High Surf in Hawaii, April 20 to 21, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST WED APR 20 2011

.SYNOPSIS...
LIGHT TRADE WINDS WILL PREVAIL ACROSS THE ALOHA STATE THROUGH
THURSDAY NIGHT. CLOUDS AND ISOLATED SHOWERS WILL BE POSSIBLE
PRIMARILY OVER LEEWARD AND INTERIOR SECTIONS OF THE ISLANDS DURING
THE AFTERNOON DUE TO LOCAL DAYTIME SEA BREEZES...WHILE THE EASTERLY
WINDS WILL CARRY LIGHT SHOWERS TO WINDWARD FACING SLOPES MAINLY AT
NIGHT. EXPECT SLIGHTLY STRONGER TRADE WIND SPEEDS FROM FRIDAY INTO
THE WEEKEND...WITH BRIEF TRADE SHOWERS FOCUSED MAINLY OVER WINDWARD
AND MAUKA AREAS.

&&

.DISCUSSION...
THE REMNANTS OF A WEAK FRONTAL BOUNDARY THAT BROUGHT RAINFALL TO
KAUAI AND NIIHAU EARLIER THIS WEEK HAVE SHIFTED BACK TOWARD THE
NORTHWEST. THIS NORTH NORTHEAST TO SOUTH SOUTHWEST ORIENTED FEATURE
APPEARS TO BE LOCATED ABOUT 80 MILES NORTHWEST OF LIHUE EARLY THIS
MORNING. A 1024 MB SURFACE HIGH...WHICH IS CENTERED NEAR 37N
147W...IS MOVING TOWARD THE EAST SOUTHEAST AT ABOUT 10 MPH. THE
PRESSURE GRADIENT SOUTH OF THIS HIGH HAS CAUSED THE LIGHT EASTERLY
TRADE WIND FLOW TO AFFECT THE ISLANDS DURING THE PAST 24 TO 30
HOURS. THESE EASTERLY WINDS HAVE ALSO BEEN RESPONSIBLE FOR PUSHING
THE REMNANT FRONTAL BOUNDARY TO THE NORTHWEST OF KAUAI.

WATER VAPOR IMAGERY INDICATES A NORTHEAST TO SOUTHWEST ORIENTED
UPPER TROPOSPHERIC TROUGH IS LOCATED ABOUT 190 MILES WEST NORTHWEST
OF KAUAI. THIS FEATURE...WHICH HAS BEEN PROPAGATING EASTWARD AT
ABOUT 15 MPH...SENT SOME STREAKS OF CIRRUS CLOUDS OVER THE ISLANDS
EARLIER TONIGHT. HOWEVER...WITH RELATIVELY DRY CONDITIONS OVER THE
AREA...THIS TROUGH ALOFT IS HAVING LITTLE SIGNIFICANT AFFECT ON OUR
WEATHER EARLY THIS MORNING.

WITH RIDGING REMAINING FAR NORTH OF THE AREA...LIGHT EASTERLY WINDS
WILL PREVAIL ACROSS THE ISLANDS THROUGH THURSDAY NIGHT. SINCE THE
WIND SPEEDS ARE RELATIVELY LIGHT...WE CAN EXPECT LOCAL AFTERNOON SEA
BREEZES AND NIGHT TIME LAND BREEZES TO DEVELOP OVER THE INDIVIDUAL
ISLANDS THROUGH THURSDAY NIGHT. THE LATEST MODEL OUTPUT SHOWS
PRECIPITABLE WATER /PW/ VALUES WILL REMAIN SLIGHTLY BELOW NORMAL
THROUGH THE FORECAST PERIOD. NIGHTS AND MORNINGS ARE EXPECTED TO BE
MOSTLY DRY...ALTHOUGH WINDWARD FACING SLOPES MAY HAVE SOME BRIEF
LIGHT SHOWERS CARRIED BY THE WEAK EASTERLY FLOW. THE CONVERGENCE OF
THE LOCAL SEA BREEZE CIRCULATIONS OVER THE ISLANDS WILL CAUSE CLOUD
BUILD UPS AND ISOLATED SHOWERS TO DEVELOP OVER INTERIOR AND LEEWARD
SECTIONS OF THE ISLANDS. THE PASSAGE OF THE WEAK UPPER LEVEL TROUGH
ACROSS THE STATE MAY ADD A LITTLE MORE VIGOR TO THE AFTERNOON
CONVECTION THIS AFTERNOON. HOWEVER...THE SHOWER COVERAGE IS EXPECTED
TO BE MORE LIMITED ON THURSDAY DUE TO THE DEVELOPMENT OF WEAK
RIDGING ALOFT ABOVE THE STATE.

