High Surf in Hawaii, April 26 to 27, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST MON APR 26 2010

.SYNOPSIS...
HIGH PRESSURE PASSING BY TO THE NORTH OF THE ALOHA STATE WILL
MAINTAIN BREEZY TRADES AND A RATHER DRY WEATHER PATTERN THROUGH
WEDNESDAY. THE SECOND HALF OF THE WEEK MAY BE SOMEWHAT DAMP...AS
WINDS ACROSS THE ISLAND CHAIN SHIFT TO A SOUTHEASTERLY DIRECTION.

&&

.DISCUSSION...
THE TIGHTENING OF THE PRESSURE GRADIENT SOUTH OF A NEARLY EAST TO
WEST ORIENTED SURFACE RIDGE....WHICH IS LOCATED ABOUT 400 MILES
NORTH OF HONOLULU...CAUSED OUR TRADE WIND SPEEDS TO INCREASE SINCE
EARLY SUNDAY MORNING. FOR EXAMPLE...PRESSURE RISES OF 1 TO 2
MILLIBARS COMPARED WITH OBSERVATIONS FROM 24 HOURS AGO ARE OCCURRING
AT MOST ISLAND STATIONS EARLY THIS MORNING. WATER VAPOR IMAGERY
INDICATES SIGNIFICANT MIDDLE AND UPPER TROPOSPHERIC RIDGING IS ALSO
SITUATED JUST NORTH OF THE WESTERN END OF THE FIFTIETH STATE.

A 1025 MB SURFACE HIGH ALONG THE SAME SURFACE RIDGE IS LOCATED ABOUT
720 MILES NORTHWEST OF LIHUE. AS THIS FEATURE CONTINUES TO MOVE
EASTWARD AT ABOUT 15 MPH...EXPECT THE TRADE WINDS TO REMAIN MODERATE
TO LOCALLY STRONG AT LEAST INTO WEDNESDAY MORNING. IN ADDITION TO
MAINTAINING THE BREEZY TRADE WINDS...AS THE HIGH CONTINUES MOVING
EASTWARD AND PASSES NORTH OF THE STATE...IT WILL CAUSE OUR WEATHER
TO BE RATHER DRY FOR THE NEXT FEW DAYS. ANY ISOLATED LIGHT TRADE
SHOWERS THAT DO OCCUR WILL MAINLY FAVOR WINDWARD AND MAUKA SECTIONS
OF THE ISLANDS. THE GUSTY TRADE WINDS MAY ALSO CARRY SOME LIGHT
SHOWERS OVER TO LEEWARD AREAS OF SOME OF THE SMALLER ISLANDS. THE
AREAL COVERAGE OF SHOWERS WILL FOLLOW A DIURNAL PATTERN WITH A
MAXIMUM MAINLY DURING THE EARLY MORNING AND NIGHTTIME HOURS.

THE PRESENCE OF THE MID-LEVEL RIDGING NORTH OF THE STATE ON
TUESDAY...WILL LIKELY CAUSE AN INCREASE IN THE EASTERLY WINDS OVER
MOUNT HALEAKALA AND TO A LESSER EXTENT ACROSS THE BIG ISLAND
SUMMITS. THEREFORE...A WIND ADVISORY MAY BE REQUIRED FOR THE SUMMIT
OF HALEAKALA ON TUESDAY AND/OR WEDNESDAY.

BY THURSDAY...AS THE UPPER-LEVEL RIDGING PROPAGATES TO A POSITION
NORTHEAST OF THE ISLANDS...A DEEP UPPER-LEVEL TROUGH IS EXPECTED TO
BEGIN APPROACHING THE ISLANDS FROM THE WEST-NORTHWEST. AT THE SAME
TIME...THE SURFACE HIGH WILL HAVE MOVED TO A POSITION FAR NORTHEAST
OF THE STATE. THIS WILL CAUSE OUR WINDS TO SHIFT TO A MORE
SOUTHEASTERLY DIRECTION...WHICH WILL LIKELY INCREASE THE LOW-LEVEL
MOISTURE ACROSS MOST OF THE STATE. THE FORECAST MODELS SUGGEST THAT
SOME OF THIS MOISTURE MAY BE ASSOCIATED WITH AN OLD SHEAR LINE
SAGGING DOWN TOWARD THE ISLANDS FROM THE NORTHEAST. THE GFS MODEL
INDICATES THE MOISTURE FROM THIS FORMER SHEAR LINE MAY REMAIN JUST
NORTHEAST OF THE ISLANDS. HOWEVER...THE LATEST RUNS OF THE NAM AND
ECMWF MODELS CONTINUE TO BRING THIS SLUG OF MOISTURE DOWN OVER THE
ISLANDS. FOR NOW...WE WILL MAINTAIN WETTER CONDITIONS IN THE OUTLOOK
FOR THE END OF THE WORK WEEK.

