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Hawaiian Islands Synoptic Discussion and Guidance

089
FXHW60 PHFO 280652
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
852 PM HST WED MAY 27 2015

.SYNOPSIS...
A SURFACE RIDGE BUILDING NORTHWEST OF THE STATE WILL BRING WEAK
TRADE WINDS ACROSS THE WESTERN ISLANDS THROUGH THE REST OF THE
WORK WEEK...BUT WINDS WILL REMAIN LIGHT ENOUGH TO ALLOW LOCALIZED
LAND AND SEA BREEZES. A PERSISTENT TROUGH LINGERING NEAR THE BIG
ISLAND WILL MAINTAIN EVEN LIGHTER WINDS WITH WIDESPREAD SEA
BREEZES OVER THE EASTERN HALF OF THE STATE. ABUNDANT MOISTURE
LINGERING AROUND THE STATE WILL PROVIDE A RATHER CLOUDY AND
SHOWERY WEATHER PATTERN AT TIMES...PARTICULARLY EACH AFTERNOON AND
EARLY EVENING. TRADE WINDS SHOULD INCREASE SOMEWHAT ACROSS THE
STATE DURING THE WEEKEND...BUT LINGERING MOISTURE WILL KEEP
SHOWERS ACTIVE ESPECIALLY ACROSS WINDWARD AND MAUKA AREAS.

&&

.DISCUSSION...
A HIGHLY ANOMALOUS WEATHER PATTERN PREVAILS OVER THE NORTH PACIFIC
OCEAN...WITH A LARGE VERTICALLY STACKED LOW PRESSURE AREA AT THE
SURFACE AND ALOFT SITTING NEAR THE POSITION WHERE THE NORTHEAST
PACIFIC SURFACE HIGH USUALLY RESIDES AT THIS TIME OF YEAR. A
COMPLEX SURFACE TROUGH EXTENDS SOUTH AND WEST FROM THIS LOW TO
NEAR THE BIG ISLAND...WITH A PERSISTENT SMALL-SCALE CIRCULATION
WITHIN THIS TROUGH EVIDENT IN SATELLITE/RADAR DATA JUST TO THE
SOUTH OF THE BIG ISLAND. ABUNDANT CLOUDINESS ASSOCIATED WITH THE
TROUGH CONTINUES TO BLANKET THE BIG ISLAND AND MOST OF MAUI
COUNTY. FURTHER TO THE WEST...A RIDGE EXTENDING EASTWARD FROM HIGH
PRESSURE CENTERED OUT NEAR THE INTERNATIONAL DATELINE IS
PROVIDING WEAK TRADE WIND TYPE FLOW OVER THE WESTERN END OF THE
STATE. MIMIC-TPW SATELLITE IMAGERY DEPICTS A LARGE AREA OF
ENHANCED MOISTURE AROUND AND EAST OF THE BIG ISLAND...WITH ANOTHER
SMALLER POCKET OF ENHANCED MOISTURE NEAR AND WEST OF KAUAI. A
SOMEWHAT DRIER AIRMASS PREVAILS IN BETWEEN...BUT ESTIMATED PWATS
ARE ABOVE NORMAL EVEN IN THAT AREA. THE 00Z HILO SOUNDING
REFLECTED A DEEP MOIST LAYER UP TO A HIGH 11KFT INVERSION AND WELL
ABOVE NORMAL PWAT OF 1.51 INCHES. THE 00Z LIHUE SOUNDING FOUND A
DRIER AIRMASS IN PLACE AT THAT TIME...WITH THE INVERSION BASE OF
7KFT AND PWAT OF 1.16 INCHES BOTH NEAR NORMAL VALUES.

OVERNIGHT...CLOUDY CONDITIONS APPEAR LIKELY TO CONTINUE ACROSS
MAUI AND THE BIG ISLAND...DUE TO THE LINGERING LOW PRESSURE TROUGH
IN THAT AREA. KAUAI WILL PROBABLY REMAIN QUITE CLOUDY AS WELL WITH
ANOTHER AREA OF MOISTURE LINGERING THERE. OAHU SHOULD SEE
IMPROVING CONDITIONS DURING THE NIGHT AS DEVELOPING LAND BREEZES
PUSH CLOUDS AND SHOWERS MOSTLY OFFSHORE. WILL ADJUST SKY COVER
AND POPS FOR SOME AREAS THIS EVENING...BUT NO MAJOR FORECAST
CHANGES ARE PLANNED.

