High Surf in Hawaii, March 17 to 20, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST THU MAR 17 2011

.SYNOPSIS...
GUSTY TRADE WINDS WILL RETURN TODAY...PERSIST THROUGH FRIDAY...
DIMINISH SLIGHTLY OVER THE WEEKEND...AND THEN STRENGTHEN AGAIN BY
THE MIDDLE OF NEXT WEEK. ALTHOUGH CONDITIONS WILL REMAIN RELATIVELY
DRY...WITH MOST SHOWERS LIMITED TO WINDWARD SLOPES...PASSING
MOISTURE FROM A DISSIPATED FRONT WILL INCREASE RAINFALL SOMEWHAT
OVER THE WEEKEND.

&&

.DISCUSSION...
ALTHOUGH PRESSURE TRENDS SUGGESTED LITTLE CHANGE IN WIND SPEED FROM
24 HOURS AGO...LOCAL OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS
REPORTED ONLY MODERATE TO STRONG TRADE WINDS BLOWING IN MOST
LOCATIONS. SOUNDING AND RADAR DATA SHOWED SIGNIFICANT MOMENTUM
THROUGH THE MIDDLE OF THE MIXED LAYER BELOW THE INVERSION...BUT
NOCTURNAL DECOUPLING PROBABLY JUST PREVENTED THIS MOMENTUM FROM
MIXING DOWN TO THE SURFACE EFFICIENTLY. DAYTIME SURFACE HEATING
SHOULD LEAD TO MORE EFFICIENT MIXING...AND THUS ANOTHER DAY OF
GUSTY TRADES.

OTHERWISE...THE SOUNDING FROM LIHUE RECORDED A DRY AIR MASS UNDER
UNDER A LOW INVERSION...WHILE THE HILO SOUNDING REPORTED AN ELEVATED
INVERSION ABOVE A TYPICAL TO MOIST AIR MASS. THE LATTER LIKELY
SHOWED THE TRANSIENT EFFECTS OF A PASSING GRAVITY WAVE...OR A MINOR
CENTER OF VORTICITY EMBEDDED IN THE TRADE FLOW. WITH MASSIVE
RIDGING ALOFT...CONDITIONS ACROSS THE STATE LIKELY WILL TREND
TOWARD THE LIHUE SOUNDING THROUGH THE SHORT RUN AT LEAST.

GUIDANCE ALSO CONTINUES TO AGREE FIRMLY THAT STRONG TRADE WINDS WILL
DOMINATE ISLAND WEATHER WELL INTO NEXT WEEK...WITH MINOR CHANGES IN
SPEED AND DIRECTION REPRESENTING THE MAIN VARIABILITY FROM DAY TO
DAY. THE SOLID AND BUILDING RIDGING ALOFT SHOULD PROVIDE STABILITY
THROUGH THE PERIOD...AND SUPPORT TO THE SUBTROPICAL HIGH NEAR THE
SURFACE. DAILY VARIATIONS LIKELY WILL ARISE FROM SHIFTS IN THE
POSITION OF THIS HIGH...MORE SUBTLE CHANGES IN THE LOCAL
STEEPNESS OF THE PRESSURE GRADIENT...AND THE EFFICIENCY WITH WHICH
MOMENTUM MIXES DOWN.

AFTER RETURNING TODAY...STRONG TRADES WILL WEAKEN SLIGHTLY INTO THE
WEEKEND...AND THEN INTENSIFY AGAIN BY THE MIDDLE OF NEXT WEEK. AS
OFTEN HAPPENS WITH TRADE WINDS...MORE RECENT RUNS HAVE BEEN REDUCING
THE AMPLITUDE OF THESE CHANGES...SO THESE EFFECTS MAY NOT BE AS
NOTICEABLE IN PRACTICE. THIS ASPECT OF THE FORECAST WILL REQUIRE
CONTINUED ATTENTION.

IN ANY CASE...A DRY AIR MASS AND PLENTIFUL SUBSIDENCE SHOULD KEEP
RAINFALL ON THE LOW SIDE...WITH MOST SHOWERS DRIVEN BY OROGRAPHIC
FORCING OVER WINDWARD SLOPES. THE REMNANTS OF A FRONT WILL BRING
INCREASED ATMOSPHERIC MOISTURE OVER THE WEEKEND...AND MODELS HINT AT
A SHORT WAVE COINCIDENTALLY SKIRTING THE SOUTH EDGE OF THE MID-LEVEL
RIDGE...BUT AGAIN THE GREATEST INCREASES IN RAINFALL WILL REMAIN
LIMITED TO WINDWARD. DRIER WEATHER SHOULD RETURN EARLY NEXT WEEK
ONCE THE MOIST AIR MASS...AND THE SHORT WAVE...PASS WEST OF THE
STATE.

&&

.MARINE...
STRONG TRADE WINDS HAVE BUILT UP A LARGE SWELL...REQUIRING A HIGH
SURF ADVISORY FOR EAST-FACING SHORES. THIS ADVISORY MAY NEED TO BE
EXTENDED THROUGH THE WEEKEND.

STRONG TRADE WINDS...AND ROUGH SEAS...WILL CONTINUE TO REQUIRE
A SMALL CRAFT ADVISORY FOR MOST OF THE COASTAL WATERS. WHILE WE DO
NOT EXPECT TO ISSUE A GALE WARNING...ONE MIGHT BE REQUIRED IF WINDS
STRENGTHEN FURTHER. THE SMALL CRAFT ADVISORY IS IN EFFECT THROUGH
EARLY FRIDAY MORNING...BUT IS LIKELY TO BE EXTENDED FOR MANY AREAS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY FOR STRONG AND GUSTY EAST WINDS UNTIL 6 AM HST FRIDAY
FOR ALL AREAS EXCEPT THE BIG ISLAND SUMMITS.

HIGH SURF ADVISORY UNTIL 6 PM HST FRIDAY FOR EAST-FACING SHORES OF
ALL ISLANDS EXCEPT LANAI AND NIIHAU.

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

&&

RYSHKO