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Hawaiian Islands Synoptic Discussion and Guidance

189
FXHW60 PHFO 021947
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST TUE SEP 2 2014

.SYNOPSIS...
LIGHT TO MODERATE TRADE WINDS WILL CONTINUE FOR MUCH OF THE UPCOMING
WEEK. THE TRADES WILL FOCUS SHOWERS MAINLY ACROSS WINDWARD AREAS...
ESPECIALLY DURING NIGHTS AND MORNINGS...WITH SOME AFTERNOON SHOWERS
ACROSS LEEWARD SECTIONS AS WELL.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL HIGH OVER/JUST SOUTH OF THE
STATE...WITH A SHORTWAVE TROUGH FAR TO THE NORTHWEST. AT THE
SURFACE...HIGH PRESSURE IS LOCATED FAR NORTH-NORTHEAST OF THE
STATE...WITH A RIDGE PASSING SEVERAL HUNDRED MILES TO OUR NORTH. LOW
PRESSURE...FORMER TC MARIE...IS FAR NORTHEAST OF THE STATE AND
MOVING GENERALLY WESTWARD. 12Z SOUNDINGS FROM HILO AND LIHUE SHOW
INVERSIONS RANGING FROM 7KFT AT LIHUE TO 10KFT AT HILO. PRECIPITABLE
WATERS ALSO RANGE FROM 1.4 TO 1.7 INCHES RESPECTIVELY. EARLY MORNING
MIMIC TOTAL PRECIPITABLE WATER IMAGERY SHOWS AN AREA OF HIGHER
MOISTURE EXTENDING UP TO 250 MILES NORTHEAST AND EAST OF THE
STATE...WITH DRIER AIR FARTHER UPSTREAM.

THE PRESSURE GRADIENT ACROSS THE ISLANDS REMAINS ON THE LIGHT SIDE
DUE TO OUR SEPARATION FROM THE HIGH AND INFLUENCE FROM FORMER MARIE.
HOWEVER...INDICATIONS ARE THAT TRADES ARE MORE ENTRENCHED THAN
YESTERDAY. THE GRADIENT BETWEEN BUOYS 51100 NORTHEAST OF THE STATE
AND 51004 SOUTHEAST OF THE STATE HAS RISEN BY OVER A MILLIBAR OVER
THE PAST 24 HOURS. THE ASCAT IMAGERY FROM LAST NIGHT SHOWED AROUND
10 KNOTS OF WIND NEAR KAUAI...10 TO 15 KNOTS NEAR OAHU AND MAUI
COUNTY...AND 15 TO 20 KNOTS NEAR THE BIG ISLAND. THE HIGH
RESOLUTION WRF FROM LAST NIGHT...AS WELL AS THE 12Z GLOBAL
MODELS...SHOW THE STRONGER WINDS SPREADING WESTWARD ACROSS THE STATE
TODAY/TONIGHT AS WELL. THERE MAY BE ENOUGH OF A STRONGER GRADIENT
FOR TRADE WINDS TO DOMINATE...WITH LOCALIZED SEA BREEZES CONFINED
TO SHELTERED LEEWARD AREAS. EVEN WITH MORE PERSISTENT TRADES...WE
WILL LIKELY SEE INCREASED CLOUD BUILDUPS ACROSS INTERIOR AND LEEWARD
SECTIONS DURING THE AFTERNOON AND EARLY EVENING.

AN AREA OF LOW CLOUDS IS CURRENTLY LOCATED NORTHEAST OF MAUI AND THE
BIG ISLAND...AND IS MOVING WESTWARD AT ABOUT 10 MPH. THESE CLOUDS
ARE AN INDICATOR OF THE HIGHER MOISTURE JUST UPSTREAM OF THE
STATE...AND WILL HELP TO ENHANCE TRADE WIND SHOWERS MAINLY ACROSS
WINDWARD SECTIONS TONIGHT INTO WEDNESDAY. GFS FORECAST
SOUNDINGS/MOISTURE PLOTS SHOW DRIER AND MORE STABLE CONDITIONS
SPREADING ACROSS THE ISLANDS DURING THE DAY ON WEDNESDAY...WITH
LOWER RAINFALL CHANCES.