ONLY SUBTLE CHANGES ARE FORECAST IN THE PATTERN THROUGH THE
REMAINDER OF THE WORK WEEK. MODEL CONSENSUS INDICATES THAT
GENERALLY ABOVE NORMAL MOISTURE WITH WEAK/HIGH INVERSIONS WILL
LINGER ACROSS THE STATE THROUGH THE PERIOD...DUE TO A COMBINATION
OF THE TROUGH TO OUR EAST AND MOISTURE DRIFTING SOUTHWARD FROM A
DISSIPATED FRONT. THE DEEP LOW TO OUR NE WILL SHIFT SLOWLY
EASTWARD AND WEAKEN...WITH A WEAK SFC TROUGH SLOWLY FILLING NEAR
THE BIG ISLAND...WHILE HIGH PRESSURE WILL REMAIN FAR TO OUR WNW
NEAR THE DATELINE. THE RESULTING PRESSURE GRADIENT WILL BE
STRONGER NEAR KAUAI/OAHU...AND WEAKER ACROSS THE EASTERN HALF OF
THE STATE. SEA BREEZES AND LAND BREEZES WILL BE MORE LOCALIZED
NEAR KAUAI/OAHU THAN FARTHER EAST...BUT WILL STILL CONTRIBUTE TO
ISLAND-SCALE EFFECTS. ENHANCED CONVERGENCE ALONG THE LEEWARD SEA
BREEZES WILL CONTRIBUTE TO INTERIOR/LEEWARD CLOUDS AND SHOWERS
DURING THE AFTERNOONS AND EARLY EVENINGS...WHILE THE WEAK TRADES
WILL PRODUCE SOME WINDWARD SHOWERS AS WELL ESPECIALLY OVER KAUAI
AND OAHU. MORE WIDESPREAD CLOUDINESS AND SHOWERS MAY LINGER OVER
THE BIG ISLAND THROUGH THURSDAY DUE TO THE PROXIMITY OF THE SFC
TROUGH...BUT CONDITIONS SHOULD SLOWLY IMPROVE TOWARD THE END OF
THE WEEK AS THE TROUGH GRADUALLY WEAKENS.

DURING THE WEEKEND AND INTO EARLY NEXT WEEK...THE TROUGH TO OUR
NE AND E WILL FINALLY WEAKEN AND MOVE FURTHER AWAY...ALLOWING A
MORE ZONAL FLOW PATTERN ALOFT TO DEVELOP OVER THE CENTRAL
PACIFIC. A SFC HIGH BUILDING TO OUR NORTH WILL BRING GENTLE TO
MODERATE TRADES BACK ACROSS THE ENTIRE STATE. A SOMEWHAT MORE
TYPICAL TRADE WIND WEATHER PATTERN SHOULD RETURN WITH CLOUDS AND
SHOWERS FOCUSED MAINLY ACROSS WINDWARD/MAUKA AREAS...BUT CONTINUED
ABOVE NORMAL MOISTURE AND HIGH INVERSIONS MAY ALLOW FOR MORE
SHOWERS AND CLOUDS LEEWARD AND INTERIOR ALONG LOCAL SEA BREEZES
THAN WOULD USUALLY BE THE CASE.

THE LATEST AVAILABLE GFS AND ECMWF RUNS CONTINUE TO DIVERGE
SOMEWHAT BY THE MIDDLE OF NEXT WEEK. THE GFS BRINGS ANOTHER
MID/UPPER LEVEL TROUGH AND SFC FRONT SOUTH OF 30N...WEAKENING AND
VEERING THE FLOW OVER HAWAII...ALTHOUGH THE FRONT ITSELF REMAINS
WELL NORTH OF THE STATE. THE ECMWF KEEPS THESE MID-LATITUDE
FEATURES WEAKER AND FURTHER NORTH...ALLOWING FOR A STRONGER
SUBTROPICAL SFC RIDGE TO OUR NORTH AND MAINTAINING MODERATE TRADE
WIND FLOW OVER THE ISLANDS. MODEL TRENDS AND EARLY JUNE
CLIMATOLOGY APPEAR TO FAVOR THE EC SOLUTION AT THIS TIME.

&&

.AVIATION...
A TROUGH OF LOW PRESSURE OVER THE BIG ISLAND CONTINUES TO PRODUCE
WIDESPREAD CLOUDINESS AND SCATTERED SHOWERS THERE. AREAS OF MVFR
AND LOCAL IFR WILL REMAIN POSSIBLE ACROSS WINDWARD SECTIONS
OVERNIGHT...AND AIRMET SIERRA FOR MOUNTAIN OBSCURATION IS POSTED
FROM UPOLU POINT TO PHTO TO VOLCANO. ELSEWHERE...A WEAK PRESSURE
PATTERN IS IN PLACE...WHICH WILL ALLOW SEA AND LAND BREEZES TO
DOMINATE THE WEATHER PATTERN OVERNIGHT AND THURSDAY. CANNOT RULE
OUT SOME BRIEF MVFR CIGS/VSBYS IN SHRA...BUT PREDOMINANTLY VFR
CONDITIONS ARE EXPECTED.

&&

.MARINE...
THE CURRENT SOUTH SWELL WILL CONTINUE TO SUBSIDE...THOUGH A SERIES
OF SMALLER SOUTH SWELLS WILL MAINTAIN SMALL TO MODERATE SURF
ALONG SOUTH FACING SHORES THROUGH THE REST OF THE WEEK. LATEST
WAVEWATCH GUIDANCE CONTINUES TO SHOW A SIGNIFICANT SOUTH SWELL
ARRIVING SATURDAY NIGHT...THEN PEAKING LATE SUNDAY INTO MONDAY...
WITH RESULTING SURF LIKELY REACHING ADVISORY LEVELS FOR SOUTH
FACING SHORES. THIS SWELL WAS PRODUCED BY A STRONG GALE LOW EAST
OF NEW ZEALAND WITH A LONG FETCH AIMED TOWARD HAWAII A FEW DAYS
AGO. WILL MONITOR OBSERVATIONS FROM THE AUNUU BUOY NEAR AMERICAN
SAMOA AS THIS SWELL WILL BE PEAKING AT THAT LOCATION THURSDAY AND
THURSDAY NIGHT.

OTHERWISE...SURF WILL REMAIN WELL BELOW ADVISORY LEVELS THROUGH
THE NEXT SEVERAL DAYS WITH ONLY SMALL SWELLS EXPECTED.

&&

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

&&

$$

JACOBSON





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