197
FXHW60 PHFO 021352
AFDHFO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
352 AM HST MON NOV 2 2009
SYNOPSIS
MODERATE TRADE WINDS WILL BRING A FEW WINDWARD SHOWERS TODAY...
BEFORE A COLD FRONT USHERS IN A WEATHER CHANGE LATE TONIGHT AND
TUESDAY. THE COLD FRONT WILL BRING A PERIOD OF SHOWERS TO MOST
ISLANDS...AND A SLIGHT CHANCE OF THUNDERSTORMS TO KAUAI AND
OAHU...AS IT MOVES DOWN THE ISLAND CHAIN. STRONG NORTHEAST WINDS
WILL FOLLOW IMMEDIATELY BEHIND THE FRONT...AND WILL BRING IN A
COOLER AIR MASS THAT WILL LINGER FOR A COUPLE OF DAYS. A RETURN TO
TRADE WIND WEATHER IS EXPECTED FOR THE SECOND HALF OF THE WORK
WEEK...AND NEXT WEEKEND.
DISCUSSION
A STRONG HIGH PRESSURE CELL IS CURRENTLY LOCATED WELL NW OF THE
ISLANDS...WITH A WEAKER HIGH WELL TO THE NE. A STALLED FRONTAL
BOUNDARY IS LOCATED ABOUT 200 TO 250 MILES NW OF KAUAI...WITH
STRONG TO NEAR GALE N AND NE WINDS LOCATED NW OF THIS BOUNDARY.
MODERATE TO LOCALLY STRONG ENE TRADE WINDS CURRENTLY EXIST OVER THE
ISLANDS...SUPPORTED BY THE HIGH TO THE NE AND IT/S ASSOCIATED RIDGE.
NOT EXPECTING BIG CHANGES TO THE CURRENT REGIME TODAY...WITH THE ENE
TRADES SUPPLYING CLOUDS...AND A FEW SHOWERS...TO WINDWARD AREAS.
CHANGES TO ISLAND WEATHER ARE EXPECTED LATER TONIGHT AND TUESDAY AS
THE HIGH TO THE NW EDGES EASTWARD...AND THE STALLED FRONTAL BOUNDARY
IS INVIGORATED AS IT IS SHUNTED SE TOWARD...AND OVER...THE ISLANDS.
WHETHER TO CALL THE BOUNDARY A FRONT OR SHEAR LINE ALWAYS SEEMS TO
BE A POINT OF CONTENTION AMONGST FORECASTERS...BUT THE PUBLIC IS
SURE TO NOTICE THE COOLER AIR MASS FOLLOWING THE PASSAGE OF THE
BOUNDARY...BROUGHT IN BY STRONG NE WINDS...AND HAVE THUS DECIDED TO
CONTINUE TO CALL THE BOUNDARY A FRONT.
OVERNIGHT MODEL RUNS INDICATE A SLIGHTLY SLOWER-MOVING AND WEAKER
BOUNDARY...RESULTING IN SLIGHTLY LATER ARRIVAL TIMES AS COMPARED TO
PREVIOUS RUNS. LATEST FORECAST BRINGS THE BOUNDARY OVER KAUAI AFTER
DARK TONIGHT...REACHING OAHU DURING THE DAY TUESDAY...MAUI COUNTY
TUESDAY EVENING...BEFORE WASHING OUT NEAR THE BIG ISLAND TUESDAY
NIGHT AND WEDNESDAY. THE SURFACE BOUNDARY WILL BE ACCOMPANIED BY A
MID AND UPPER-LEVEL TROUGH...WHICH COULD PROVIDE SUFFICIENT
DYNAMICAL SUPPORT FOR THUNDERSTORMS...HENCE THE SLIGHT CHANCE IN THE
FORECAST. GIVEN RECENT MODEL TRENDS...CHANCES APPEAR TO BE
DIMINISHING...SO HAVE PARED BACK THE THUNDERSTORM THREAT AND LIMITED
IT TO KAUAI AND OAHU /AND SURROUNDING WATERS/ TONIGHT AND EARLY
TUESDAY.
