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Hawaiian Islands Synoptic Discussion and Guidance

934
FXHW60 PHFO 010139 CCA
AFDHFO

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE HONOLULU HI
338 PM HST SAT APR 30 2016

.SYNOPSIS...
TRADE WINDS WILL PERSIST THROUGH THE FORECAST PERIOD...AND CLOUDS
AND SHOWERS ARE EXPECTED TO FAVOR WINDWARD AND MAUKA AREAS. AFTER
DECREASING TODAY...WIND SPEEDS WILL REBOUND BEGINNING TONIGHT OR
SUNDAY AS HIGH PRESSURE BUILDS TO OUR NORTH. ANOTHER FRONT MAY
APPROACH THE ISLANDS FROM THE NORTHWEST LATE NEXT WEEK INTO THE
FOLLOWING WEEKEND.

&&

.DISCUSSION...
THERE IS LITTLE CHANGE IN FORECAST PHILOSOPHY FROM THIS MORNING.
A FRONT 400 TO 500 MILES NORTHWEST OF THE MAIN HAWAIIAN ISLANDS IS
ACTING TO WEAKEN THE SUBTROPICAL RIDGE EXTENDING EAST TO WEST
ROUGHLY ALONG 30N. THIS RIDGE...GOVERNING THE STRENGTH OF TRADE
FLOW ACROSS LOCAL HAWAIIAN WATERS...IS ANCHORED BY HIGH PRESSURE
CENTERS OFF THE NORTHERN CALIFORNIA COAST AND FAR NORTHWEST OF THE
MAIN HAWAIIAN ISLANDS. PATCHY BROKEN LOW CLOUDS EMBEDDED WITHIN
TRADE FLOW ARE MOVING ACROSS THE ISLANDS THIS AFTERNOON...FAVORING
WINDWARD AND MAUKA AREAS. PW VALUES ARE BETWEEN 1.2 AND 1.4
INCHES...BUT A CAPPING INVERSION NEAR 8000 FEET AND THE LACK OF A
CONVECTIVE TRIGGER...BESIDES LOCAL TERRAIN...KEEP TOTAL RAINFALL
AMOUNTS LIGHT. WE EXPECT THIS GENERAL TRADE WIND SHOWER PATTERN TO
CONTINUE INTO NEXT WEEK.

MODELS SHOW THE FRONT FAR NORTHWEST OF THE MAIN HAWAIIAN ISLANDS
WILL MOVE EASTWARD AND WEAKEN THROUGH SUNDAY AS HIGH PRESSURE
BEHIND IT BUILDS IN STRENGTH. BY THE MIDDLE OF NEXT WEEK...TRADE
WINDS ALONG THE SOUTHERN FLANK OF THIS NEW HIGH MAY PUSH REMNANT
FRONTAL MOISTURE ACROSS THE ISLANDS...INCREASING TRADE SHOWER
COVERAGE. BEFORE THIS HOWEVER...WE EXPECT THE NEW HIGH WILL CAUSE
TRADE WIND SPEEDS TO REBOUND TO SMALL CRAFT ADVISORY THRESHOLD
ACROSS MORE OF THE WINDIER LOCAL WATERS BEGINNING SUNDAY. ANOTHER
FRONT MAY APPROACH THE ISLANDS TOWARD NEXT WEEKEND. THE LATEST GFS
RUN KEEPS THIS FRONT SEVERAL HUNDRED MILES NORTHWEST OF KAUAI AS
LATE AS NEXT SATURDAY MORNING...FARTHER WEST THAN THE LAST MODEL
RUN DEPICTED. GIVEN THE UNEVEN SOLUTIONS...WE WILL KEEP TRADE
FLOW IN THE GRIDS THROUGH NEXT WEEKEND AS MODELS RESOLVE THIS
FEATURE.

&&

.AVIATION...
A STABLE TRADE WIND FLOW WILL CONTINUE THROUGH SUNDAY WITH AN
INVERSION MAINLY NEAR 8000 FT. EXPECT AREAS OF MVFR CIGS OVER
WINDWARD AREAS OF MOST ISLANDS...AND WITH NO ORGANIZED AREAS OF
MOISTURE NOTED UPSTREAM...THE CHANCE FOR EXTENSIVE MOUNTAIN
OBSCURATION TONIGHT IS LOW. RADAR VAD WINDS CONTINUE TO SHOW
EASTERLY TRADE WINDS OF 20 KT OR LESS...BUT AS WINDS INCREASE
TONIGHT AND SUNDAY...AIRMET TANGO MAY BE NEEDED FOR LOW LEVEL
TURB.

&&

.MARINE...
TRADE WINDS ARE FORECAST TO INCREASE STARTING TONIGHT OR
SUNDAY...SO THE SMALL CRAFT ADVISORY HAS BEEN EXPANDED TO COVER
THE TYPICALLY WINDY ZONES ADJACENT TO THE ISLANDS OF MAUI COUNTY
AND THE BIG ISLAND. THIS ADVISORY WILL LIKELY BE EXTENDED FOR
THESE WATERS THROUGH THE MIDDLE OF NEXT WEEK.

A NORTHWEST SWELL WILL GRADUALLY LOWER TONIGHT. ANOTHER
REINFORCING SMALL NORTHWEST SWELL ARRIVING LATE TONIGHT WILL
LIKELY PEAK SUNDAY...THEN SLOWLY LOWER EARLY NEXT WEEK. A NEW
SMALL LONGER PERIOD WEST-NORTHWEST SWELL IS EXPECTED ON WEDNESDAY
AND THURSDAY. ELSEWHERE...EXPECT A NOTICEABLE BUMP IN SURF HEIGHTS
ALONG SOUTH FACING SHORES FROM THIS WEEKEND THROUGH EARLY NEXT
WEEK.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 AM HST MONDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.

&&

$$

PUBLIC/MARINE...POWELL
AVIATION...WROE

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