THE GFS AND ECMWF MODELS HINT THAT THE DAMP WEATHER PATTERN WILL
PERSIST INTO NEXT WEEKEND. BOTH MODELS DEPICT A LATE SEASON SURFACE
FRONT...WHICH IS A REFLECTION OF THE UPPER LEVEL TROUGH...
APPROACHING KAUAI FROM THE WEST NORTHWEST. THE LATEST RUN OF THE
ECMWF MODEL STALLS THIS SURFACE FEATURE NEAR KAUAI DURING THE
WEEKEND. THE GFS MODEL SUGGESTS THE FRONT WILL BE RELATIVELY WEAK
WHEN IT REACHES KAUAI NEXT SUNDAY. THEREFORE...IT IS UNCERTAIN AT
THIS TIME HOW FAR DOWN THE ISLAND CHAIN THE FRONT WILL MOVE.
HOWEVER...BOTH THE ECMWF AND GFS MODELS INDICATE THERE WILL BE A
CONTINUATION OF MOIST SOUTHEASTERLY FLOW ACROSS THE ISLANDS AHEAD OF
THIS FEATURE. FOR THE CURRENT FORECAST PACKAGE...IT IS ASSUMED THIS
FRONT WILL AT LEAST REACH KAUAI BEFORE IT STALLS. BUT...THERE IS LOW
CONFIDENCE IN THIS FORECAST. THERE IS ALSO A GREAT DEAL OF
UNCERTAINTY ABOUT THE POSSIBILITY OF EXCESSIVE RAINFALL OCCURRING
WITH THIS SYSTEM. THE LATEST MODEL RUNS HINT THAT LOCALLY HEAVY
RAINFALL MAY OCCUR ACROSS SOME PARTS OF THE STATE NEXT WEEKEND.

&&

.MARINE...
A LONG PERIOD SOUTH SWELL CONTINUES ALONG SOUTH FACING SHORES OF ALL
HAWAIIAN ISLANDS EARLY THIS MORNING. THE WAVEWATCH III MODEL IS
FORECASTING THE SIGNIFICANT WAVE HEIGHT OF THE SWELL WILL BE CLOSE
TO 4 FEET WITH A PERIOD OF ABOUT 17 SECONDS FROM A DIRECTION OF 190
DEGREES LATER THIS MORNING. THE SPECTRAL ENERGY AT BUOYS 2 AND
3...LOCATED SOUTH OF THE HAWAIIAN ISLANDS...HAVE BEEN SHOWING WAVE
PERIODS OF 17 AND 19 SECONDS SINCE LATE SUNDAY AFTERNOON. THE KILO
NALU BUOY...WHICH IS MOORED IN SHALLOW WATER ALONG THE SOUTH SHORE
OF OAHU...ALSO INDICATES A PEAK WAVE PERIOD OF NEAR 19 SECONDS HAS
BEEN PRESENT SINCE EARLY SUNDAY AFTERNOON. AS A RESULT OF THIS LARGE
SWELL...A HIGH SURF ADVISORY HAS BEEN ISSUED FOR THE SOUTH FACING
SHORES OF ALL HAWAIIAN ISLANDS. THE WAVEWATCH III MODEL INDICATES
THAT ELEVATED SURF WILL LIKELY CONTINUE AT LEAST THROUGH TUESDAY
AFTERNOON.

AS A CONSEQUENCE OF THE TIGHT PRESSURE GRADIENT ACROSS THE ISLAND
CHAIN...THE TRADE WINDS REMAIN LOCALLY STRONG IN THE TYPICALLY WINDY
MARINE ZONES ADJACENT TO MAUI AND THE BIG ISLAND. LOCALLY GENERATED
WIND WAVES WILL ALSO LIKELY INCREASE TO NEAR 10 FEET IN SOME OF THE
SAME MARINE ZONES TODAY. THEREFORE...A SMALL CRAFT ADVISORY /SCA/
REMAINS POSTED FOR THE PAILOLO AND ALENUIHAHA CHANNELS...MAALAEA BAY
AND THE WATERS SOUTH OF THE BIG ISLAND. THE LATEST FORECAST SUGGESTS
THESE WINDY CONDITIONS WILL PERSIST AT LEAST THROUGH EARLY WEDNESDAY
MORNING. ELSEWHERE...THE TRADE WIND GENERATED WAVES AND THE SOUTH
SWELL DESCRIBED ABOVE COMBINED WITH A NEW MODERATE-SIZED NORTHWEST
SWELL ARRIVING LATER TODAY WILL LIKELY INCREASE THE SIGNIFICANT WAVE
HEIGHTS TO NEAR 10 FEET OVER OTHER PARTS OF THE HAWAIIAN COASTAL
WATERS ADJACENT TO KAUAI AND OAHU. THEREFORE...A SCA HAS ALREADY
BEEN ISSUED FOR POTENTIALLY HAZARDOUS SEAS IN THESE MARINE ZONES FOR
TONIGHT.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY THROUGH 600 PM HST TUESDAY FOR SOUTH FACING
SHORES OF ALL HAWAIIAN ISLANDS.

SMALL CRAFT ADVISORY THROUGH 900 AM HST WEDNESDAY FOR THE PAILOLO
AND ALENUIHAHA CHANNELS...MAALAEA BAY AND THE WATERS SOUTH OF THE
BIG ISLAND.

SMALL CRAFT ADVISORY FROM 600 PM HST THIS EVENING THROUGH 600 AM HST
TUESDAY MORNING FOR THE WATERS SURROUNDING KAUAI AND OAHU...
INCLUDING THE KAUAI AND KAIWI CHANNELS.

&&

HOUSTON