Special Weather Statement, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
223 PM MDT Fri Aug 6 2021

.SHORT TERM...(This evening through Saturday night)
Issued at 220 PM MDT Fri Aug 6 2021

Convection is ongoing as a robust shortwave moves east across the
state today. We have already seen thunderstorms producing strong
wind gusts and expect that to be the main concern into the evening.
SPC now has a slight risk east of the Divide for wind where the most
favorable shear and instability exists. Additional convection should
develop across these areas soon. This activity will decrease by late
evening, mostly ending by midnight with the loss of heating and
forcing. Still seeing the possibility for showers to linger across
the Bighorns and Johnson County through early morning. Elsewhere,
the other concern today is still fire weather across southern areas
through the Wind Corridor. With the mixing and strong pressure
gradient, winds have been gusting above critical criteria. While
relative humidity may bottom out around 15-18 percent, will have the
Red Flag Warning continue through 8 PM for these locations. Ridging
will build back in Saturday leading to a generally dry and sunny
day.

.LONG TERM...(Sunday through Friday)
Issued at 220 PM MDT Fri Aug 6 2021

The start of the period on Sunday has a W-WSW flow over the area
with an area of low pressure over the Pacific Northwest. The EC
has this feature further to the north and a bit further west than
the GFS. The EC has the eastward progression much more than it was
showing yesterday. Both models have this moving across the
northern tier of the CONUS and being in southern SK (EC) or near
the ND/MB/SK border (GFS) by 12Z Tuesday. The main weather story
over WY with this feature looks to be the wind with a tight
pressure gradient on Monday, especially on the GFS. There is also
jet support to the south of the low. In regards to precipitation,
the bulk looks to stay to the north over MT. The most likely
places to see any activity would be across the northern 1/3 to 1/2
of the forecast area and from Sunday afternoon through Monday
night, otherwise expect it to be dry. Even where it does rain, QPF
amounts look to be quite low so it is possible this may be almost
a non-event precipitation-wise. The rest of the long-term looks
to be quiet with no further precipitation. It may still be rather
windy on Tuesday, but afterwards the low is expected to push
further to the northeast and a ridge looks to become dominant
again over the southwest bringing lighter winds to the state.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday)
Issued at 1155 AM MDT Fri Aug 6 2021

Expect VFR conditions to prevail throughout the period.  Winds look
to be an issue for many locations until around 01Z-05Z.  The most
wind looks to be from KRKS over to KCPR. Outflow winds and winds
from heavier thunderstorms may also impact terminals until around
00Z with waves of precipitation moving through the area.

&&

.FIRE WEATHER...
Issued AT 220 PM MDT Fri Aug 6 2021

Scattered showers and thunderstorms will continue this afternoon and
evening, especially east of the Divide, as a strong shortwave moves
east across the state. These will continue to produce strong, gusty
outflow winds through the evening, decreasing after sunset. Also
still have a Red Flag Warning in effect through 8 PM for southern
areas through the Wind Corridor where strong west winds and low
relative humidity have resulted in near to critical fire weather
conditions.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 8 PM MDT this evening for WYZ277-279-280-
289.

&&

SHORT TERM...Myers
LONG TERM...Clayton
AVIATION...Clayton
FIRE WEATHER...Myers

Hazardous Weather Outlook
National Weather Service Riverton WY
1119 AM MDT Fri Aug 6 2021

WYZ001>020-022>030-071730-
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-
1119 AM MDT Fri Aug 6 2021

...There is a Slight Risk of Severe Thunderstorms Mainly East of
the Continental Divide this Afternoon and Evening...

...Red Flag Warning Continues for Portions of Southwest and
Central Wyoming Until 8 PM...

This Hazardous Weather Outlook is for Western and Central
Wyoming.

.DAY ONE...This Afternoon and Tonight

Scattered afternoon and evening thunderstorms will produce
outflow wind gusts of 40 to 50 mph, with localized damaging
outflow winds around 60 mph. The best chance for damaging winds
will be east of the Continental Divide.

Westerly wind 20 to 30 mph with gusts 35 to 45 mph will combine
with low relative humidities to produce critical fire weather
conditions from Sweetwater County to Natrona County through 8 PM.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

Sunday and Monday...Elevated fire weather conditions possible due
to hot, dry, and breezy conditions. A cold front Sunday night
into Monday could lead to gusty northwest wind and increased fire
danger.

.SPOTTER INFORMATION STATEMENT...

Weather spotters are encouraged to report significant weather
conditions.

Special Weather Statement
National Weather Service Riverton WY
112 PM MDT Fri Aug 6 2021

WYZ005>007-061945-
Owl Creek and Bridger Mountains-Southeast Big Horn Basin-Southwest
Big Horn Basin-
112 PM MDT Fri Aug 6 2021

...Strong thunderstorms will impact portions of central Hot Springs
and west central Washakie Counties through 145 PM MDT...

At 111 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 6 miles northeast of Hamilton Dome to 9 miles
west of Lucerne to 6 miles west of Thermopolis. Movement was east at
40 mph.

HAZARD...Winds in excess of 40 mph and half inch hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Strong thunderstorms will be near...
  Thermopolis and Hot Springs State Park around 120 PM MDT.
  Lucerne around 125 PM MDT.
  Kirby around 130 PM MDT.

