Severe Thunderstorm Warning 10, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
127 PM MDT Mon May 21 2018

.SHORT TERM...(This evening through Thursday)
Issued at 119 PM MDT Mon May 21 2018

A Pacific low pressure trough extending south from the Pacific
Northwest into the Great Basin will deepen and continue to move
east spreading moisture and instability across the area producing
scattered showers and isolated thunderstorms with small hail and
gusty winds this evening.  Clouds and precipitation will become
more widespread on Tuesday with areas of heavy rain along and east
of the Continental divide. Temperatures will cool on Tuesday
with elevated relative humidity as precipitation becomes more
widespread on Tuesday.  Showers will continue with decreasing
thunderstorm activity on Wednesday as temperatures continue to
cool.  The heaviest precipitation will be over central, north and
east areas, with the least in the south. Some showers and
thunderstorms will linger over the area on Thursday with some
warming.

.LONG TERM...(Thursday night through Monday)
Issued at 119 PM MDT Mon May 21 2018

We should see gradually building heights to start the medium range
period beyond our pesky early to midweek upper low which finally
exits stage north later Thursday. There is still some moisture and
instability to start the period Thursday evening for some lingering
convection. Even with warming mid level temps Friday, there is still
some forecast mid level moisture resulting in some building capes
over the far west and possibly far nrn Johnson Friday afternoon. So
some thunderstorm threat in these areas. Otherwise, dry and warm.
The next upper low then begins to enter onto the CA coast Friday
night into Saturday before slowly heading our way over the Memorial
Day weekend. Mean ridge holds to the west so initial feeling is that
system will bump into the ridge and lift more ne across Western WY
late in the Memorial Day weekend or possibly stall out just to our
west which is what the new Euro shows. It initially starts to lift
nnewd on Sunday but then redevelops swd over UT on Memorial Day. The
GFS just stalls it out over UT Sunday through Memorial Day. Either
way, an unsettled period with a high probability of showers and
thunderstorms for much of the area late in the weekend into Memorial
Day. Still a long way off so it will have to be watched closely over
the next couple days. In addition, the warming ahead of this system
with cause rapid snowmelt and if this is followed by more
significant rain, we could see serious river rises on what will be
high water levels to begin with.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 119 PM MDT Mon May 21 2018

West of the Continental Divide...KJAC/KBPI/KPNA/KRKS

VFR conditions to prevail through the forecast period. Scattered
showers and thunderstorms will continue over the west through 06Z.
Then isolated showers to linger through 12Z Tuesday. Local MVFR
conditions will occur with some thunderstorms. Mountains will be
obscured at times. Wind gusts to 30 knots and small hail will occur
with stronger storms. Showers and thunderstorms will be more
numerous Tuesday afternoon and evening.

East of the Continental Divide...KCPR/KCOD/KRIW/KLND/KWRL

VFR conditions to prevail through the forecast period. Scattered
showers and thunderstorms will continue over and near the mountains
through 06Z. Then isolated showers to linger through 12Z Tuesday.
Local MVFR conditions will occur with some thunderstorms. Mountains
will be obscured at times. KCOD airport looks to have the best
chance of seeing thunderstorm activity this afternoon and evening.
Showers and thunderstorms will be on the increase Tuesday afternoon
with widespread showers and thunderstorms Tuesday night. Some storms
will be quite strong to severe, especially eastern zones.

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 119 PM MDT Mon May 21 2018

A Pacific low pressure trough extending south from the Pacific
Northwest into the Great Basin will deepen and continue to move
east spreading moisture and instability across the area producing
scattered showers and isolated thunderstorms with small hail and
gusty winds this evening.  Clouds and precipitation will become
more widespread on Tuesday with areas of heavy rain along and east
of the Continental divide. Temperatures will cool on Tuesday
with elevated relative humidity as precipitation becomes more
widespread on Tuesday.  Showers will continue with decreasing
thunderstorm activity on Wednesday as temperatures continue to
cool.  The heaviest precipitation will be over central, north and
east areas, with the least in the south. Some showers and
thunderstorms will linger over the area on Thursday with some
warming. Mixing heights will be favorable even with relatively
light transport winds to produce good or better smoke dispersal
each afternoon through Thursday except in areas receiving heavier
precipitation where poor smoke dispersal will occur.

&&

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

SHORT TERM...Baker
LONG TERM...Skrbac
AVIATION...Skrbac
FIRE WEATHER...Baker

Hazardous Weather Outlook
National Weather Service Riverton WY
210 AM MDT Mon May 21 2018

WYZ001>020-022>030-221100-
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-
210 AM MDT Mon May 21 2018

This Hazardous Weather Outlook is for Western and Central Wyoming.

.DAY ONE...Today and Tonight

Isolated afternoon and evening thunderstorms mainly over the
northern and western portions.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Tuesday...Widespread showers and thunderstorms by late afternoon,
continuing overnight. Heavy rain with localized flooding with the
stronger storms, especially east of the Divide. Some stronger
storms will have hail and strong wind gusts.

Wednesday...A chance of afternoon and evening thunderstorms,
especially over western and northern portions. Locally heavy rain
possible over the northwest.

Thursday...A chance of afternoon and evening thunderstorms over
and near the mountains. Isolated thunderstorms elsewhere.

Sunday...A chance of thunderstorms in the west. Isolated
thunderstorms elsewhere.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today or tonight.

Special Weather Statement
National Weather Service Riverton WY
238 PM MDT MON MAY 21 2018

WYZ003-004-212115-
North Big Horn Basin-Cody Foothills-
238 PM MDT MON MAY 21 2018

...SIGNIFICANT WEATHER ADVISORY FOR WEST CENTRAL BIG HORN AND EAST
CENTRAL PARK COUNTIES UNTIL 315 PM MDT...

At 237 PM MDT, Doppler radar was tracking a strong thunderstorm 8
miles southeast of Ralston, which is 11 miles south of Powell, moving
northeast at 10 mph.

Nickel size hail and winds in excess of 40 mph will be possible with
this storm along with very heavy rain.

This strong thunderstorm will be near...
  Ralston around 305 PM MDT.
  Powell around 315 PM MDT.

Other locations impacted by this storm include Park County
Fairgrounds and Northwest College.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

To report severe weather, contact your nearest law enforcement
agency. They will relay your report to the National Weather Service
office in Riverton.

&&

LAT...LON 4447 10871 4457 10894 4481 10888 4465 10846
TIME...MOT...LOC 2037Z 207DEG 10KT 4460 10879

13

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