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Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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Hawaiian Islands Synoptic Discussion and Guidance

639
FXHW60 PHFO 030141
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST WED SEP 2 2015

.SYNOPSIS...
A MOIST AND SOMEWHAT UNSTABLE SOUTHERLY FLOW WILL PRODUCE VERY HUMID
CONDITIONS WITH SPOTTY HEAVY SHOWERS AND A SLIGHT CHANCE OF A
THUNDERSTORMS DURING THE NEXT COUPLE OF DAYS. WEAK TRADE WINDS AND
INCREASINGLY STABLE CONDITIONS WILL GRADUALLY BUILD FROM THE EAST
ON FRIDAY AND SATURDAY...THOUGH HOT AND HUMID WEATHER WILL PERSIST.
WEAK TRADE WINDS MAY HOLD INTO EARLY NEXT WEEK.

&&

.DISCUSSION...
HEAVY SHOWERS AND THUNDERSTORMS ARE FLARING THIS AFTERNOON WITH THE
GREATEST ACTIVITY OBSERVED ON MAUI AND THE BIG ISLAND. THIS
CONVECTION IS BEING TRIGGERED BY DAY TIME HEATING UNDER A MOIST AND
UNSTABLE ATMOSPHERE. AFTERNOON SOUNDINGS SHOW A DEEP...RATHER MOIST
PROFILE WITH PRECIPITABLE WATER VALUES WELL ABOVE NORMAL AT 1.7 TO 2
INCHES. AN UPPER LEVEL TROUGH PARKED JUST WEST AND SOUTHWEST OF
KAUAI IS MAINTAINING THE INSTABILITY...WHICH IS HELPING TO PRODUCE
BURSTS OF LIGHTNING ON THE BIG ISLAND. THUNDERSTORM ACTIVITY WILL
TAPER OFF LATE IN THE AFTERNOON...THOUGH A CONTINUED HEAVY SHOWER
AND THUNDERSTORM THREAT WILL REMAIN FOR MAINLY THE SMALLER ISLANDS
TONIGHT.

SPOTTY HEAVY SHOWERS AND THUNDERSTORMS UNDER AN UNSTABLE AND MOIST
SOUTHERLY FLOW WILL PERSIST THROUGH THURSDAY. THE GFS HINTS SUGGESTS
THAT A ZONE OF MAXIMUM LOW LEVEL CONVERGENCE TO THE SOUTH OF
IGNACIO WILL BE OVER THE CENTRAL ISLANDS TONIGHT THEN SHIFT TO THE
WESTERN END OF THE STATE ON THURSDAY. CHANCES OF RAINFALL WILL BE
GREATEST IN THIS ZONE OF WEAK CONVERGENCE...AND AN UPPER LEVEL
TROUGH PARKED JUST WEST OF KAUAI WILL PRODUCE SUFFICIENT INSTABILITY
TO KEEP THE THREAT OF HEAVY SHOWERS AND THUNDERSTORMS. THIS THREAT
WILL BE GREATEST THURSDAY AFTERNOON UNDER MAXIMUM DAY TIME HEATING.

ASIDE FROM THE HEAVY RAIN AND THUNDERSTORM THREAT...ANOTHER CONCERN
WILL BE THE VERY HUMID CONDITIONS. THE ISLANDS ARE LOCKED UNDER A
TROPICAL AIRMASS WITH DEW POINTS HOVERING IN THE MID 70S TO EVEN THE
UPPER 70S AT TIMES. WHILE MAX TEMPS WILL BE NEAR TO SLIGHTLY ABOVE
NORMAL...THE HUMIDITY WILL CREATE HEAT INDEX VALUES IN THE VERY
UNCOMFORTABLE RANGE OF THE UPPER 90S...AND OVERNIGHT TEMPS WILL
STRUGGLE TO GET BELOW THE UPPER 70S.

AS IGNACIO MOVES FAR NORTH OF THE STATE ON FRIDAY AND SATURDAY...
WEAK EASTERLY WINDS AND WEAK RIDGING ALOFT WILL SLOWLY BUILD OVER
THE ISLANDS FROM THE EAST. WHILE THIS MAY INHIBIT HEAVY SHOWER
DEVELOPMENT...IT WILL PRODUCE LITTLE RELIEF FROM THE HOT AND HUMID
CONDITIONS.

HEADING INTO NEXT WEEK...FORECAST CONFIDENCE DECREASES DUE TO
UNCERTAINTY IN THE EXACT POSITION OF HURRICANE JIMENA. THIS FEATURE
SHOULD BE SEVERAL HUNDRED MILES NORTHEAST OF THE STATE...CLOSE
ENOUGH TO PREVENT TRADE WINDS FROM INCREASING SIGNIFICANTLY. AS A
RESULT...EXPECT CONTINUED HOT AND HUMID CONDITIONS UNDER LIGHT WINDS.

&&

.AVIATION...
SEE TC SIGMET SERIES TANGO FOR TROPICAL CYCLONE IGNACIO.

