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Hawaiian Islands Synoptic Discussion and Guidance

791
FXHW60 PHFO 220143
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST TUE MAY 21 2013

.SYNOPSIS...
A WEAKENING LOW TO OUR NORTH AND A TROUGH JUST WEST OF KAUAI WILL
KEEP WINDS LIGHT ACROSS THE STATE...LEADING TO OFFSHORE LAND BREEZES
AT NIGHT AND ONSHORE SEA BREEZES DURING THE DAY. CLOUDS AND SHOWERS
FOCUSED ACROSS INTERIOR SECTIONS OF THE ISLANDS EARLY EVENING WILL
DIMINISH THIS EVENING...THEN ARE POSSIBLE AGAIN WEDNESDAY AFTERNOON.
AS THE LOW MOVES WEST AND DISSIPATES...TRADE WINDS WILL RETURN
BEGINNING THURSDAY AND STRENGTHEN THROUGH THE WEEKEND.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL LOW OVER 600 MILES NORTH
NORTHEAST OF THE STATE...WITH AN UPPER TROUGH EXTENDING ABOUT 200
MILES NORTHWEST OF KAUAI. AT THE SURFACE...LOW PRESSURE IS LOCATED
LESS THAN 500 MILES NORTH OF KAUAI...WITH A TROUGH MOVING WELL WEST
OF THE ISLAND. 00Z RAOB SHOW INVERSIONS HAVING STRENGTHENED OVER
THE PAST 24 HOURS...BUT STILL REMAINING ELEVATED AROUND 8-9KFT.
PRECIPITABLE WATERS ALSO RANGE FROM 1.1 INCHES AT LIHUE TO 1.5
INCHES AT HILO. EARLY AFTERNOON MIMIC TOTAL PRECIPITABLE WATER
IMAGERY SHOWS HIGHER MOISTURE FROM OAHU EASTWARD...PART OF A
CONVERGENCE BAND THAT EXTENDS FAR NORTHEAST OF THE STATE.

WITH THE WEAK SURFACE FEATURES NEARBY...THE PRESSURE GRADIENT ACROSS
THE ISLANDS REMAINS QUITE WEAK. THE PRESSURE DIFFERENCE BETWEEN BUOY
51101 TO THE NORTHWEST OF KAUAI AND BUOY 51003 TO THE SOUTH OF KAUAI
HAS RISEN TO AROUND 1MB...WHICH IS STILL SLIGHTLY LESS THAN THE
VALUES OBSERVED YESTERDAY. WIDESPREAD SEA BREEZES FORMED
TODAY...AND LAND BREEZES ARE EXPECTED AGAIN TONIGHT. THIS TYPE OF
CONVECTIVE PATTERN IS EXPECTED TO CONTINUE THROUGH WEDNESDAY...WITH
AFTERNOON CLOUDS AND SHOWERS AND SOME CLEARING AT NIGHT. A BAND OF
HIGH CLOUDS HAS FILLED IN OVER THE SMALLER ISLANDS EARLIER IN THE
DAY AHEAD OF THE UPPER TROUGH...BUT ARE BEGINNING TO PULL AWAY TO
THE EAST NORTHEAST. DESPITE OFFSHORE LAND BREEZES TONIGHT...WE MAY
NOT SEE THE DEGREE OF CLEARING AS IN PAST NIGHTS AS MOISTURE AND
CLOUDS FROM THE CONVERGENCE BAND EAST AND NORTHEAST OF THE STATE
SPREAD WESTWARD.

THE LOW NORTH OF THE STATE IS EXPECTED TO MOVE WESTWARD AND
DISSIPATE...WITH THE SURFACE TROUGH MOVING FARTHER WEST OF KAUAI.
THE GRADIENT REMAINS LIGHT THROUGH WEDNESDAY...WHICH WILL ALLOW FOR
ANOTHER DAY OF ONSHORE SEA BREEZES AND AFTERNOON CLOUDS AND SHOWERS.
THE ATMOSPHERE STABILIZED TODAY...WHICH LIMITED RAINFALL AMOUNTS
ACROSS THE SMALLER ISLANDS...AND WILL STABILIZE FURTHER ON
WEDNESDAY. HOWEVER...THE MOISTURE SPREADING WESTWARD MAY HELP TO
ENHANCE SHOWERS DURING THE AFTERNOON.

HIGH PRESSURE NORTHEAST OF THE STATE WILL REASSERT ITSELF ACROSS THE
ISLANDS BY THURSDAY. AS THIS HIGH MOVES SOUTHWARD AND THE RIDGE
REESTABLISHES ITSELF NORTH OF THE STATE...WE SHOULD SEE
STRENGTHENING TRADE WINDS THROUGH THE WEEKEND. HOWEVER...AN UPPER
LEVEL LOW DEVELOPING TO OUR NORTHEAST WILL HELP TO DESTABILIZE THE
AIRMASS BY THE WEEKEND. WHILE THE LATEST MODEL RUNS SHOW THIS
FEATURE CONFINED TO UPPER LEVELS...WITH TRADES PERSISTING AT THE
SURFACE...IT WILL HELP TO ERODE THE TRADE WIND INVERSION AND ENHANCE
TRADE WIND SHOWERS. THIS PATTERN IS EXPECTED TO LINGER INTO EARLY
NEXT WEEK.

&&

.AVIATION...
VFR CONDITIONS PREVAIL AT MOST OF THE TAF SITES WITH SOME LOCAL MVFR
IN THE VICINITY OF PHTO DUE TO VOLCANIC EMISSIONS. THESE CONDITIONS
WILL LIKELY PERSIST THROUGH THE NIGHT. DAYTIME HEATING HAS CAUSED
SOME MVFR CONDITIONS TO OCCUR MAINLY OVER THE BIG ISLAND DUE TO
CLOUDS AND SHRA. CONDITIONS WILL LIKELY IMPROVE OVERNIGHT.

&&

.MARINE...
THE CURRENT SOUTH SWELL REMAINS ELEVATED...AND WAVEWATCH GUIDANCE
SHOWS ADDITIONAL ENERGY AFFECTING THE AREA TONIGHT/WEDNESDAY. BASED
ON CURRENT OBSERVATIONS AND A GRADUAL DOWNWARD TREND...WE WILL
EXTEND THE HIGH SURF ADVISORY THROUGH WEDNESDAY. BEYOND
WEDNESDAY...SURF WILL CONTINUE TO GRADUALLY DIMINISH THROUGH THE
WEEK. A NEW LONGER-PERIOD WEST-NORTHWEST SWELL WILL CONTINUE TO
BUILD TONIGHT AND PEAK WEDNESDAY INTO THURSDAY. WHILE THIS SWELL IS
LARGER THAN NORMAL FOR THIS TIME OF YEAR...SURF IS EXPECTED TO
REMAIN BELOW ADVISORY LEVELS FOR NORTH AND WEST FACING SHORES.

TRADE WINDS ARE EXPECTED TO BE STRONG ENOUGH BY THE WEEKEND FOR
SMALL CRAFT WIND CONDITIONS TO DEVELOP OVER THE WINDIER AREAS AROUND
MAUI COUNTY AND THE BIG ISLAND.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 PM HST WEDNESDAY FOR SOUTH FACING SHORES
OF ALL ISLANDS.

WIND ADVISORY UNTIL 6 AM HST WEDNESDAY FOR BIG ISLAND SUMMITS.

&&

$$

PUBLIC/MARINE...BRAVENDER
AVIATION...BURKE

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

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