Other locations in the path of these storms include Winchester.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Heavy rainfall is also occurring with these storms and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

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

&&

LAT...LON 4396 10806 4374 10792 4358 10789 4361 10840
      4373 10841 4388 10854
TIME...MOT...LOC 1911Z 266DEG 36KT 4383 10846 4374 10838 4361 10834

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPH

Vorst

Special Weather Statement
National Weather Service Riverton WY
136 PM MDT Fri Aug 6 2021

WYZ019-020-022-062000-
Green Mountains and Rattlesnake Range-Casper Mountain-Natrona County
Lower Elevations-
136 PM MDT Fri Aug 6 2021

...A strong thunderstorm will impact portions of southeastern Natrona
County through 200 PM MDT...

At 136 PM MDT, Doppler radar was tracking a strong thunderstorm near
Alcova, or 34 miles southwest of Casper, moving northeast at 50 mph.

HAZARD...Winds in excess of 40 mph and half inch hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

This strong thunderstorm will be near...
  Alcova around 145 PM MDT.
  Bates Hole around 155 PM MDT.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

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.

If on or near Alcova Reservoir, get out of the water and move
indoors or inside a vehicle. Remember, lightning can strike out to
10 miles from the parent thunderstorm. If you can hear thunder, you
are close enough to be struck by lightning. Move to safe shelter
now! Do not be caught on the water in a thunderstorm.

&&

LAT...LON 4243 10664 4243 10701 4247 10709 4284 10643
      4256 10615
TIME...MOT...LOC 1936Z 239DEG 45KT 4250 10682

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPH

Vorst

Special Weather Statement
National Weather Service Riverton WY
209 PM MDT Fri Aug 6 2021

WYZ020-022-062045-
Casper Mountain-Natrona County Lower Elevations-
209 PM MDT Fri Aug 6 2021

...A strong thunderstorm will impact portions of east central Natrona
County through 245 PM MDT...

At 209 PM MDT, Doppler radar was tracking a strong thunderstorm over
Beartrap Meadows County Park, or 8 miles south of Casper, moving
northeast at 55 mph.

HAZARD...Wind gusts up to 50 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

This storm will remain over mainly rural areas of northern Casper
Mountain and east central Natrona County Lower Elevations Counties,
including the following locations... Edness K Wilkins State Park,
Casper Mountain, Casper Mountain County Park, Rotary Park and Casper
Natural Gas Processing Plant.

This includes Interstate 25 between mile markers 176 and 184.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential 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 800 PM MDT for
central Wyoming.

&&

LAT...LON 4267 10608 4259 10632 4274 10645 4294 10608
      4283 10607 4275 10608 4272 10607
TIME...MOT...LOC 2009Z 239DEG 49KT 4272 10627

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...50 MPH

Vorst

Special Weather Statement
National Weather Service Riverton WY
212 PM MDT Fri Aug 6 2021

WYZ019-020-062045-
Green Mountains and Rattlesnake Range-Natrona County Lower
Elevations-
212 PM MDT Fri Aug 6 2021

...A strong thunderstorm will impact portions of south central
Natrona County through 245 PM MDT...

At 212 PM MDT, Doppler radar was tracking a strong thunderstorm near
Independence Rock, or 38 miles east of Jeffrey City, moving east at
35 mph.

HAZARD...Winds in excess of 40 mph and half inch hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
  Alcova.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

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.

If on or near Pathfinder and Alcova Reservoirs, get out of the water
and move indoors or inside a vehicle. Remember, lightning can strike
out to 10 miles from the parent thunderstorm. If you can hear
thunder, you are close enough to be struck by lightning. Move to
safe shelter now! Do not be caught on the water in a thunderstorm.

A Severe Thunderstorm Watch remains in effect until 800 PM MDT for
central Wyoming.

&&

LAT...LON 4243 10721 4254 10723 4268 10663 4243 10658
TIME...MOT...LOC 2012Z 262DEG 32KT 4247 10708

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPH

Vorst

Special Weather Statement
National Weather Service Riverton WY
221 PM MDT Fri Aug 6 2021

WYZ020-062100-
Natrona County Lower Elevations-
221 PM MDT Fri Aug 6 2021

...A strong thunderstorm will impact portions of east central Natrona
County through 300 PM MDT...

At 220 PM MDT, Doppler radar was tracking a strong thunderstorm over
Antelope Hills, or 16 miles north of Casper, moving northeast at 25
mph.

HAZARD...Winds in excess of 40 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

This strong thunderstorm will be near...
  Antelope Hills around 225 PM MDT.

This includes Interstate 25 between mile markers 197 and 213.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

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 800 PM MDT for
central Wyoming.

&&

LAT...LON 4295 10640 4311 10651 4338 10607 4302 10608
TIME...MOT...LOC 2020Z 242DEG 23KT 4307 10637

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...40 MPH

Vorst

Special Weather Statement
National Weather Service Riverton WY
231 PM MDT Fri Aug 6 2021

WYZ012-014-025-062100-
Wind River Mountains West-Upper Green River Basin Foothills-Teton and
Gros Ventre Mountains-
231 PM MDT Fri Aug 6 2021

...A strong thunderstorm will impact portions of north central
Sublette County through 300 PM MDT...

At 231 PM MDT, Doppler radar was tracking a strong thunderstorm over
Warren Bridge, or 16 miles northwest of Pinedale, moving north at 20
mph.

HAZARD...Winds in excess of 40 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
  New Fork Lake.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4292 11004 4297 11023 4328 11015 4317 10980
TIME...MOT...LOC 2031Z 201DEG 18KT 4301 11012

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...40 MPH

Vorst