Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210446
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1046 PM MDT Tue Aug 20 2019

.SHORT TERM...This afternoon through Wednesday

The weak boundary that pushed into areas east of the Divide this
morning has kept RH values slightly higher in northeastern and
central areas this afternoon. Light showers are beginning to pop
up across the north and slight chances will linger into the
evening hours in the far north. The biggest impact of the boundary
will likely be seen tonight with max RH values ten to twenty
percent higher than last night. A weak pressure gradient will keep
winds light through Wednesday.

On Wednesday, the set up will be similar with areas west of the
Divide and the southwest staying warm and dry with RH values in
the teens to possibly single digits. With winds staying light
highlights are unlikely. East of the Divide will once again have
slightly higher RH values. A shortwave along with moisture
advection will increase precipitation chances in the afternoon
hours, mainly in the Absaroka and Bighorn Mountains. Some of these
storms could be strong with gusty winds being the main threat.

.LONG TERM...Wednesday night through Tuesday

Isolated to scattered showers and thunderstorms will end over areas
east of the Divide Wednesday evening. By Thursday, the trough will
begin to weaken and fill as it moves over southern Canada and MT. A
cold front associated with this trough will make its way east of the
Divide Thursday afternoon, stalling out along the Divide by early
Friday morning. Precipitation chances Thursday afternoon and evening
look to be over northern portions of the forecast area for now.

A more zonal pattern will return in the wake of this trough, with
hints of a flat ridge rebuilding over the Great Basin by Sunday. The
PFJ will be over the Northern Tier states in this pattern. Stronger
winds will return Saturday and Sunday over wind prone locations, due
to the proximity of the jet and tightened pressure gradient. These
conditions will continue into Monday, as model progs are resolving a
cold trough moving over the Northern Rockies by Tuesday. This will
be in response to the ridge shifting over the West Coast and
building up toward the Northwest Territories. If this forecast
verifies, this would effectively kill any chance for the monsoon
season to return to the Intermountain West.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1045 PM MDT Tue Aug 20 2019

Isolated showers will remain possible through the predawn hours
across Johnson County. A moist, easterly surface flow could result
in low clouds (MVFR) developing/advecting westward into Johnson and
Natrona counties between 10Z and 13Z. If low clouds indeed form and
become widespread enough, they could last through midday (~17Z).
This would impact the KCPR terminal. Due to the widespread rainfall
east of Natrona County this evening will introduce MVFR ceilings for
Wednesday morning.

The focus shifts to robust afternoon and evening thunderstorms
developing along and east of the Divide. Looks like widely scattered
thunderstorms will develop over/near the Absaroka/Wind River
mountains in the early afternoon and shift eastward through the
afternoon and evening. The best chances for thunderstorms will be
terminals near the mountains (KCOD and KLND), but will have VCTS at
all terminals. Brief MVFR/IFR conditions are possible with any
convection.

Areas west of the Divide are expected to remain mainly dry with
light winds.

&&

.FIRE WEATHER...

Isolated to widely scattered showers and thunderstorms will end
over northern portions this evening. A boundary will move over
Johnson/Natrona counties late this afternoon and into the Wind
River Basin early this evening. Easterly winds and increasing
dewpoints will occur behind this boundary. Gusts up to 30 mph will
occur with these winds this evening over Natrona and Fremont
counties. These conditions will also sweep through the Bighorn
Basin through the overnight hours tonight. These conditions will
continue Wednesday east of the Divide, with isolated to scattered
thunderstorms returning mainly over the mountains during the
afternoon. West of the Divide will remain hot and dry this
afternoon and Wednesday, with poor RH recovery tonight. Winds
will remain on the light to moderate side across all areas.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Bourque
LONG TERM...LaVoie
AVIATION...Murrell
FIRE WEATHER...LaVoie


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