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Hawaiian Islands Synoptic Discussion and Guidance

216
FXHW60 PHFO 301349
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
349 AM HST SAT APR 30 2016

.SYNOPSIS...
EXPECT THE TRADE WINDS TO WEAKEN SLIGHTLY TODAY AS A WEAK FRONT
PASSES TO THE NORTH OF THE ISLANDS. A NEW SURFACE HIGH WILL BUILD
NORTH OF THE REGION STARTING SUNDAY...WHICH WILL INCREASE TRADE
WINDS AGAIN FROM LATE THIS WEEKEND THROUGH THE MIDDLE OF NEXT
WEEK. TRADE SHOWERS WILL REMAIN FOCUSED OVER THE WINDWARD SIDES
OF THE ISLANDS THROUGH NEXT WEEK. AN INCREASE IN TRADE SHOWERS IS
POSSIBLE OVER PARTS OF THE STATE AROUND MID-WEEK.

&&

.DISCUSSION...
THE ROUNDED WESTERN END OF A SURFACE RIDGE IS LOCATED ABOUT 375
MILES NORTH OF LIHUE EARLY THIS MORNING. THE PRESSURE GRADIENT
SOUTH OF THIS FEATURE IS MAINTAINING TRADE WINDS ACROSS THE
ISLANDS THIS MORNING. HOWEVER...A SURFACE FRONT LOCATED ABOUT 540
MILES NORTH-NORTHWEST OF LIHUE IS MOVING TOWARD THE EAST-
SOUTHEAST. AS A RESULT...IT WILL CAUSE THE SURFACE RIDGE TO
GRADUALLY ERODE FROM THE WEST OVER THE NEXT 12 TO 18 HOURS.
THEREFORE...THE PRESSURE GRADIENT IS EXPECTED TO BECOME WEAKER BY
LATER TODAY. THIS WILL LIKELY CAUSE THE TRADE WIND SPEEDS TO
DIMINISH SLIGHTLY DURING THE NEXT 6 TO 12 HOURS.

ALOFT...WATER VAPOR IMAGERY SUGGESTS THAT THE ISLANDS ARE UNDER
MID-TROPOSPHERIC RIDGING. THERE IS ALSO UPPER LEVEL SUBSIDENCE
DIRECTLY ABOVE MOST OF THE STATE THIS MORNING. THIS IS
CONTRIBUTING TO THE STABLE ATMOSPHERIC CONDITIONS THAT ARE
CURRENTLY BEING OBSERVED ACROSS THE ISLAND CHAIN. NOTE THAT
SATELLITE IMAGERY SHOWS AN AREA OF BROKEN LOW CLOUDS AND
SCATTERED SHOWERS ALONG THE WINDWARD BIG ISLAND AND MAUI.
OTHERWISE...EXPECT BRIEF TRADE SHOWERS OVER SOME OF THE WINDWARD
FACING SLOPES AND HIGHER TERRAIN ON THE SMALLER ISLANDS FROM
MOLOKAI TO OAHU.

BY SUNDAY...THE FORECAST MODELS DEPICT A NEW SURFACE HIGH BUILDING
AS IT MOVES TO A POSITION FAR NORTH OF THE ISLANDS. THIS WILL
AGAIN TIGHTEN THE PRESSURE GRADIENT OVER THE REGION...WITH TRADES
REBOUNDING TO LOCALLY BREEZY SPEEDS. EXPECT TRADE SHOWERS TO
REMAIN FOCUSED OVER WINDWARD AND MAUKA SECTIONS OF THE STATE NEXT
WEEK. THE MODEL GUIDANCE ALSO SHOWS THAT THE REMNANT MOISTURE
ASSOCIATED WITH THE FRONT MAY EVENTUALLY BE CARRIED BACK TOWARD
ISLANDS BY THE TRADE WIND FLOW. IF SO...EXPECT THIS AREA OF
ENHANCED MOISTURE TO REACH THE ISLANDS AROUND WEDNESDAY...WHICH
MAY CAUSE AN INCREASE IN TRADE SHOWERS STARTING MID-WEEK.

LOOKING TOWARD THE END OF NEXT WEEK...THE FORECAST MODELS APPEAR
TO AGREE THAT A LATE-SEASON FRONT WILL SLOWLY APPROACH KAUAI FROM
THE NORTHWEST FROM WEDNESDAY THROUGH THURSDAY. HOWEVER...THE GFS
MODEL APPEARS TO PUSH THIS FRONT CLOSER TO KAUAI ON FRIDAY THAN
THE ECMWF MODEL. UNTIL THESE MODEL DIFFERENCES ARE RESOLVED IN
FUTURE RUNS...THERE REMAINS A GREAT DEAL OF UNCERTAINTY IN
REGARDS TO THE WEATHER WE MIGHT EXPECT AS WE APPROACH NEXT
WEEKEND. REGARDLESS OF THE ACTUAL SCENARIO THAT OCCURS...IT IS
LIKELY THAT THE TRADE WINDS WILL WEAKEN BY FRIDAY...ESPECIALLY
OVER THE WESTERN END OF THE STATE. IN FACT...THE FLOW WILL LIKELY
SHIFT A BIT MORE TO THE EAST-SOUTHEAST OR SOUTHEAST AHEAD OF THE
FRONT. IF THIS DEVELOPS...IT SUGGESTS THAT WARM AND MUGGY
CONDITIONS MAY DEVELOP ACROSS PORTIONS OF THE REGION AS WE HEAD
INTO NEXT WEEKEND.

&&

.AVIATION...
MODERATE TRADE WINDS WILL CONTINUE TODAY. WIND SPEEDS ARE EXPECTED
TO REMAIN BELOW THE THRESHOLD FOR WIDESPREAD MODERATE LOW LEVEL
TURB...AND NO AIRMETS ARE IN EFFECT AT THIS TIME. CLOUDS AND
SHOWERS WILL REMAIN MOSTLY OVER WINDWARD AREAS...AND AIRMET SIERRA
FOR MOUNTAIN OBSCURATION COULD BECOME NECESSARY FOR SOME WINDWARD
SLOPES THIS MORNING.

&&

.MARINE...
THE SMALL CRAFT ADVISORY /SCA/ CONTINUES FOR THE ALENUIHAHA
CHANNEL TODAY. TRADE WINDS ARE FORECAST TO INCREASE AGAIN
STARTING TONIGHT OR SUNDAY...SO THE SCA WILL LIKELY BE POSTED
AGAIN FOR THE TYPICALLY WINDY ZONES ADJACENT TO THE ISLANDS OF
MAUI COUNTY AND THE BIG ISLAND. THIS SCA WILL LIKELY CONTINUE FOR
THESE WATERS THROUGH THE MIDDLE OF NEXT WEEK ONCE IT IS ISSUED.

THE WAIMEA BAY AND PAUWELA BUOYS INDICATE THE SMALL NORTHWEST
SWELL HAS ARRIVED IN THE ISLANDS. THIS SWELL WILL PEAK TODAY...
THEN 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 PM HST THIS EVENING FOR ALENUIHAHA
CHANNEL.

&&

$$

DISCUSSION/MARINE...HOUSTON
AVIATION...JACOBSON

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



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