BY THE END OF THE WEEK...BOTH GFS AND ECMWF CONTINUE TO SHOW A
TONGUE OF MOISTURE TRAILING FROM FORMER MARIE REACHING THE ISLANDS.
THE GFS IS FASTER/MORE AGGRESSIVE THAN THE ECMWF...BUT BOTH SHOW THE
AREA OF CLOUDS/MOISTURE CURRENTLY EXTENDING DOWN TO 25N BETWEEN
140W/150W REACHING THE ISLANDS LATE THURSDAY/FRIDAY. FORECAST
SOUNDINGS/CROSS SECTIONS SHOW INCREASING MOISTURE DEPTH BACK TO NEAR
10KFT. DURING THIS TIMEFRAME...WINDS MAY ALSO WEAKEN A BIT. THE
REMNANT OF MARIE WILL MOVE WESTWARD AND WEAKEN AS IT ENCROACHES UPON
THE RIDGE. THE GRADIENT ACROSS THE ISLANDS MAY WEAKEN
THURSDAY/FRIDAY...BEFORE STRENGTHENING FURTHER SATURDAY INTO SUNDAY
AS THE LOW DISSIPATES AND WE FEEL MORE OF A DIRECT IMPACT FROM THE
SURFACE HIGH.

&&

.AVIATION...
THROUGHOUT THE DAY...TRADE WINDS WILL GRADUALLY BEGIN TO BUILD BACK.
ISOL MVFR OVER WINDWARD AND MOUNTAIN AREAS MAY OCCUR IN PASSING
SHOWERS. OTHERWISE...VFR CONDITIONS WILL PREVAIL. THERE ARE NO
AIRMETS IN EFFECT.

&&

.MARINE...
ELEVATED SURF WILL CONTINUE ALONG S FACING SHORES THIS WEEK.
ALTHOUGH SURF IS CURRENTLY BELOW ADVISORY LEVELS...FORERUNNERS OF A
VERY LONG PERIOD S SWELL /20-25 SECONDS/ ARE EXPECTED TO BUILD TODAY
AND TONIGHT...ALTHOUGH AT LOW HEIGHTS. LATEST OBSERVATIONS FROM THE
CDIP BUOY AT BARBERS POINT SHOW INITIAL ENERGY FROM THIS SWELL IS
BEGINNING TO ARRIVE...BUT THE BULK OF THE SWELL ENERGY IS DOMINATED
BY THE FADING SHORTER PERIOD SOUTH SWELL. AS THIS NEW SWELL FILLS
IN...IT WILL PRODUCE SURF NEAR ADVISORY LEVELS ALONG S FACING
SHORES...LIKELY FROM WEDNESDAY INTO THURSDAY. WILL HOLD OFF ON THE
ISSUANCE OF AN ADVISORY TO SEE IF BUOY DATA SHOWS GREATER AMPLITUDE
SWELL HEIGHTS...BUT AN ADVISORY MAY BE NECESSARY THIS AFTERNOON FOR
SURF NEARING ADVISORY LEVELS ON WEDNESDAY.

FORECAST MODELS SHOW A FAIRLY LARGE GALE LOW DEVELOPING NW OF THE
STATE THIS WEEK...GENERATING A NW SWELL THAT ARRIVES LATE
MONDAY/TUESDAY. IT IS TOO EARLY FOR SPECIFICS AS THE LOW HAS YET TO
DEVELOP...BUT RESULTANT SURF LOOKS TO REMAIN BELOW ADVISORY LEVEL
HEIGHTS ALONG N AND W FACING SHORES.

WINDS AND SEAS WILL REMAIN BELOW SMALL CRAFT ADVISORY THRESHOLDS FOR
AT LEAST THE NEXT SEVERAL DAYS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

DISCUSSION/MARINE...BRAVENDER
AVIATION...EATON



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