Special Weather Statement, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
525 PM MDT Tue Jul 13 2021

.SHORT TERM...(This evening through Wednesday night)
Issued at 101 PM MDT Tue Jul 13 2021

Storms are expected to get ramped up later this afternoon. This
morning's action was primarily just showers across the south, west,
and into the Wind River Basin. These are moving into Natrona county
as of 1130 AM, but beginning to wane. There will be a brief pause in
activity around noon, but things will start to ramp up pretty
quickly after that and continue into the later part of the evening.
SPC is continuing with the slight risk of severe across the eastern
zones and northern Bighorn Basin, however, an enhanced risk was
added for zones just to the east, across east-central WY and
southwest SD with a 30% hatched wind risk. There is a chance that
this threat could cross into our area. The greatest area of concern
continues to be where the slight risk areas is.

A shortwave continues to rotate south out of Montana with an
associated cold front. With hot, dry weather, and unstable
atmosphere, this will lead to a rather eventful severe weather day.
Diving deeper into the setup, MUCAPE is around 1000 to 1500 J/kg
across Johnson county and the northern Bighorn basin, with decent
shear. There is also 300 to 500 J/kg of CIN, which will hinder
convective development initially. Once the cap gets broken, storms
will develop quickly and strong to severe storms are likely. Again,
the biggest threat will be wind, and and storms will likely be
enhanced by outflows. large hail is also a possibility, especially
across northern Johnson county. Another thing that may help to
prevent convective development, at least initially will be some
stabilization from the earlier showers across the Wind River basin
and Natrona county. As the sun comes back out through the clouds and
the surface begins to heat through the afternoon, we expect
destabilization to occur and storms to begin to fire along the
terrain.

The dynamics will move east through the evening, along with the
severe weather threats. Some storms will stick around through the
late evening, likely ending between 9 PM and midnight. The severe
weather threat will mostly end by 9 PM, however.

West of the Divide, some storms will be possible, but much drier air
exists in that area, so dry thunderstorms are more of a threat. Low
RHs and gusty winds will create critical fire weather conditions by
early this afternoon. Red Flag Warnings remain in effect for that
area for the hot, dry, windy conditions, along with the treat for
possible dry lightning. A weak thunderstorm over Rock Springs this
morning has already shown us that dry thunderstorms are definitely
a threat today across the south west and west.

Tomorrow, looking like mainly dry and warm across the region.
Today's cold front will help to moderate temperatures a bit,
especially east of the Divide. Winds will also remain mainly light
tomorrow as ridging begins to set in again. This will somewhat
reduce the fire weather concerns, at least temporarily.

.LONG TERM...(Thursday through Tuesday)
Issued at 101 PM MDT Tue Jul 13 2021

The overall pattern across the western US changes little during
the next several days, but looks to amplify as we go into the
weekend. The H5 ridge remains in place along the Intermountain
West, with broad westerly flow aloft across WY. Since the ridge is
not yet strengthening, convection will be able to fire along the
higher elevations during peak daytime heating. While activity will
be widely scattered, there will be the threat for occasional
lightning and very gusty winds. Starting Saturday, the ridge will
buckle northward, with models settling the center of the ridge
across western WY through the weekend. The stronger ridging will
help cap off convection, and will bring higher temperatures
statewide into early next week. The main question in this
forecast is whether a cutoff low dropping across the central
Plains will block this ridge from shifting east, or remaining in
place through the remainder of next week, continuing the heat
across the state.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 525 PM MDT Tue Jul 13 2021

Expect some thunderstorms to be around for the beginning of the TAF
period for areas across the north and east.  Any storm may contain
gusty winds and some hail.  Some brief MVFR/IFR conditions may occur
at these sites.  Any thunderstorms should be out of the area between
01Z-03Z.  Otherwise, expect the clouds to dissipate during the
period with improving conditions.

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 101 PM MDT Tue Jul 13 2021

Scattered showers and thunderstorms will continue across the state
today, with the dry lightning threat continuing mainly for far
northern WY and southern WY. Any storms that do form will also
bring a threat for gusty winds. Southern WY will also see humidity
levels well below 15 percent through the day, with widespread
gusts above 30 mph.

The system bringing the increase in thunderstorm activity will
move east tonight, with lighter winds returning on Wednesday. The
only areas with gusty winds will be limited to far southwest WY
across zones 277 and 279. Isolated thunderstorms will try to
develop on the higher elevations of the western and northern
mountains Wednesday afternoon, but will be very isolated in
nature.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 9 PM MDT this evening for WYZ275>279-286-
414.

Red Flag Warning until 11 PM MDT this evening for WYZ281.

&&

SHORT TERM...Fisher
LONG TERM...Straub
AVIATION...Clayton
FIRE WEATHER...Straub

Hazardous Weather Outlook
National Weather Service Riverton WY
315 AM MDT Tue Jul 13 2021

WYZ001>020-022>030-141200-
Yellowstone National Park-Absaroka Mountains-Cody Foothills-
North Big Horn Basin-Southwest Big Horn Basin-
Southeast Big Horn Basin-Owl Creek and Bridger Mountains-
Bighorn Mountains West-Bighorn Mountains Southeast-
Northeast Johnson County-Southeast Johnson County-
Teton and Gros Ventre Mountains-Jackson Hole-
Wind River Mountains West-Wind River Mountains East-
Upper Wind River Basin-Wind River Basin-Lander Foothills-
Green Mountains and Rattlesnake Range-
Natrona County Lower Elevations-Casper Mountain-Star Valley-
Salt River and Wyoming Ranges-Upper Green River Basin Foothills-
Upper Green River Basin-South Lincoln County-
Rock Springs and Green River-Flaming Gorge-East Sweetwater County-
315 AM MDT Tue Jul 13 2021

...Red Flag Warnings this afternoon and evening across portions of
northern and southern Wyoming...

This Hazardous Weather Outlook is for Western and Central Wyoming.

.DAY ONE...Today and Tonight.

Strong to severe thunderstorms possible across northern
portions east of the Divide during the afternoon and evening.

Critical fire weather likely across portions of northern and
southern Wyoming this afternoon. Please see Red Flag Warning for
details.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

Wednesday through Friday...Isolated thunderstorms possible across
the area.

Saturday and Sunday...Well above normal temperatures likely, with
high temperatures over 100 possible in some of the lower
elevations.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will be needed today.

Special Weather Statement
National Weather Service Riverton WY
610 PM MDT Tue Jul 13 2021

WYZ009-011-020-140045-
Southeast Johnson County-Bighorn Mountains Southeast-Natrona County
Lower Elevations-
610 PM MDT Tue Jul 13 2021

...SIGNIFICANT WEATHER ADVISORY FOR NORTH CENTRAL NATRONA AND SOUTH
CENTRAL JOHNSON COUNTIES UNTIL 645 PM MDT...

At 610 PM MDT, Doppler radar was tracking a strong thunderstorm near
Red Wall, or 23 miles southwest of Kaycee, moving east at 40 mph.

Pea size hail and winds in excess of 30 mph will be possible with
this storm.

This storm will remain over mainly rural areas of north central
Natrona and south central Johnson Counties.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Very heavy rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

A Severe Thunderstorm Watch remains in effect until 1100 PM MDT for
central and north central Wyoming.

LAT...LON 4341 10713 4359 10696 4350 10650 4308 10657
TIME...MOT...LOC 0010Z 286DEG 33KT 4345 10694

Fisher

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