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Hawaiian Islands Synoptic Discussion and Guidance

982
FXHW60 PHFO 201959
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
959 AM HST SAT SEP 20 2014

.SYNOPSIS...
LIGHT WINDS WILL MAINTAIN A PATTERN OF LOCAL NIGHTTIME LAND AND
AFTERNOON SEA BREEZE CIRCULATIONS ACROSS THE ALOHA STATE THROUGH
THIS WEEKEND. IN THIS REGIME...EXPECT MOSTLY CLEAR NIGHTS AND
MORNINGS...WITH CLOUD BUILD UPS AND ISOLATED SHOWERS DEVELOPING
OVER LEEWARD AND INTERIOR SECTIONS EACH AFTERNOON. A RETURN OF
LIGHT TO MODERATE TRADES IS EXPECTED EARLY NEXT WEEK...WITH BRIEF
PASSING SHOWERS FAVORING WINDWARD AND MAUKA AREAS THROUGH MID-
WEEK. THE TRADE WINDS MAY BECOME WETTER FROM THURSDAY INTO NEXT
WEEKEND AS MOISTURE ASSOCIATED WITH A WEAKENING FRONT PUSHES DOWN
OVER THE ISLANDS.

&&

.DISCUSSION...
A WEAK NORTH TO SOUTH ORIENTED SURFACE TROUGH CROSSES THE KAIWI
CHANNEL THIS MORNING...MOVING SLOWLY WESTWARD. THE WEAK WINDS
WEST OF THIS FEATURE ARE FROM THE NORTHEAST...AND FROM THE
SOUTHEAST TO THE EAST OF THE TROUGH AXIS. ELSEWHERE...A WEAK
SURFACE FRONT LOCATED ABOUT 425 MILES NORTH OF LIHUE IS ALSO
DISRUPTING THE PRESSURE GRADIENT ACROSS THE REGION. AS A
RESULT...THE BACKGROUND LOW-LEVEL FLOW ACROSS THE ISLANDS REMAINS
VERY WEAK. THIS IS ALLOWING ALTERNATING LOCAL NIGHTTIME LAND
BREEZE AND AFTERNOON SEA BREEZE CIRCULATIONS TO DOMINATE THE
WEATHER PATTERN ACROSS EACH OF THE INDIVIDUAL ISLANDS.

ELSEWHERE...WEAK TROPICAL DISTURBANCES ARE EVIDENT IN SATELLITE
IMAGERY JUST SOUTH OF THE BIG ISLAND. ALTHOUGH THESE FEATURES ARE
NOT HAVING A SIGNIFICANT IMPACT ON OUR LOCAL WINDS...THESE SYSTEMS
ARE PROVIDING A SOURCE OF DEEP TROPICAL MOISTURE THAT HAS BEEN
ADVECTING UP OVER PARTS THE BIG ISLAND SINCE FRIDAY. THE OUTER
FRINGES OF THIS MOISTURE CONTINUE TO STREAM UP OVER PARTS OF THE
BIG ISLAND THIS MORNING. AS A RESULT...THE EARLY MORNING SOUNDING
FROM HILO SHOWS A VERY MOIST AND RELATIVELY UNSTABLE AIR MASS
REMAINS IN PLACE. THE SOUTHEASTERN AND WINDWARD SECTIONS OF THE
BIG ISLAND AND ADJACENT COASTAL WATERS WILL CONTINUE TO HAVE AN ACTIVE
SHOWER PATTERN THROUGH AT LEAST SUNDAY MORNING DUE TO THE PASSAGE
OF THESE TROPICAL DISTURBANCES TO THE SOUTH OF THE AREA.

A WEAK MID-LEVEL TROUGH PASSING NORTH OF THE ISLANDS WILL
EVENTUALLY BE REPLACED BY MID-LEVEL RIDGING STARTING SOME TIME ON
SUNDAY. THE SURFACE FRONT NORTH OF THE ISLANDS WILL MOVE SLOWLY
SOUTHWARD TODAY...BUT WILL STALL NORTH OF THE AREA SOMETIME ON
SUNDAY. AS A RESULT...THE PRESSURE GRADIENT OVER THE ISLANDS WILL
REMAIN WEAK THROUGH THIS WEEKEND MAINTAINING THE LOCAL LAND AND
SEA BREEZE CIRCULATION PATTERN THROUGH SUNDAY. THEREFORE...EXPECT
GENERALLY CLEAR SKIES EACH NIGHT AND MORNING...FOLLOWED BY CLOUD
BUILD UPS AND ISOLATED SHOWERS DEVELOPING BY LATE MORNING AND
LINGERING INTO THE EARLY EVENING HOURS. AFTERNOON CLOUDS COULD
PRODUCE SOME BRIEF HEAVY DOWNPOURS TODAY...BUT THESE MAY BE A LITTLE
LESS LIKELY ON SUNDAY AS A SLIGHTLY MORE STABILITY RETURNS ALOFT.

