Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 171704
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1103 AM MDT SUN MAY 17 2015

.SHORT TERM...SUNDAY THROUGH TUESDAY NIGHT

ANOTHER DAY...ANOTHER REPORT OF SEVERE WEATHER. YESTERDAY THE ACTION
WAS AGAIN IN PARK COUNTY WHERE THERE WERE SEVERAL FUNNEL CLOUDS AND
EVEN A CONFIRMED TOUCHDOWN OF A WEAK TORNADO. TODAY LOOKS LIKE
ANOTHER UNSETTLED DAY AS WELL AS AN UPPER LEVEL LOW CONTINUES TO
SLOWLY MOVE ACROSS THE STATE AND SOME MOISTURE WRAPS AROUND FROM A
SURFACE LOW IN THE NORTHERN PLAINS. THIS MEANS ANOTHER MOSTLY
CLOUDY DAY WITH MORE SHOWERS AND A FEW THUNDERSTORMS. HOWEVER...IT
WILL BE ABOUT 5 TO 10 DEGREES COOLER EAST OF THE DIVIDE AFTER THE
PASSAGE OF A COLD FRONT AND AS A RESULT...ALMOST ALL THE CAPE AND
NEGATIVE LIFTED INDICES ARE MAINLY ACROSS WESTERN AND SOUTHERN
AREAS SO WE CONFINED THUNDER TO THOSE LOCATIONS. WE CAN`T RULE OUT
A RUMBLE OF THUNDER ELSEWHERE...BUT IT SHOULD BE MORE STABLE
TODAY. THERE IS STILL SOME SHEAR THOUGH SO I WOULD NOT BE
SURPRISED TO SEE A REPORT OF A COLD AIR FUNNEL IN SPOTS THOUGH. AS
FOR SNOW LEVEL...THE MODELS HAVE TRENDED A BIT WARMER WITH 700
MILLIBAR TEMPERATURES BOTTOMING OUT AT MINUS 3 WHICH WOULD BUT THE
LEVELS AROUND 7000 FEET WITH MOST NOT STICKING EXCEPT FOR THE
HIGHER ELEVATIONS. IN ADDITION...AREAS THAT HAVE GUSTY WINDS WITH
COLD ADVECTION FOLLOWING A COLD FRONT...LIKE THE NORTHERN BIG HORN
BASIN AND JOHNSON COUNTY WILL SEE GUSTY NORTH TO NORTHWEST WINDS
THROUGH THE DAY...BUT NOT TO HIGH WIND LEVEL. WE WILL CONTINUE THE
SPS AS THAT SEEMS SUFFICIENT. THE COVERAGE OF SHOWERS SHOULD
DECREASE TONIGHT AS SHORTWAVE RIDGING BUILDS ACROSS THE AREA
TONIGHT.

FOR MONDAY...IT LOOKS LIKE THE DRIEST OF THE THREE DAYS OF THE SHORT
TERM ACROSS THE BODY OF THE FORECAST AREA. ANOTHER SHOT OF MOISTURE
WILL MOVE IN FROM THE SOUTHWEST...BUT SHOULD BE A MAINLY CONFINED TO
AREAS WEST OF THE DIVIDE THROUGH MOST OF THE DAY. EVEN
THERE...ACTIVITY SHOULD BE SCATTERED SO MOST OF THE DAY SHOULD BE
RAIN FREE. MOISTURE WILL BEGIN TO SPREAD ACROSS THE DIVIDE DURING
THE EVENING HOURS AND INTO THE NIGHT. THIS DAY ALSO LOOKS TO BE THE
MILDEST DAY OF THE NEXT THREE.

DEEPER MOISTURE THEN MOVES INTO THE AREA AS ANOTHER UPPER LEVEL LOW
MOVES TOWARDS WYOMING FOR TUESDAY AND SPREADS MORE MOISTURE ACROSS
THE AREA. THIS LOOKS LIKE THE WETTEST AND COOLEST DAY OF THE THREE.
WITH DEEPER MOISTURE AND LARGELY STABLE AIR ACROSS AREAS EAST OF THE
DIVIDE FOR TUESDAY...WE DECIDED TO GO WITH JUST PLAIN RAIN AND NOT
SHOWERS. WEST OF THE DIVIDE...WHERE THINGS MAY BE MORE UNSTABLE WE
KEPT THINGS MORE CONVECTIVE THOUGH. AND ONCE AGAIN...WITH CHILLY AIR
IN PLACE SOME OF THE HIGHER ELEVATIONS COULD EVEN SEE SOME SNOW ON
TUESDAY NIGHT.

.LONG TERM...WEDNESDAY THROUGH SUNDAY

SYNOPSIS...THE OVERALL PATTERN OF COOL AND SHOWERY WEATHER WILL
CONTINUE THROUGH THIS PERIOD WITH THE NEXT PACIFIC STORM SYSTEM
IMPACTING THE AREA PRIMARILY ON FRIDAY AND SATURDAY.

DISCUSSION...BLOCKING RIDGE OVER WESTERN CANADA FORECAST TO REMAIN
THROUGH MEMORIAL DAY WEEKEND AND BEYOND.  PACIFIC STORM SYSTEMS
MOVING INTO THE NORTHEAST PACIFIC WILL CONTINUE TO SPLIT AROUND THIS
RIDGE WITH THE SOUTHERN END OF THE SPLIT TAKING THE STORMS SOUTH OFF
THE WEST COAST BEFORE TURNING EAST INTO SOUTHERN CALIFORNIA...THEN
LIFTING NE ACROSS THE GREAT BASIN AND CENTRAL ROCKIES...WITH STORMS
SPACED AROUND 72 HOURS APART.  THIS COOL AND WET PATTERN FOR THE
AREA IS EXPECTED TO CONTINUE WELL INTO THE FINAL WEEK OF MAY.

