Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 301717
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1117 AM MDT SAT MAY 30 2015

.SHORT TERM...TODAY THROUGH MONDAY NIGHT

WE ARE CURRENTLY EXPERIENCING ONE OF THE DRIEST NIGHTS IN A WHILE.
WITH SATURATED SOILS IN MANY AREAS ALONG WITH MOSTLY CLEAR SKIES
AND LIGHT WINDS...WILL LEAVE FOG IN SOME OF THE BASINS AND VALLEYS
FOR THIS MORNING.

ANOTHER PROMINENT CLOSED LOW PRESSURE CENTER IS CURRENTLY SPINNING
ALONG THE BRITISH COLUMBIA COAST WITH A TRAILING WEAK SHORTWAVE NOW
ALONG THE PACIFIC COAST. THIS SHORTWAVE WILL RAPIDLY TRACK EAST
ACROSS MT TODAY THUS FLATTENING THE WEAK RIDGE OVER US WITH MOST OF
THE ENERGY FROM THIS WEAK SHORTWAVE TO OUR NORTH...THE BEST CAPES
WILL BE OVER THE WEST CENTRAL AND NORTH TODAY. OVERALL...IT WILL
NOT BE THAT UNSTABLE TODAY SO HAVE PAINTED THE HIGHEST SCATTERED
POPS TODAY OVER THE WESTERN AND NORTHERN MTNS WITH JUST SHOWERS AND
ISOLATED THUNDERSTORMS IN THESE AREAS EXPECTED FOR THIS AFTERNOON
AND EVENING. WITH LIGHT STEERING WINDS...MOST OF THE CONVECTION WILL
BE ANCHORED TO THE MTNS. TEMPERATURES WILL BE AROUND 10 DEGREES
WARMER TODAY WITH HIGHS IN THE 70S AT THE LOWER ELEVATIONS EAST OF
THE DIVIDE AND 60S WEST.

ON SUNDAY THE RIDGE WILL REAMPLIFY. THIS WILL ENHANCE THE SOUTHWEST
FLOW ON THE BACKSIDE OF THE HIGH USHERING IN THE WARMEST AIR OF THE
CALENDER YEAR THUS FAR WITH MANY AREAS EAST OF THE DIVIDE AT THE
LOWER ELEVATIONS REACHING THE 80S FOR THE FIRST TIME AND 70S WEST.
THIS SOUTHWEST FLOW WILL ALSO USHER IN SOME SOUTHERN STREAM MOISTURE
THAT WILL COMBINE WITH INCREASING INSTABILITY. AN OLD SURFACE
BOUNDARY THAT WILL STILL BE DRAPED ACROSS NORTHEAST WY FROM THE
FROPA THAT OCCURRED THURSDAY NIGHT WILL SEPARATE HIGHER DEW POINTS
IN THE 50S AND HIGH TEMPS IN THE 70S IN JOHNSON COUNTY WITH HIGHS IN
THE 80S AND DEW POINTS IN THE 40S ELSEWHERE...EXCEPT HIGHS IN THE
70S WEST OF THE DIVIDE AS MENTIONED ABOVE. THIS BOUNDARY WILL ACT AS
A FOCUS FOR THUNDERSTORMS OVER JOHNSON COUNTY. IN ADDITION...WITH A
SMALLER TEMP/DEW POINT SPREAD EXPECT MORE IN THE WAY OF LOCALLY
HEAVY RAIN AND SMALL HAIL IN JOHNSON COUNTY SUNDAY AFTERNOON AND
EVENING. ELSEWHERE...GUSTY WINDS AND LIGHTNING SHOULD BE THE
DOMINANT FEATURES WITH THE THUNDERSTORMS MOVING IN WITH THE
SOUTHWEST UNSTABLE FLOW.

ON MONDAY THE RIDGE CONTINUES TO AMPLIFY RESULTING IN SCORCHING
TEMPS COMPARED TO WHAT WE HAVE BECOME ACCUSTOMED TO LATELY WITH SOME
AREAS IN THE BIG HORN BASIN REACHING 90 DEGREES. A WELL DEFINED
SHORTWAVE WILL TRACK NORTHEAST ACROSS THE AREA IN THE SOUTHWEST FLOW
ON THE BACKSIDE OF THE RIDGE RESULTING HIGHER BASED BUT STRONGER
THUNDERSTORMS SO WILL THROW IN SMALL HAIL AND GUSTY WINDS EVERYWHERE
FOR NOW. SPC JUST MISSES A SLIGHT RISK FOR JOHNSON COUNTY BY ABOUT A
HALF MILE MONDAY.

ONE SIDE NOTE...INCREASED SOUTHWEST WINDS THROUGH THE PERIOD...
MAINLY IN THE ABSAROKAS AND EAST SLOPE OF THE WIND RIVERS...TO
ACCOUNT FOR THE INCREASED SOUTHWEST FLOW ON THE BACKSIDE OF THE
RIDGE.

