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Hawaiian Islands Synoptic Discussion and Guidance

974
FXHW60 PHFO 241330
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST THU APR 24 2014

.SYNOPSIS...
BREEZY AND GUSTY TRADE WINDS WILL STRENGTHEN GRADUALLY INTO
FRIDAY...AND THEN DIMINISH FROM SUNDAY INTO NEXT WEEK. A MOISTER
AIR MASS WILL BRING MORE SHOWERS AROUND FRIDAY...BUT OTHERWISE
RAINFALL WILL BE LIGHT...AND FOCUSED MAINLY OVER WINDWARD SLOPES...
THOUGH THE WIND WILL CARRY SOME SHOWERS LEEWARD BEFORE THEY
DISSIPATE.

&&

.DISCUSSION...
SURFACE OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS REPORTED
MODERATE TRADE WINDS BLOWING...WHILE PRESSURE TRENDS SUGGESTED
LITTLE CHANGE THROUGH THE SHORT RUN. CLOUD COVER UPWIND OF THE STATE
WAS FAIRLY TYPICAL OVERALL...WITH AN ARC OF MORE EXTENSIVE LOW
CLOUDS AFFECTING MAINLY KAUAI...BUT SPREADING SOUTH TOWARD OAHU AND
MAUI COUNTY. MOST PRECIPITATION OVER LAND REMAINED LIMITED TO
WINDWARD SLOPES...WHERE IT RECEIVED SUPPORT FROM OROGRAPHIC FORCING.
THREE-HOUR ACCUMULATION FOLLOWED THE CLOUD TRENDS.

ALOFT...WEAK TROUGHING TO THE SOUTHWEST MOVED FURTHER AWAY FROM THE
ISLANDS...WHILE BROAD RIDGING EXPANDED OVER THE STATE FROM THE NORTH
NORTHWEST. GUIDANCE PREDICTED THIS MID-LEVEL RIDGE TO PERSIST
THROUGH ABOUT SATURDAY...PROVIDING SUBSTANTIAL SUPPORT TO THE
SUBTROPICAL RIDGE AT THE SURFACE TO THE NORTHEAST. INCREASING WIND
SPEEDS MAY APPROACH THE CRITERION FOR A WIND ADVISORY AROUND FRIDAY.
LOCAL STABILITY ALSO SHOULD BUILD WITH TIME...BUT THE FULL INFLUENCE
OF THE RIDGE HAD NOT YET BEGUN TO TAKE EFFECT...LEAVING THE
INVERSION HIGHER THAN USUAL FOR THE TIME BEING.

DRY TRADE-WIND CONDITIONS WILL PREVAIL THROUGH MOST OF THE FORECAST
PERIOD. HOWEVER...OROGRAPHIC FORCING WILL CONTINUE TO SQUEEZE OUT
AT LEAST A LITTLE RAIN OVER WINDWARD SLOPES...ESPECIALLY FROM FRIDAY
INTO FRIDAY NIGHT...WHEN MOST SOLUTIONS PREDICTED A SOMEWHAT MOISTER
AIR MASS TO PASS.

FROM LATE SUNDAY INTO THE EARLY PART OF NEXT WEEK...THE RIDGE ALOFT
WILL SHIFT QUITE FAR EAST...LIMITING ITS INFLUENCE LOCALLY. SOME
MODELS HINTED THAT THE OLD MID-LEVEL LOW MAY RETURN FROM THE WEST AS
AN OPEN...IF SHALLOW...TROUGH ON A BROADER AND DEEPER TROUGH
APPROACHING FROM THE NORTH NORTHWEST. THIS LATE IN THE SEASON...THE
DEEPER TROUGH WILL BE VERY UNLIKELY TO HAVE MUCH LOCAL EFFECT...BUT
THE SHALLOWER ONE MAY BE ABLE TO WEAKEN THE SUBTROPICAL RIDGE...AND
THUS THE TRADE WINDS...THROUGH THIS PERIOD. BY THE TIME THAT THE
SURFACE FRONT ASSOCIATED WITH THE DEEPER TROUGH ARRIVES AROUND
WEDNESDAY...IT SHOULD BE LIMITED TO LITTLE MORE THAN A SLIGHT WIND
SHIFT...AND A BIT OF CONVERGED MOISTURE SUPPORTING THE WINDWARD
SHOWERS.

MORE TYPICAL TRADE-WIND CONDITIONS WILL RETURN THROUGH THE SECOND
HALF OF NEXT WEEK AS ANOTHER MID-LEVEL RIDGE BUILDS IN FROM THE WEST.

&&

.MARINE...
A SMALL CRAFT ADVISORY /SCA/ REMAINED IN EFFECT FOR ALL WATERS IN
ANTICIPATION OF STRENGTHENING WINDS AND BUILDING COMBINED SEAS
THROUGH FRIDAY AFTERNOON...BUT MAY BE REDUCED IN AREA TO THE
TYPICALLY WINDY WATERS AROUND THE BIG ISLAND AND MAUI ON SATURDAY
NIGHT OR SUNDAY. THE NORTHEASTERLY WINDS WILL REACH THEIR MINIMUM
SPEEDS LATE SUNDAY OR MONDAY...AND THEN INCREASE AGAIN BY THE MIDDLE
OF NEXT WEEK.

THE PERSISTENT TRADE WINDS WILL BUILD SEAS...PRODUCING ROUGH SURF
NEAR THE ADVISORY LEVEL ALONG EAST-FACING SHORES BY FRIDAY. THE
INCREASING SEAS ALSO WILL CONTRIBUTE TO THE NEED FOR AN EXPANDED
SCA. SEAS WILL DIMINISH WITH THE WINDS LATE SATURDAY AND SUNDAY.

A NORTHWEST SWELL THAT BUILD OVERNIGHT WILL PRODUCE SIGNIFICANT
WAVE HEIGHTS JUST BELOW THE NORTH-SHORE ADVISORY CRITERION ON THE
SMALLER ISLANDS TODAY. THE SWELL WILL DROP FRIDAY AND SATURDAY...BUT
A SMALL REINFORCING NORTHWEST SWELL MAY ARRIVE LATE SUNDAY INTO
TUESDAY.

SOUTH-FACING SHORES HAVE BEEN RECEIVING SOME SWELL ENERGY FROM THE
SOUTHWEST. THIS SWELL WILL FADE ON FRIDAY...BUT SMALL SWELL FROM
THE SOUTH SOUTHEAST MAY ARRIVE OVER THE WEEKEND.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 PM HST FRIDAY FOR ALL HAWAIIAN
WATERS-

&&

$$

RYSHKO



Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office