A MODERATELY STRONG SURFACE HIGH PASSING FAR NORTH OF THE ISLANDS
WILL INCREASE THE PRESSURE GRADIENT OVER THE AREA MONDAY...ALLOWING
THE LOW-LEVEL TRADE WIND FLOW TO RETURN. LIGHT TO MODERATE TRADE
WINDS WILL LIKELY PERSIST THROUGH THE REMAINDER OF THE WEEK. THE
FORECAST MODELS SHOW A NEW FRONT MOVING DOWN TOWARD THE AREA
AROUND THURSDAY. THIS FEATURE APPEARS TO BE DROPPING DOWN TOWARD
THE ISLAND CHAIN AS AN EAST TO WEST ORIENTED BAND OF CLOUDS AND
SHOWERS. AS A RESULT...IT APPEARS THIS FEATURE WILL IMPACT THE
ISLANDS FROM THURSDAY INTO FRIDAY. MOST OF THE MOISTURE WILL
LIKELY BE FOCUSED OVER WINDWARD SECTIONS OF THE STATE...AND IT
WILL POSSIBLY LINGER INTO NEXT WEEKEND. THE SLIGHTLY STRONGER LOW-
LEVEL TRADE WIND FLOW MAY TRANSPORT A FEW OF THESE SHOWERS OVER TO
LEEWARD AREAS ON THE SMALLER ISLANDS...ESPECIALLY NIGHTS AND
MORNINGS.

NOTE THAT THE LIGHT WINDS WILL ALLOW VOG AND SMOKE FROM THE BIG
ISLAND/S ACTIVE VOLCANO/LAVA FLOWS TO POOL NEAR THE SURFACE OVER
PARTS OF THE BIG ISLAND DURING THE OVERNIGHT AND EARLY MORNING HOURS
THIS WEEKEND...BEFORE CLEARING BY MID-MORNING. STRONGER WINDS NEXT
WEEK WILL LIKELY PREVENT A STRONG GROUND-BASED INVERSION FROM
DEVELOPING...AND THE POOLING OF PARTICULATES NEAR THE SURFACE SHOULD
BE LESS PRONOUNCED.

LOCAL SEA SURFACE TEMPERATURES /SSTS/ ARE ANOMALOUSLY WARM FOR
MID-SEPTEMBER...WHICH IS CONTRIBUTING TO THE ONGOING MUGGY
CONDITIONS. SURFACE DEW POINTS REMAIN IN THE UPPER 60/S TO LOWER
70/S THIS MORNING. THE SSTS ARE NOT EXPECTED TO CHANGE SIGNIFICANTLY
IN THE NEAR FUTURE. WITH THE RETURN OF THE TRADE WINDS NEXT WEEK...
AIR TEMPERATURES MAY COOL SLIGHTLY. HOWEVER...THE DEW POINTS WILL
LIKELY REMAIN RELATIVELY HIGH DUE TO THE ADDITION OF MOISTURE FROM
THE ADJACENT WARM OCEAN WATERS.


&&

.AVIATION...
VFR CONDITIONS PREVAIL ACROSS MOST OF THE AREA THOUGH THERE IS
LIKELY SOME MVFR CONDITIONS OVER THE SOUTHEAST BIG ISLAND SLOPES.
DAYTIME HEATING ALONG WITH SEA BREEZES WILL LIKELY CAUSE CLOUDS TO
BUILD UP OVER SOME AREAS THIS AFTERNOON ESPECIALLY INTERIOR AND
MOUNTAIN AREAS WITH A BIT MORE WIDESPREAD MVFR CONDITIONS EXPECTED IN
-SHRA BR WITH THE POSSIBILITY OF SOME BRIEF IFR CONDITIONS IN +SHRA.
IMPROVING CONDITIONS ARE EXPECTED LATER THIS EVENING AND OVERNIGHT.


&&

.MARINE...
THE FORECAST FOR WINDS AND SEAS REMAIN BELOW SMALL CRAFT ADVISORY
/SCA/ CRITERIA INTO EARLY NEXT WEEK. THERE IS A POSSIBILITY TRADE
WIND SPEEDS MAY REACH THE SCA THRESHOLD ACROSS THE WINDIER MARINE
ZONES ADJACENT TO THE ISLANDS OF MAUI AND HAWAII COUNTIES BY
MID-WEEK. A SLIGHT CHANCE OF THUNDERSTORMS HAS BEEN ADDED TO THE
FORECAST FOR THE COASTAL WATERS SOUTH OF THE BIG ISLAND TODAY DUE
TO THE DEEP TROPICAL MOISTURE STREAMING UP OVER THIS MARINE
ZONE...WHICH IS EXPERIENCING CONDITIONALLY UNSTABLE ATMOSPHERIC
CONDITIONS.

SURF WILL LIKELY REMAIN BELOW THE HIGH SURF ADVISORY CRITERIA ALONG
ALL SHORES OF ALL ISLANDS THROUGH NEXT FRIDAY. HOWEVER...SMALL
SWELLS FROM THE NORTHWEST...NORTHEAST...EAST AND SOUTH ARE EXPECTED
TO PROVIDE SMALL BACKGROUND SURF ALONG MOST SHORELINES OF THE STATE
THROUGH THIS WEEKEND. NEW NORTH AND NORTHWEST SWELLS WILL ALSO
AFFECT MOST ISLANDS ON WEDNESDAY AND THURSDAY...WITH A NORTH
NORTHEAST SWELL POSSIBLE THURSDAY AND FRIDAY. TINY LONG-PERIOD
SWELLS FROM THE SOUTH WILL MAINTAIN SURF HEIGHTS NEAR OR BELOW
SEASONAL AVERAGES ALONG SOUTH FACING SHORES NEXT WEEK.


&&

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

&&

$$


DISCUSSION/MARINE...FOSTER
AVIATION...BURKE


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



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