THE NEXT IN THIS SERIES OF STORMS WILL SHEAR OUT ACROSS
CENTRAL/EASTERN WYOMING INTO THE HIGH PLAINS ON WEDNESDAY WITH UPPER
RIDGE PUSHING UP FROM WEST TEXAS ACROSS THE DIVIDE WEDNESDAY NIGHT.
AREAS EAST OF THE DIVIDE WILL SEE SCT-NMRS SHOWERS UNDER THE UPPER
LOW DURING THE DAY WEDNESDAY WHILE THE SOUTH AND WEST WILL SEE A BIT
MORE INSTABILITY AND SOME THUNDERSTORMS BLOSSOMING IN THE
AFTERNOON.  THERE SHOULD BE A LETUP IN PRECIPITATION WEDNESDAY NIGHT
INTO THURSDAY MORNING BETWEEN SYSTEMS.

THE NEXT STORM SYSTEM IN THIS SERIES WILL PUSH INTO SOUTHERN
CALIFORNIA LATE THURSDAY WITH SOUTHERLY DIFLUENT DOWNSTREAM ACROSS
THE GREAT BASIN AND CENTRAL ROCKIES THURSDAY AFTERNOON/EVENING.
MOISTURE AND INSTABILITY AND THUS SHOWER AND THUNDERSTORM ACTIVITY
WILL BE BEST ACROSS THE SOUTH AND WEST...UP TO NEAR A CPR-RIW LINE
WITH A MORE STABLE AIRMASS OVER NE WYOMING...WHERE SHOWERS AND
THUNDERSTORMS SHOULD WEAKEN AS THEY MOVE UP FROM THE SOUTH.

THE MAIN UPPER LOW IS EXPECTED TO LIFT NE ACROSS THE GREAT BASIN AND
CENTRAL ROCKIES FRIDAY AND SATURDAY WITH AT LEAST SCATTERED SHOWERS
ACROSS THE AREA WITH SOME THUNDERSTORMS IN THE AFTERNOON/EVE.

THIS SYSTEM WILL THEN LIFT ACROSS THE PLAINS ON SUNDAY.  BOTH GFS
AND ECMWF SHOW AN UPPER LOW BECOMING TRAPPED UNDER THE WESTERN
CANADIAN RIDGE OVER THE NW U.S. SUNDAY...BUT CONFIDENCE IS LOW IN
THIS SCENARIO AND WOULD PREFER PERSISTENCE IN THIS BLOCKING PATTERN
WITH WITH NEXT SOUTHERN STREAM LOW MAKING LANDFALL ON THE S CA COAST
AROUND TUE MAY 26TH.

&&

.AVIATION.../18Z ISSUANCE/

ISOLATED TO WIDELY SCATTERED SHOWERS WILL OCCUR THROUGH 06Z AT MOST
TERMINALS...PERSISTING UNTIL THE END OF THE FORECAST PERIOD BETWEEN
KRKS AND KCPR. BETTER COVERAGE EXPECTED ACROSS THE CENTRAL AND
SOUTHERN PORTIONS OF THE FORECAST AREA. VFR TO OCCASIONAL MVFR
CONDITIONS WILL OCCUR THROUGH THE PERIOD...WITH IMPROVEMENT
GENERALLY FROM NORTH TO SOUTH THIS AFTERNOON. AREAS OF MVFR
CONDITIONS DUE TO LOW CIGS WILL RETURN AGAIN LATER THIS EVENING AS
IT COOLS OFF IN A RATHER MOIST ATMOSPHERE. VCFG MAY ALSO BE EXPECTED
ON AN ISOLATED BASIS AFTER 09Z MONDAY. FREQUENT MOUNTAIN
OBSCURATION WILL CONTINUE THROUGH THE FORECAST PERIOD WITH LOW CIGS
ACCOMPANYING LIGHT RAIN AND/OR SNOW. ISOLATED THUNDERSTORMS WILL
ALSO BE POSSIBLE...MAINLY SOUTH OF A KPNA TO KCPR LINE THIS
AFTERNOON AND EARLY EVENING.

&&

.FIRE WEATHER...

FIRE DANGER RELATIVELY LOW INTO THE MIDDLE OF THE WEEK AS SEASONAL
TO COOL AND RELATIVELY MOIST CONDITIONS PERSIST. HOWEVER...IT WILL
BE UNSTABLE AGAIN TODAY IN THE SOUTH AND WEST...OFFERING UP CHANCES
FOR MORE THUNDERSTORMS AND ASSOCIATED LIGHTNING. SNOWS IS LIKELY TO
RETURN TO THE HIGHER ELEVATIONS AGAIN THIS WEEKEND WITH SNOW LEVELS
LOWERING TO AROUND 7000 FEET AT TIMES. AFTER A SHORT BREAK SUNDAY
NIGHT...MORE SHOWERS ARE POSSIBLE WEST OF THE DIVIDE FOR MONDAY WITH
A WETTING RAIN A DECENT POSSIBILITY FOR MANY AREAS ON TUESDAY.     &&

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

$$

SHORT TERM...HATTINGS
LONG TERM...AEM
AVIATION...BRAUN
FIRE WEATHER...HATTINGS









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