.LONG TERM...TUESDAY THROUGH SATURDAY

SHORTWAVES IN A LOW AMPLITUDE TROUGH WILL IMPACT THE FORECAST AREA
TUESDAY AND WEDNESDAY WITH DAILY CHANCES OF CONVECTION. HOWEVER
MODELS DIFFER SIGNIFICANTLY WITH POSITION OF ANY UPPER LEVEL JET
STREAKS OR WEAK FRONTAL BOUNDARIES...WHICH MIGHT AID IN DEVELOPMENT
AND STRENGTH OF CONVECTION. THE 00Z GFS AND GEM HAVE A DRIER
SOLUTION THAN PREVIOUS RUNS FOR TUESDAY...BUT THE ECMWF CONTINUES TO
GIVE A DECENT SHOT AT CONVECTION...ESPECIALLY OVER THE NORTHWEST AND
NORTH.

BY THURSDAY...MEDIUM RANGE MODELS ARE IN DECENT AGREEMENT WITH THE
BROAD TROUGH EARLY IN THE WEEK BECOMING MORE AMPLIFIED ACROSS THE
WEST. THE ECMWF IS THE MOST AMPLIFIED WITH AN UPPER LOW DIGGING
ACROSS CENTRAL CALIFORNIA...WHILE THE GFS SHOWS AN UPPER LOW OVER
THE PACIFIC NORTHWEST. THE GFS THEN BRINGS AN OPEN TROUGH ACROSS THE
ROCKIES FRIDAY NIGHT/SATURDAY...WHILE THE ECMWF SHOWS MORE OF A
NEARLY STATIONARY UPPER LOW OVER THE GREAT BASIN FRIDAY AND
SATURDAY. AT THE SURFACE...GLOBAL MODELS SHOW A WEAK...QUASI
STATIONARY FRONTAL BOUNDARY DRAPED ACROSS THE FORECAST AREA. THIS
FRONTAL BOUNDARY COMBINED WITH THIS UPPER LEVEL ENERGY NEAR THE AREA
WILL KEEP CHANCES OF DAILY CONVECTION IN THE FORECAST.

EXPECT SOME BOUTS OF STRONG TO POSSIBLY SEVERE CONVECTION DURING THE
LONG TERM...ESPECIALLY ALONG AND EAST OF THE SURFACE BOUNDARY. ALSO
CONSECUTIVE WARM TO VERY WARM DAYS IN THE SHORT TERM INTO THE LONG
TERM...WILL PROMOTE HIGH ELEVATION SNOWMELT. THIS COMBINED WITH
LOCALIZED HEAVY RAINFALL FROM THUNDERSTORMS AND ALREADY SATURATED
GROUNDS WILL KEEP FLOODING A CONCERN.

&&

.AVIATION.../18Z ISSUANCE/

EXPECT VFR CONDITIONS OVER THE REGION THROUGH 18Z SATURDAY. ISOLATED
SHOWERS AND STORMS WILL OCCUR OVER AND NEAR THE MOUNTAINS FROM 20Z
TODAY UNTIL 02Z SUNDAY. A FEW SHOWERS WILL MOVE EAST OFF THE
MOUNTAINS AND ACROSS THE LOWER ELEVATIONS FROM 21Z TODAY UNTIL 03Z
SUNDAY. PATCHY FOG AFTER 9Z TONIGHT IN AREAS THAT RECEIVE A SHOWER
BUT PINPOINTING THAT IS DIFFICULT AT THIS TIME.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

VFR CONDITIONS WILL PREVAIL OVER THE REGION. ISOLATED SHOWERS AND
STORMS WILL OCCUR AFTER 19Z OVER THE WESTERN MOUNTAINS. SOME OF
THESE SHOWERS OR STORMS WILL MOVE OVER THE LOWER ELEVATIONS AFTER
20Z TODAY UNTIL 03Z SUNDAY...ESPECIALLY IN THE VICINITY OF KJAC.
PATCHY FOG MAY DEVELOP NEAR KJAC AFTER 9Z TONIGHT.
&&

.FIRE WEATHER...

DRIER AND WARMER WEATHER FOR MUCH OF THE AREA TODAY EXCEPT A FEW
THUNDERSTORMS COULD OCCUR IN THE NORTH. ISOLATED TO SCATTERED
SHOWERS WILL RETURN SUNDAY AFTERNOON ACROSS NORTHERN AND CENTRAL
PORTIONS. THIS WILL MARK A RETURN TO ANOTHER ACTIVE WEATHER
PATTERN THROUGH THE MIDDLE OF NEXT WEEK. SMOKE DISPERSAL EACH DAY
WILL BE GOOD TO EXCELLENT...EXCEPT FOR SHORT PERIODS IN THE VERY
EARLY MORNING HOURS. A HAINES OF SIX WILL AFFECT PORTIONS OF
SOUTHERN AREAS EARLY NEXT WEEK.

&&

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

$$

SHORT TERM...LIPSON
LONG TERM...WM
AVIATION...HATTINGS
FIRE WEATHER...LIPSON








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