High Surf in Hawaii, April 19 to 23, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1015 AM HST SUN APR 19 2015

.SYNOPSIS...
HIGH PRESSURE PASSING NORTH OF THE STATE WILL BRING WINDY CONDITIONS
THE NEXT COUPLE OF DAYS...WITH BREEZY CONDITIONS PREVAILING FOR THE
REMAINDER OF THE WEEK. THE TRADE WINDS WILL FOCUS CLOUDS AND SHOWERS
OVER WINDWARD SLOPES AND COASTS...BUT A FEW SHOWERS WILL SPREAD
LEEWARD...WITH SHOWERS MOST ACTIVE DURING NIGHTS AND MORNINGS. A
DISTURBANCE NORTHWEST OF THE ISLANDS MAY TURN WINDS TOWARD THE
SOUTHEAST LATER IN THE WEEK...POTENTIALLY BRINGING INCREASED SHOWERS
TO KAUAI.

&&

.DISCUSSION...
LATEST SURFACE ANALYSIS FEATURES TWO HIGH PRESSURE CENTERS WELL N
OF THE ISLANDS...ONE TO THE NE AND THE OTHER TO THE NW. THE HIGH TO
THE NE IS TRACKING SW AND WEAKENING WHILE THE HIGH TO THE NW IS
TRACKING E AND STRENGTHENING. MID AND UPPER LEVEL ANALYSIS INDICATES
A RIDGE DIRECTLY OVERHEAD OF THE ISLANDS...WITH EARLY MORNING
SOUNDINGS INDICATING THAT THE BASE OF THE SUBSIDENCE INVERSION IS
BETWEEN 6 AND 8 KFT. DESPITE THE STABLE ATMOSPHERE...MOISTURE
EMBEDDED IN THE TRADE FLOW IS DEEP ENOUGH TO FUEL A FEW OPEN OCEAN
SHOWERS...WHILE OROGRAPHIC FORCING OF THE STRONG TRADE WIND FLOW IS
ALSO AIDING IN PROMOTING SHOWER DEVELOPMENT. FOR THE REMAINDER OF
TODAY...THE EXPECTATION IS THAT CLOUDS AND SHOWERS WILL DIMINISH ON
KAUAI AND OAHU AS UPSTREAM MOISTURE WANES...BUT A CLUSTER OF BROKEN
LOW CLOUDS UPSTREAM OF MAUI AND THE BIG ISLAND WILL LIKELY KEEP
WINDWARD AREAS OF THOSE ISLANDS A LITTLE CLOUDIER AND WETTER
THROUGH THE DAY. SOME OF THIS MOISTURE IS EXPECTED TO FUEL TRADE
SHOWERS OVER KAUAI AND OAHU TONIGHT.

THE HIGH TO THE NW WILL TRACK STEADILY TOWARD THE E AND STRENGTHEN
OVER THE NEXT 24-48 HOURS...TOPPING OUT NEAR 1034 MB FAR NE OF THE
ISLANDS ON TUESDAY. FROM WEDNESDAY THROUGH THE END OF THE WEEK...
THE HIGH WILL SLOWLY WEAKEN AS IT DRIFTS S. THUS WE ARE LOOKING FOR
TRADE WINDS TO INCREASE A NOTCH TODAY AND TONIGHT...REMAINING WINDY
THROUGH TUESDAY...WITH WINDS EASING JUST A LITTLE LATER IN THE WEEK.
IN THE SHORT-TERM...THE WINDS MAY REACH ADVISORY LEVEL IN SOME
WIND-PRONE AREAS ACROSS THE ISLANDS UNDER THE STABLE AIR MASS.

FOR THE MOST PART MODELS AGREE ON THE EVOLUTION OF THE SURFACE
HIGH...BUT THERE ARE SOME DIFFERENCES WITH RESPECT TO OTHER
FEATURES. ALTHOUGH THE MODEL SOLUTIONS ARE A LITTLE MORE CLOSELY
ALIGNED WITH RESPECT TO THE ANTICIPATED DEVELOPMENT AND EVOLUTION OF
A CUT-OFF LOW TO THE NW OF THE ISLANDS TUESDAY/WEDNESDAY...ENOUGH OF
A DIFFERENCE EXISTS BETWEEN THE GFS AND ECMWF TO HAVE IMPLICATIONS
FOR HAWAIIAN WEATHER. THE GFS BRINGS A MODERATE INSTABILITY AND
MOISTURE INCREASE ASSOCIATED WITH THE LOW OVER THE ISLANDS...AROUND
MID-WEEK...AND ALSO BRINGS IT CLOSE ENOUGH TO VEER WINDS TO SSE FROM
TUESDAY INTO THE WEEKEND. MEANWHILE...THE NEWLY-ARRIVED ECMWF
GUIDANCE HAS TRENDED A LITTLE CLOSER TO THE GFS. ONGOING FORECAST
FAVORS THE OLDER ECMWF SOLUTION...WHICH ESSENTIALLY KEEPS A FAIRLY
STABLE AND BREEZY TRADE WIND FLOW IN PLACE THROUGH THE WEEK. WILL
LIKELY NUDGE THE WIND FORECAST A LITTLE MORE TOWARD THE SE FOR THE
LATTER PART OF THE WEEK IN DEFERENCE TO THIS. THE MID LEVEL RIDGE
CURRENTLY OVER THE ISLANDS WILL SHIFT SLIGHTLY E THIS WEEK...KEEPING
FAIRLY STABLE CONDITIONS IN PLACE NEAR THE BIG ISLAND...WITH KAUAI
WEATHER POTENTIALLY INFLUENCED BY THE LOW AFTER MID-WEEK.

