Special Weather Statement, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
548 AM MDT Fri Aug 6 2021

.SHORT TERM...(Today through Saturday)
Issued at 321 AM MDT Fri Aug 6 2021

A well-defined shortwave trough traverses the Rockies today, causing
active weather across western and central Wyoming over the next 24
hours. Falling heights lead to slightly cooler temperatures, about 5
F cooler than yesterday. Synoptic lift ahead of the wave encourages
greater thunderstorm coverage this afternoon and evening, and given
deep layer shear of 30 to 45 knots combined with 500 to 700 J/kg
of CAPE over the northern basins, isolated strong to severe storms
are possible. Current SPC outlooks highlight the region in a
Marginal Risk for severe weather, mainly due to damaging outflow
wind potential, and this lines up well with local forecaster
thinking.

Further south, from Uinta through Natrona county, afternoon rain and
thunderstorms will be less widespread; however, a strong pressure
gradient and deep mixing lead to sustained west winds 25 to 30 mph,
and frequent gusts to 45 mph. Additionally, the relatively dry air
mass allows afternoon relative humidity to drop into the the
upper teens. Given the winds well above critical fire weather
criteria, low humidity, and receptive fuels, a Red Flag Warning
covers this region for the afternoon and evening hours.

Rain and thunderstorms decrease in coverage after sunset. One
exception is the Bighorn Basin and Bighorn Mountains, where several
high-resolution models show a subtle vorticity maximum causing
showers to continue through Saturday morning. Expect another
slightly cooler than normal day Saturday, with much less active
weather thanks to brief ridging in the wake of Friday's system.
Mostly clear skies and light winds should lead to favorable
weather for most outdoor activities.

.LONG TERM...Saturday through Friday

Flat ridging should control the weather from Saturday evening
through most of Sunday. However, a strong cold front will be
approaching from the north and west. This will tighten the pressure
gradient up and bring a gusty wind to much of the area. With low
humidity expected, this could be a day of elevated to critical fire
weather. Now, as for the cold front, it will sweep across the state
Sunday night into Monday and bring in some cooler air. As for the
chance of showers and thunderstorms, it looks rather limited at this
point. Almost all of the models keep the best moisture and upper
level forcing over Montana. In addition, the system does not have a
lot of moisture to work with. We do have some slight POPs in
northern Wyoming from Sunday night into Monday but most areas will
not see rain from this system. It will bring wind though, especially
in areas favored by the northwest flow / cold advection pattern.
This especially includes the northern Big Horn Basin, Wind River
Basin and northern Johnson County where things could get blustery
post frontal. And, this could bring another day of elevated to
critical fire weather on Monday. Lower temperatures may mitigate
this somewhat though.

Flat zonal flow will then dominate for Tuesday and bring a dry and
warm day. Again, with the fast flow, wind may be gusty and could
bring another day of elevated fire weather to the the area.
Uncertainty then increases for the middle of next week. The GFS
brings another cold front into the area and a cooler day for
Wednesday and even Thursday. The European on the other hand, keeps
ridging over the area and deflects the cooler air the north and east
with warm to hot temperatures continuing over the area. Given the
uncertainty, for now we played the middle ground with a slight hedge
toward the European model, keeping temperatures warm. The GFS would
be a nice, if brief taste of September with highs in the 70s and
lows possibly falling into the 40s at night, stay tuned.
Temperatures should then see a warming trend toward the end of the

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 538 AM MDT Fri Aug 6 2021

Despite a very active weather pattern, VFR conditions prevail at
terminal sites today, but brief MVFR conditions are possible due
to thunderstorms. Scattered showers and storms currently over the
region continue this morning. Multiple rounds of rainfall are
expected in most areas. Northwest WY will see an increase in
showers and storm coverage after 06/1400z, lasting through
06/2000z. For the Upper Green and Wind River basins, storms begin
after 06/1800z and continue through around 07/0100z. Showers and storms
decrease in coverage after sunset; however, the Bighorn Basin
including KWRL and KCOD may see showers throughout the night.

In addition to the gusty winds near any thunderstorms, a tight
pressure gradient causes strong wind gusts of 35 to 45 mph during
the afternoon and evening, especially in the wind corridor from
Uinta to Natrona county, including at KRKS and KCPR. These winds
decrease after sunset.

