Severe Thunderstorm Warning 35, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
1138 AM MDT Sun Jul 5 2020

.SHORT TERM...This afternoon through Monday night

Hot and dry conditions will once again be the norm today, as the
region remains sandwiched between a large area of high pressure over
the Southern Plains/Southern Rockies and a longwave trough over
western Canada.

A frontal boundary will play a greater role today, with the boundary
stretching through northeast Natrona County, along the Bridger/Owl
Creek Mountains and the rest of the Divide. North of this boundary
is where the better moisture and focus for afternoon convection will
occur. Strong, to possibly severe, storms will be possible in this
area, with hail and strong winds. South of the boundary, humidity
levels will once again crater to critical levels. Values of 8 to 12
percent will be commonplace. Conditions are expected to remain
largely dry in these areas, with the best thunderstorm chances near
the front. This would be over Fremont and western Natrona counties.
A Rangeland Fire Danger statement was reissued again earlier this
morning for Fremont, Sublette, Sweetwater and southern Lincoln
counties. Wind gusts will range between 20 to 25 mph in most
areas again this afternoon, while areas west of the Divide will
have some gusts up to 30 mph.

Current indications have the front making it to the Divide
late this afternoon and continuing to push south into portions of
Sweetwater County through the overnight hours. Storms will be ending
and moving toward the northeast through the evening. Showers could
linger over Johnson County through the overnight hours tonight.

The front will begin to wash out through the morning Monday, as it
retreats back northward into northern WY. Thunderstorm chances will
be confined to this area, with the best chances over northern
Johnson County. Strong, damaging wind gusts will be the likely
threat, with large hail a possibility. Critically low RH's will
spread back over central portions, with elevated to critical fire
behavior once again likely over central and southern portions.

Storms will gradually end Monday evening, with dry conditions across
the forecast area after midnight.

.LONG TERM...Tuesday through Sunday

On Tuesday, a stronger shortwave in the southwest flow lifting
northeast across Nevada to Western Montana. This shortwave looks
to increase winds aloft and push drier air farther northward
across the county warning area. This will result in critical fire
weather thresholds being met across much of the area as stronger
winds aloft easily mix down due to steep lapse rates, in
combination with very low RH. A Fire Weather Watch has been issued
for Tuesday afternoon through Wednesday evening. Any convection
should be confined to the far north/northeast.

In wake of the system on Tuesday, a weak cold front is progged to be
across N. Wyoming on Wednesday. This should lower fire concerns
north of the front. However south of the front, elevated to critical
fire weather conditions remain. Winds will be weaker than what
occurs Tuesday, but still look to reach low-end critical fire
weather thresholds based on progged mid- level flow. Fire highlights
may need to be extended into southern Johnson County. North of the
boundary will be a shot of diurnal isolated convection.

A weak transitory ridge is expected Thursday ahead of another trough
pulling into the Pacific Northwest. The front on Wednesday will wash
out with slightly warmer temps across the entire forecast area which
should result in dry conditions across the entire area. Models are
showing varying speeds in the mid-levels for a more marginal day for
critical fire weather wind thresholds. Relative humidity thresholds
should be met in most locations.

Friday is trending toward similar conditions to what occurs Tuesday
as another trough pulls northeast across N. Rockies of Idaho and
Montana. This will result in increasing mid-level flow and
increasing surface gradient while also keeping the forecast area in
a dry southwest flow.

Next weekend looks to remain hot with continued fire weather
concerns. The GFS hints at some precipitation on Sunday over the
west/southwest from moisture pulling north/northeast from the Great
Basin. For now will keep the forecast dry.

&&

.AVIATION...For the 18Z TAFS through 18Z Monday

Isolated to locally scattered afternoon and evening thunderstorms
are expected across the north and east primarily impacting KCOD,
KCPR and KWRL. Some of the storms could produce large hail and
very strong gusty winds this afternoon into early evening. Some of
this activity could linger across Johnson County well into the
night. VFR conditions are expected at area terminals, except for
very brief MVFR/IFR conditions in thunderstorms.

&&

.FIRE WEATHER...

Hot and dry weather will continue. Winds are expected to remain
somewhat light, with gusts of 25 to 30 mph occurring once again
across southern portions this afternoon. A frontal boundary will
stretch through northeast Natrona County, along the Bridger/Owl
Creek Mountains and the rest of the Divide by midday Sunday. The
boundary is expected to reach the Divide by late afternoon, pushing
south into zone 279 tonight. RH levels will be in the 20s and 30s
north of this boundary. Thunderstorms this afternoon are expected in
areas north of the front and along it. Strong, to possibly severe,
storms will be possible in this areas as well. RH values between 8
and 14 percent will occur over southern portions this afternoon. The
low RH and hot temperatures will continue to be the greatest concern
this week. A Haines Index of 5 or 6 will also be in place for all,
but far western portions this week. As a result, a Fire Weather
Watch has been issued across central zones for Tuesday and Wednesday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Tuesday afternoon through Wednesday
evening for WYZ280-283-285-300.

&&

SHORT TERM...LaVoie
LONG TERM...Murrell/Hulme
VIATION...Hulme
FIRE WEATHER...LaVoie

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