&&

.AVIATION...
A PATCH OF LOW CLOUDS WITH EMBEDDED SHOWERS THAT MOVED ASHORE IN
MANY AREAS THIS MORNING HAS CLEARED SUCH THAT AIRMET SIERRA FOR MTN
OBSCURATION WAS CANCELLED...BUT ENOUGH LINGERS NEAR WINDWARD BIG
ISLAND TO SUPPORT ITS CONTINUATION...AT LEAST FOR NOW. AIRMET TANGO
FOR LO LVL TURB BLO 080 WILL CONT FOR THE FORESEEABLE FUTURE FOR
ALL ISLANDS. WHILE VFR CONDITIONS WILL PREVAIL AT THE TERMINALS...
BRIEF MVFR CIG/VIS IN SHRA CAN BE EXPECTED AT WINDWARD LOCATIONS...
MOST NOTABLY PHTO/

&&

.MARINE...
A SMALL CRAFT ADVISORY /SCA/ IS IN EFFECT THROUGH TUESDAY FOR ALL
HAWAIIAN WATERS...DUE TO WINDS AND SEAS ASSOCIATED WITH
STRENGTHENING HIGH PRESSURE PASSING N OF THE ISLANDS. WINDS ARE
EXPECTED TO INCREASE A NOTCH TODAY...WITH NEAR GALE FORCE WINDS
EXPECTED IN CHANNELS AROUND MAUI AND THE BIG ISLAND. IN FACT...
LATEST GUIDANCE FROM HI-RES MODELS INDICATES GALE FORCE WINDS IN THE
ALENUIHAHA CHANNEL FROM TONIGHT INTO MONDAY NIGHT. THUS THE MARINE
FORECAST HAS BEEN MODIFIED TO UPGRADE THE SCA TO A GALE WARNING...
BEGINNING LATER TODAY...FOR THIS CHANNEL. ALTHOUGH WINDS MAY EASE
AFTER TUESDAY...ONLY A SLIGHT DOWNTREND IS EXPECTED...AND THE SCA
WILL LIKELY NEED TO BE EXTENDED.

THE STRONG TRADE WINDS WILL ALSO BRING NEAR ADVISORY LEVEL SURF
ALONG E FACING SHORES THE NEXT FEW DAYS...WITH RATHER ROUGH AND
CHOPPY SURF UNDER THE WINDS. LATEST NEAR SHORE BUOY OBSERVATIONS
INDICATE 8-9 FOOT SEAS WITH A DOMINANT PERIOD OF 8-9 SECONDS...WHICH
IS VERY CLOSE TO THE VALUES WE NORMALLY LOOK FOR WHEN ISSUING HIGH
SURF ADVISORIES FOR TRADE WIND SWELLS. WILL LIKELY BE ISSUING AN
ADVISORY AS WAVE WATCH ANTICIPATES A SLIGHT INCREASE IN WAVE
HEIGHTS.

SMALL NW SWELLS ARE EXPECTED TO PEAK TODAY AND THEN AGAIN ON
TUESDAY...WITH PEAK SURF HEIGHTS WELL BELOW ADVISORY LEVELS. A
LONG-PERIOD SSW SWELL IS EXPECTED TO SLOWLY BUILD INTO TUESDAY...AND
THE CDIP/PACIOOS BUOY NEAR LANAI HAS BEEN DETECTING FORERUNNERS IN
THE 20-SECOND BAND ALREADY THIS MORNING. THIS SWELL WILL PEAK AROUND
TUESDAY...AND THEN SLOWLY DIMINISH THROUGH THE REST OF THE
WEEK...WITH NEAR ADVISORY LEVEL SURF ALONG S FACING SHORES DURING
THE PEAK OF THE SWELL.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
GALE WARNING FROM 4 PM TODAY TO 6 AM TUESDAY FOR ALENUIHAHA CHANNEL.

SMALL CRAFT ADVISORY UNTIL 6 AM HST TUESDAY FOR ALL HAWAIIAN WATERS.

&&

BIRCHARD