Severe Thunderstorm Warning 23, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
1134 AM MDT Thu Jun 25 2020

.SHORT TERM...This afternoon through Friday night

Weak showers developed over the Bighorn Basin shortly after dawn
this morning. Additional showers over the Green River Basin are
moving over the Wind River Mountains and into the Wind River Basin
by 15Z. A more widespread area of showers over eastern ID,
associated with a surface low along the cold front, is starting to
spread over the Star and Jackson Valleys at the same time. The
eastern flank of the cold front looks to have moved through areas
east of the Divide late night, as surface winds are now N-NE.
Boundary conditions that have been forecasted the last couple of
days remain in place. The RIW 12Z sounding PW of 0.70 of an inch.
Precipitatable water values will range between 0.75 to an 1 inch
across the forecast area. Overnight lows for many areas were in
the middle to upper 50s to the lower 60s. Dewpoints at 14Z range
from the middle 40s to the middle 50s. This instability is also
reflected with CAPE values of 1000 to 2000 J/kg and a lifted index
around minus 4. The main limiting factor for severe storms
remains weak upper level support. The right rear quadrant of the
supporting jet streak will primarily be over southern MT today.
700mb winds on the 12Z sounding were 15-20 kt from 650-500mb.
These winds are still forecasted to be 10-20kt through today.
Locally heavy rain and small hail will be the main threats today,
although strong gusty winds cannot be ruled out. Two areas where
stronger storms could occur are Johnson/Natrona counties this
afternoon and southern Lincoln County. Both areas will be
influenced by the passage of the cold front this afternoon and
look to be where the greatest instability will be located.

Showers and storms are expected to have ended/exited the area by
midnight, except Natrona County, where they will linger for a few
more hours. Drier conditions will be in place across the forecast
area Friday. Isolated thunderstorms could be possible, especially
over the higher terrain and southern Sweetwater County. The boundary
layer will remain moist, with PW's ranging between 0.60 to 0.70 and
dewpoints in the 40s to near 50 degrees.

.LONG TERM...Sunday through Wednesday
Issued at 224 AM MDT Wed Jun 24 2020

Once the weak trough gets nudged east Saturday night, dry westerly
flow is still expected to develop for the day Sunday. Temperatures
will soar back into the 80s and 90s, with humidity readings well
below 20 percent for the lower elevations. The atmosphere continues
to buckle on Sunday, as a larger trough continues to push into the
northern Rockies, with strong ridging along the Mississippi. This
puts most of western and central WY in the southwesterly flow ahead
of the trough, which is a more agitated pattern. Right now, the
placement of the trough will focus the rain chances more across the
far western higher elevations and Yellowstone Park, with scattered
showers and thunderstorms through the afternoon, and looking more
likely to continue overnight Sunday night. Models have become more
consistent with the forecast for Monday, bringing the core of the
midlevel trough somewhere across Idaho, with southerly flow
continuing across the state keeping the unsettled pattern going for
most of western WY. Both main models are noting a cold front
sweeping across the state sometime later Monday, with the GFS during
the late afternoon, and ECMWF later in the evening. Will need to
watch this feature timing for stronger convective potential, if
it passes during peak heating.

Models continue to diverge in the longer term after Monday, so any
solution is taken with low confidence at this time.

&&

.AVIATION...For the 18Z TAFS through 18Z Friday
Issued at 1134 AM MDT Wed Jun 24 2020

Showers and thunderstorms will continue to increase in coverage over
western WY through 01Z Friday. Along and east of the divide, showers
and storms will be developing early this afternoon and continue
through 04Z Friday. Some stronger storms will be capable of
producing wind gusts to 40 knots and small hail. Severe
thunderstorms are not out of the question, but confidence is low for
any direct impacts. The terminal forecasts will have VFR conditions,
but local MVFR conditions are likely in areas of heavy rain.

&&

.FIRE WEATHER...

A weather system will cross the area today and bring showers and
thunderstorms to much of the area. Some intense storms, with
strong winds and hail, are possible in zones 280/281 and zone
277. Many areas will have a chance of a wetting rain today. Winds
are expected to be relatively light today, outside of any shower
or thunderstorm. Precipitation is expected to end across much of
the area by midnight.

&&

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

SHORT TERM...LaVoie
LONG TERM...Straub
AVIATION...Hulme
FIRE WEATHER...Hattings/LaVoie

Hazardous Weather Outlook
National Weather Service Riverton WY
226 AM MDT Thu Jun 25 2020

WYZ001>020-022>030-260830-
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-
226 AM MDT Thu Jun 25 2020

This Hazardous Weather Outlook is for Western and Central Wyoming.

.DAY ONE...Today and Tonight.

Scattered thunderstorms with the best coverage over the north.
Some strong to severe thunderstorms are possible, especially in
eastern areas.

.DAYS TWO THROUGH SEVEN...Friday through Thursday.

Saturday...Elevated fire weather conditions possible over central
and southern areas.

Sunday and Monday...Elevated to possibly critical fire weather
conditions mainly over central and southern areas. Isolated to
scattered thunderstorms, except for the south. Some thunderstorms
could be strong.

.SPOTTER INFORMATION STATEMENT...

Weather spotters are encouraged to report significant weather
conditions, according to standard operating procedures.

TEAM RIVERTON

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