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Hawaiian Islands Synoptic Discussion and Guidance

121
FXHW60 PHFO 300619
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
819 PM HST FRI APR 29 2016

.SYNOPSIS...
EXPECT THE TRADE WINDS TO WEAKEN SLIGHTLY ON SATURDAY 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 380
MILES NORTH OF LIHUE AT DUSK THIS EVENING. THE RELATIVELY TIGHT
PRESSURE GRADIENT SOUTH OF THIS FEATURE IS MAINTAINING LOCALLY
BREEZY TRADE WINDS ACROSS THE ISLANDS FOR AT LEAST A FEW MORE
HOURS. HOWEVER...A SURFACE FRONT LOCATED NEARLY 600 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 24 HOURS. THEREFORE...THE
PRESSURE GRADIENT IS EXPECTED TO BECOME WEAKER BY EARLY SATURDAY
MORNING. THIS WILL LIKELY CAUSE THE TRADE WIND SPEEDS TO DIMINISH
DURING THE NEXT 24 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 EVENING. 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 LOW CLOUDS AND SHOWERS MOVING
TOWARD THE WINDWARD BIG ISLAND AND MAUI. THERE MAY BE A SLIGHT
INCREASE IN TRADE SHOWERS OVER THE WINDWARD SIDES OF THE EASTERN
ISLANDS FROM LATER TONIGHT INTO EARLY SATURDAY. 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 BEHIND THE FRONT AS IT MOVES TO A POSITION FAR NORTH OF
THE ISLANDS. THIS WILL AGAIN TIGHTEN THE PRESSURE GRADIENT OVER
THE ISLANDS...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 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 TAUT 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 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 TO LOCALLY BREEZY TRADE WINDS WILL CONTINUE OVERNIGHT AND
SATURDAY. 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 MAY BE NEEDED FOR
SOME WINDWARD SLOPES LATER TONIGHT OR EARLY SATURDAY MORNING.

&&

.MARINE...
THE SMALL CRAFT ADVISORY /SCA/ CONTINUES FOR MOST OF THE
TYPICALLY WINDIER WATERS ADJACENT TO THE ISLANDS OF MAUI COUNTY
AND THE BIG ISLAND. SINCE THE TRADE WINDS ARE FORECAST TO DECREASE
ON SATURDAY...EXPECT THE SCA TO BE LOWERED FOR 12 TO 24 HOURS.
TRADE WINDS ARE FORECAST TO INCREASE AGAIN STARTING SUNDAY...SO
THE SCA WILL LIKELY BE POSTED AGAIN FOR THE TYPICALLY WINDY ZONES
AGAIN.

SMALL NORTHWEST SWELLS WILL CONTINUE THROUGH THIS WEEKEND. THERE
WILL ALSO BE SMALL SHORT PERIOD NORTH-NORTHWEST SWELLS EARLY NEXT
WEEK...FOLLOWED BY A NEW SMALL LONGER PERIOD WEST-NORTHWEST SWELL
ON WEDNESDAY AND THURSDAY. EXPECT A 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 SATURDAY FOR MAALAEA BAY...
THE LEEWARD WATERS OF THE BIG ISLAND NEAR SOUTH POINT...AND THE
PAILOLO AND ALENUIHAHA CHANNELS.

&&

$$

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

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