Flash Flood Warning 4, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
1126 AM MDT Mon Jun 26 2023

.DISCUSSION...This afternoon through Sunday
Issued at 1103 AM MDT Mon Jun 26 2023

The longwave trough over the PACNW will remain in place today, with
the ridge axis from the west TX high now further east over the High
Plains. A shortwave trough over NV/UT will move over the Cowboy
State today, as a result of the southwest flow aloft. Showers and
thunderstorms look to be focused further north, with convection
developing between 18Z and 19Z. Most of this will be focused from
the Gros Ventres and the northern end of the Wind River Mountains
into the Absarokas eastward. Strong to severe thunderstorms will be
possible over portions of Johnson County, with most of these looking
to impact the I-90 corridor. MLCAPE values of 1500-2000 J/kg and LI
values of -6 to -8 will be in place across Johnson County, northern
portions of Natrona County and the Wind River Basin and southern
portions of the Bighorn Basin. Dewpoints in the lower 50s already in
place across areas east of the Divide this morning. Dew points look
to increase to the mid 50s by this afternoon across Johnson/Natrona
counties, as a result of the continued east-southeast winds
advecting in Gulf moisture. All these factors will contribute to a
marginal chance for severe thunderstorms again. Convection will get
pushed into far northern portions of the CWA this evening toward
sunset, with isolated showers and thunderstorms remaining possible
over the Bighorn Basin and the northern end of the Bighorns through
the night tonight.

Showers and thunderstorms will be more widespread across the
northern half of the CWA once again Tuesday, as a low at the base of
the longwave trough begins to eject and move over the region. Any
strong/severe storm looks to be isolated, as the timing for any
upper level support will exit eastward in the morning. Additionally,
the shear as a result of the veering profile today will not be in
place Tuesday. The longwave trough will remain in place Wednesday
and Thursday, as additional upper lows continue to circulate over
the PACNW. Afternoon and evening showers and thunderstorms will
remain possible, as additional shortwaves move over the area as a
result of this pattern. This looks to change Friday, as the upper
low finally moves eastward and becomes in-phase with another upper
low over the western Ontario/Upper Midwest. Additionally, the strong
high over TX will have shifted toward the Gulf Coast and weaken as
it does so. A ridge will build over the Northern Rockies Saturday,
in the wake of the upper low's exit bringing drier and warmer
conditions to the area. Model solutions become split by Sunday as a
result of a strong upper low over western Canada. The GFS solution
progs this system having more of an impact to the CWA, bringing a
line of thunderstorms over eastern MT and clipping northern portions
of the state. The ECMWF keeps the low much shallower and thus keeps
these thunderstorms further north.

&&

.AVIATION...For the 18Z TAFS through 18Z Tuesday
Issued at 1103 AM MDT Mon Jun 26 2023

VFR conditions expected through the TAF period. Showers and
thunderstorms are expected across much of Wyoming this afternoon and
evening, with the strongest over northern Wyoming. Gusty winds and
lightning are the threats near any storms. Models are showing a few
showers/storms near KLND and KRIW, so have included VCTS through the
afternoon. Rain chances decrease late evening, with any lingering
storm outflows remaining the threat into the early night. Winds will
become lighter through the overnight hours, with more showers and
storms expected Tuesday.

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 1103 AM MDT Mon Jun 26 2023

Critical RH values and wind gusts of 25 to 30 mph will occur this
afternoon in zone 279 and the surrounding border areas. Strong to
severe thunderstorms will be possible east of the Divide today,
with the better chances in zone 281. Otherwise, minimal fire
weather concerns this week. Unsettled weather will continue,
especially over the northern half of the state, with daily
afternoon and evening rain shower and thunderstorm activity
expect. Near average temperatures and wind gusts of 20-25 mph
gusts expected. Minimum relative humidity values expected to stay
above the 40-50 percent range for the northern half of the CWA
with points south dipping to around the 20 percent mark with the
lowest being near the Colorado state line, from Tuesday onward.

&&

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

&&

DISCUSSION...LaVoie
AVIATION...Wittmann
FIRE WEATHER...Lowe/LaVoie

Hazardous Weather Outlook
National Weather Service Riverton WY
103 AM MDT Mon Jun 26 2023

WYZ001>020-022>030-271115-
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-
103 AM MDT Mon Jun 26 2023

This Hazardous Weather Outlook is for Western and Central Wyoming.

.DAY ONE...Tonight.

Strong to marginally severe thunderstorms are expected to develop
early afternoon Monday off the Bighorns in Johnson County before
quickly pushing to the east of the CWA by late afternoon. The main
severe weather threats will be larger hail over the size of quarters
and damaging winds up to 60 mph. Tornado threat is low but locally
heavy rainfall is possible as well in areas of entraining storms or
rain showers that linger through the evening across much of the
northern half of the CWA. Any lingering rain showers and
thunderstorms will weaken towards sunset Monday evening.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

Tuesday will see similar conditions as Monday but less of a severe
weather chance with weaker parameters expected. The severe weather
threat will be limited to Johnson County again during peak heating of
the afternoon being shortlived in the process. Lingering scattered
rain showers and isolated thunderstorms for the northern half of the
CWA east of the Divide expected once again into the evening hours
diminishing towards sunset once again.

Unsettled weather continues through the remainder of the long term
with scattered rain showers and isolated thunderstorms but severe
weather probability is low after Tuesday at this point in time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed.

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Special Weather Statement
National Weather Service Riverton WY
259 PM MDT Mon Jun 26 2023

WYZ010-262115-
Northeast Johnson County-
259 PM MDT Mon Jun 26 2023

...A strong thunderstorm will impact portions of central Johnson
County through 315 PM MDT...

At 257 PM MDT, Doppler radar was tracking a strong thunderstorm 10
miles south of Buffalo, moving east at 10 mph.

HAZARD...Winds in excess of 30 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 storm will remain over mainly rural areas of central Johnson
County.

This includes Interstate 25 between mile markers 287 and 295.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4426 10649 4417 10659 4416 10677 4418 10680
      4430 10671
TIME...MOT...LOC 2057Z 291DEG 9KT 4420 10666

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...30 MPH

Wittmann

Special Weather Statement
National Weather Service Riverton WY
314 PM MDT Mon Jun 26 2023

WYZ010-011-262145-
Southeast Johnson County-Northeast Johnson County-
314 PM MDT Mon Jun 26 2023

...A strong thunderstorm will impact portions of central Johnson
County through 345 PM MDT...

At 314 PM MDT, Doppler radar was tracking a strong thunderstorm 10
miles south of Buffalo, moving southeast at 15 mph.

HAZARD...Winds in excess of 30 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 central Johnson
County.

This includes Interstate 25 between mile markers 279 and 294.

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.

&&

LAT...LON 4420 10680 4430 10670 4418 10645 4405 10662
TIME...MOT...LOC 2114Z 308DEG 13KT 4419 10667

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...30 MPH

Wittmann