THE SURFACE RIDGING NORTH OF THE ISLAND CHAIN IS FORECAST TO
STRENGTHEN FRIDAY AND SATURDAY. AS A RESULT...EXPECT SLIGHTLY
STRONGER TRADES LATER THIS WEEK AND INTO THE WEEKEND. THE TRADES
WILL REMAIN RATHER DRY...WITH BRIEF WINDWARD AND MAUKA SHOWERS
POSSIBLE...ESPECIALLY DURING NIGHTTIME AND EARLY MORNING HOURS.

BY LATE SUNDAY AND MONDAY...THE SURFACE RIDGE NORTH OF THE STATE IS
EXPECTED TO BEGIN WEAKENING AS ANOTHER SURFACE FRONT AND A TROUGH
ALOFT APPROACH THE STATE FROM THE NORTHWEST. THIS WILL AGAIN CAUSE
OUR WINDS TO BECOME LIGHTER...AND VEER TO A MORE SOUTHEASTERLY
DIRECTION EARLY NEXT WEEK. THE GFS AND ECMWF FORECAST MODELS
CONTINUE TO SHOW DIVERGING SOLUTIONS FOR THE FORECAST POSITIONS OF
THIS FRONT. THEREFORE...CONFIDENCE REMAINS LOW ON THE ACTUAL IMPACT
THIS FEATURE MAY HAVE ON WEATHER ACROSS THE STATE NEXT WEEK. FOR
NOW...WE ARE SUGGESTING STRENGTHENING TRADES WILL RETURN TUESDAY AND
WILL CONTINUE AT LEAST THROUGH THE MIDDLE OF NEXT WEEK FOLLOWING THE
GFS SOLUTION.

&&

.MARINE...
FORERUNNERS FROM A NEW LONG PERIOD SOUTH SWELL HAVE BEEN AFFECTING
SOUTH FACING SHORES OF THE ISLANDS SINCE LATE TUESDAY AFTERNOON. THE
BARBERS POINT BUOY NEAR SOUTHWESTERN OAHU CONTINUES TO SHOWS
SIGNIFICANT ENERGY IN THE 18 TO 22 SECOND SPECTRAL BANDS. BUOYS
51002...51003 AND 51004 LOCATED SOUTH OF THE HAWAIIAN ISLANDS ARE
ALSO SHOWING THIS LONG PERIOD ENERGY...EVEN THOUGH THE DOMINANT WAVE
PERIOD CURRENTLY REMAINS LESS THAN 18 SECONDS. THIS LONG PERIOD
SWELL IS EXPECTED TO CONTINUE FILLING IN EARLY THIS MORNING...WHICH
WILL CAUSE BUILDING SURF HEIGHTS ALONG ALL SOUTH FACING SHORES OF
THE STATE. THEREFORE...A HIGH SURF ADVISORY HAS BEEN ISSUED FOR
SOUTH FACING SHORES OF ALL HAWAIIAN ISLANDS STARTING AROUND SUNRISE
MORNING AND CONTINUING AT LEAST THROUGH TONIGHT. THIS SOUTH SWELL IS
EXPECTED TO GRADUALLY DIMINISH THURSDAY AND FRIDAY.

THE MODERATE NORTHWEST SWELL...WHICH IS CURRENTLY AFFECTING THE
ISLANDS...IS EXPECTED TO PEAK EARLY THIS MORNING. THIS NORTHWEST
SWELL WILL GRADUALLY DIMINISH LATER TODAY AND TONIGHT. A COUPLE OF
SMALLER NORTHWEST SWELLS WILL AFFECT THE ISLANDS THIS WEEKEND.

THE CURRENT FORECAST KEEPS WINDS AND SEAS BELOW THE SMALL CRAFT
ADVISORY /SCA/ CRITERIA OVER ALL HAWAIIAN WATERS THROUGH THIS
WEEKEND. INCREASING TRADE WINDS NEXT WEEK WILL BRING THE POSSIBILITY
OF SCA CONDITIONS TO THE TYPICALLY WINDY WATERS ADJACENT TO MAUI AND
THE BIG ISLAND STARTING TUESDAY OR WEDNESDAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY FROM 600 AM THIS MORNING UNTIL 600 AM HST
THURSDAY FOR SOUTH FACING SHORES OF ALL HAWAIIAN ISLANDS.

&&

HOUSTON