpours overnight have brought brief visibilities down below
2SM, so have started adding tempo groups for lower vis for the
heaviest thunderstorm chances. Showers will continue overnight,
especially for the sites west of the Divide.

&&

.FIRE WEATHER...
Issued AT 301 AM MDT Fri Jul 30 2021

Monsoonal moisture has pretty much arrived across western WY.
Scattered showers and thunderstorms are expected today and
tomorrow across most of the state. The ample moisture in place
will help showers be efficient rainfall producers, which should
limit any dry lightning threat. But this also means that flash
flooding may be a threat for higher elevations and flood-prone
drainages. Winds remain 10 mph or less, except in the vicinity of
some thunderstorms, while moisture remains well elevated into
early next week. Drying is expected next week as the monsoon
moisture is pushed back to the west.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from noon MDT today through Saturday afternoon
for WYZ012>015-019-023>030.

&&

$$

SHORT TERM...Straub
LONG TERM...Swanson
AVIATION...Straub
FIRE WEATHER...Straub


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