NE WINDS WILL SURGE TO NEAR 25 KT AFTER THE PASSAGE OF THE SURFACE
BOUNDARY TONIGHT AND TUESDAY...AND MAY BE STRONG ENOUGH TO
NECESSITATE THE ISSUANCE OF A WIND ADVISORY THAT INCLUDES THE LOWER
ELEVATIONS...WITH THE GREATEST THREAT OF STRONG WINDS ON KAUAI AND
OAHU. NE WINDS WILL EASE PRETTY QUICKLY ON WEDNESDAY...AS THE HIGH
WEAKENS AND IT/S ASSOCIATED RIDGE MOVES CLOSER TO THE ISLANDS.
THE SURFACE BOUNDARY WILL WASH OUT NEAR THE BIG ISLAND ON WEDNESDAY
AS THE HIGH TO THE NW GRADUALLY WEAKENS WHILE CONTINUE TO DRIFT E.
THE MODERATE LOW LEVEL WIND FLOW WILL FAVOR A NE DIRECTION ON
WEDNESDAY AND THEN VEER TO THE ENE ON THURSDAY...WITH GLOBAL MODELS
INDICATING A WEAK SHOWER BAND MOVING ACROSS THE ISLAND CHAIN FROM
THE NE AS THIS OCCURS. A RETURN TO MORE TYPICAL TRADE WIND WEATHER
IS THEN EXPECTED AS WE HEAD INTO NEXT WEEKEND...BUT THIS SOLUTION IS
NOT SHARED BY ALL MODELS AND CONFIDENCE IS MODERATE.
MARINE
A DIMINISHING LONG PERIOD NW SWELL CONTINUES TO MOVE THROUGH ISLAND
WATERS THIS MORNING...WITH A HIGH SURF ADVISORY /HSA/ REMAINING IN
EFFECT FOR N SHORES OF ISLANDS FROM MAUI WESTWARD...AND FOR W FACING
SHORES OF KAUAI AND NIIHAU AS WELL. SURF HEIGHTS ARE EXPECTED TO
DROP BELOW ADVISORY LEVELS BY THIS EVENING.
THE SMALL CRAFT ADVISORY /SCA/ FLAG IS BACK UP DUE TO 25 KT WINDS
AROUND THE TIPS OF THE BIG ISLAND DETECTED BY QUIKSCAT...AND THE SCA
WILL NEED TO BE EXPANDED TO WATERS AROUND KAUAI AND OAHU AS WINDS
AND SEAS BUILD BEHIND THE FRONT TONIGHT AND TUESDAY. EVEN WITH WINDS
EXPECTED TO EASE SOMEWHAT TOWARD THE MIDDLE OF THE WEEK...THE SCA
AREAS MAY NEED TO BE EXPANDED...AS COMBINED SEAS ABOVE 10 FEET WILL
AFFECT MANY MARINE ZONES.
ALTHOUGH CONDITIONS WILL BE MARGINALIZED DUE TO THE CLOSE PROXIMITY
OF THE SWELL SOURCE AND THE NORTHERLY ANGLE OF THE WINDS...SURF WILL
BE ELEVATED ALONG NORTH FACING SHORES FROM TUESDAY INTO FRIDAY...
LIKELY REQUIRING A HSA FOR AT LEAST PART OF THAT TIME.
HFO WATCHES/WARNINGS/ADVISORIES
HIGH SURF ADVISORY UNTIL 6 PM HST TODAY FOR NIIHAU-KAUAI WINDWARD-
KAUAI LEEWARD-OAHU NORTH SHORE-OAHU KOOLAU-MOLOKAI-MAUI WINDWARD
WEST-MAUI CENTRAL VALLEY-WINDWARD HALEAKALA.
BIRCHARD
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