THUNDERSTORMS AND HEAVY SHOWERS ONCE AGAIN PLAGUED THE ISLANDS THIS
AFTERNOON. THE BULK OF THE ACTIVITY HAS BEEN FOCUSED OVER THE BIG
ISLAND AND MAUI...NEAR A WEAK CONVERGENCE BAND EXTENDING SOUTH FROM
TC IGNACIO. MOST OF THE CURRENT HEAVY SHOWERS AND THUNDERSTORMS WILL
SUCCUMB TO DIURNAL EFFECTS AND START TO DIMINISH WITH SUNSET.
HOWEVER...POTENTIAL FOR THUNDERSTORMS OVERNIGHT WILL REMAIN AS THE
CONVERGENCE BAND MENTIONED ABOVE TRACKS WEST AND BECOMES INFLUENCED
BY AN UPPER LEVEL TROUGH LOCATED JUST WEST OF KAUAI.

THIS EVENING A SLIGHT CHANCE OF THUNDERSTORMS WILL REMAIN OVER THE
CENTRAL PART OF THE STATE. BY LATE TONIGHT THE FORECAST CALLS FOR
THUNDERSTORM ACTIVITY TO SPREAD OVER KAUAI. ANY THUNDERSTORMS ARE
LIKELY TO CARRY CB TOPS TO FL350-400...MVFR TO ISOL IFR...AND THE
POSSIBILITY OF ICING AND TURB.

AIRMET SIERRA FOR MOUNTAIN OBSCURATION REMAINS FOR SECTIONS OF THE
BIG ISLAND AS OF 02Z. THIS SPECIFIC AIRMET WILL BE CANCELLED AS
CONDITIONS IMPROVE AFTER SUNSET. HOWEVER...WE CAN NOT RULE OUT THE
NEED FOR ADDITIONAL MOUNTAIN OBSCURATIONS TONIGHT. THESE WILL BE
ISSUED AS NEEDED.

&&

.MARINE...
A HIGH SURF WARNING REMAINS IN EFFECT FOR EAST FACING SHORES OF MOST
ISLANDS. THE LARGE SURF IS DUE TO A MIX OF DECLINING NORTHERLY SWELL
FROM IGNACIO AND A BUILDING EAST SWELL FROM DISTANT HURRICANE
JIMENA. THE IGNACIO SWELL REMAINS LARGEST ACROSS KAUAI AND OAHU AND
HAS DROPPED OFF SIGNIFICANTLY ELSEWHERE. NEAR SHORE CDIP BUOYS
SHOWED A STRONG ONSET OF THE EAST SWELL THIS MORNING...BUT WE HAVE
SEEN A TEMPORARY LULL THIS AFTERNOON AS SIGNIFICANT WAVE HEIGHTS
HAVE DROPPED SLIGHTLY. SURF ALONG EAST FACING SHORES WILL BE AT
BORDER LINE WARNING LEVELS TONIGHT...WITH AN EXPECTED RISE AGAIN
TOMORROW. THE HIGH SURF WARNING CURRENTLY GOES THROUGH THURSDAY
AFTERNOON...AND WE WILL NOT MAKE ANY CHANGES AT THIS TIME AS WE
WATCH BUOY TRENDS TONIGHT. EVEN IF THE SWELL DROPS SLIGHTLY...EAST
SHORE SURF WILL REMAIN ELEVATED AT OR NEAR WARNING LEVELS THROUGH
AT LEAST THE WEEKEND AND LIKELY INTO EARLY NEXT WEEK.

A SMALL CRAFT ADVISORY IS ALSO IN EFFECT THROUGH FRIDAY AFTERNOON
FOR HAZARDOUS SEAS DUE TO THE SWELLS FROM IGNACIO AND JIMENA. WINDS
ARE EXPECTED TO REMAIN BELOW ADVISORY LEVELS THIS WEEK.

LONG-PERIOD SWELLS FROM THE SOUTH-SOUTHWEST MAY PRODUCE ADVISORY
LEVEL SURF ALONG SOUTH-FACING SHORES THIS WEEKEND. PLEASE REFER TO
THE LATEST COLLABORATIVE NEARSHORE SWELL AND WIND FORECAST /SRDHFO/
FOR MORE DETAILS ON THE SWELLS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF WARNING UNTIL 6 PM HST THURSDAY FOR EAST FACING SHORES OF
KAUAI...OAHU...MOLOKAI...MAUI...AND THE BIG ISLAND.

SMALL CRAFT ADVISORY UNTIL 6 PM HST FRIDAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI CHANNEL-OAHU WINDWARD WATERS-
KAIWI CHANNEL-MAUI COUNTY WINDWARD WATERS-PAILOLO CHANNEL-ALENUIHAHA
CHANNEL-BIG ISLAND WINDWARD WATERS-BIG ISLAND SOUTHEAST WATERS.

&&

$$

DISCUSSION/MARINE...WROE
AVIATION...EATON




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