&&

.FIRE WEATHER...

A combination of low humidity and windy conditions will bring
critical fire weather across much of southern Wyoming as well as
Natrona County. Elsewhere, relative humidity should remain above
critical levels. Isolated to scattered showers and thunderstorms
will be possible today, with strong wind gusts the main threat for
any storm. The best chance for convection will be across northern
Wyoming. Mixing and smoke dispersal will range from fair to good in
the north to very good to excellent in central and southern

&&

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

&&

SHORT TERM...VandenBoogart
LONG TERM...Hattings
AVIATION...VandenBoogart
FIRE WEATHER...Hattings

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

WYZ001>020-022>030-071200-
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-
338 AM MDT Fri Aug 6 2021

...Red Flag Warning for portions of southwest and central Wyoming
this afternoon and evening...

This Hazardous Weather Outlook is for Western and Central
Wyoming.

.DAY ONE...Today and Tonight.

Scattered afternoon and evening thunderstorms, some of which could
be severe across the northern half of Wyoming, with localized
damaging outflow winds.

In addition, west winds 20 to 30 mph with gusts to 45 mph are
forecast from Uinta to Natrona county this afternoon and evening.
Low humidities combined with these gusty winds lead to critical
fire weather conditions.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

A cold front Sunday night into Monday could lead to breezy winds
and increased fire danger.

.SPOTTER INFORMATION STATEMENT...

Weather spotters are encouraged to report significant weather
conditions.

Special Weather Statement
National Weather Service Riverton WY
505 AM MDT Fri Aug 6 2021

WYZ001-002-012-013-061330-
Yellowstone National Park-Absaroka Mountains-
Teton and Gros Ventre Mountains-Jackson Hole-
Including the cities of Lake, Mammoth, Old Faithful, and Jackson
505 AM MDT Fri Aug 6 2021

...Thunderstorms with gusty outflow winds across the region...

This is a special weather statement from the National Weather
Service Office in Riverton.

* WHAT...A line of showers and thunderstorms moving northeastward
  is producing strong outflow winds with gusts 40 to 50 mph.

* WHERE...Northwest Wyoming.

* WHEN...Through 730 AM.

Special Weather Statement
National Weather Service Riverton WY
810 AM MDT Fri Aug 6 2021

WYZ012-013-023-024-061445-
Star Valley-Jackson Hole-Teton and Gros Ventre Mountains-Salt River
and Wyoming Ranges-
810 AM MDT Fri Aug 6 2021

...Strong thunderstorms will impact portions of northwestern Lincoln
and southwestern Teton Counties through 845 AM MDT...

At 809 AM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 8 miles southwest of Rafter J Ranch to near
Star Valley Rest Area. Movement was east at 35 mph.

HAZARD...Winds in excess of 40 mph and pea 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.

Strong thunderstorms will be near...
  Rafter J Ranch around 820 AM MDT.
  Jackson and Hoback around 825 AM MDT.

Other locations in the path of these storms include Meadow Creek
Guard Station.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Persons in campgrounds should consider seeking sturdy shelter until
these storms pass.

&&

LAT...LON 4344 11104 4352 11045 4283 11062 4281 11104
TIME...MOT...LOC 1409Z 254DEG 30KT 4335 11092 4286 11105

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...40 MPH

Myers

Special Weather Statement
National Weather Service Riverton WY
929 AM MDT Fri Aug 6 2021

WYZ002-012-014>016-061600-
Wind River Mountains West-Upper Wind River Basin-Wind River Mountains
East-Teton and Gros Ventre Mountains-Absaroka Mountains-
929 AM MDT Fri Aug 6 2021

...Strong thunderstorms will impact portions of northwestern Fremont
and south central Park Counties through 1000 AM MDT...

At 928 AM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 16 miles north of Pinnacle Buttes to near Union
Pass. Movement was east at 40 mph.

HAZARD...Winds in excess of 40 mph and pea 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...
  Dubois and Torrey Lake.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Persons in campgrounds should consider seeking sturdy shelter until
these storms pass.

&&

LAT...LON 4357 11009 4380 11007 4404 10999 4413 10943
      4345 10943 4343 11005
TIME...MOT...LOC 1528Z 259DEG 34KT 4398 10992 4348 10997

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...40 MPH

Myers