Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 201359
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 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 BUILDS 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 REMAINS IN THE
VICINITY OF OAHU EARLY THIS MORNING. 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
540 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 SOUTH AND SOUTHEAST 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 A MORE ACTIVE SHOWER PATTERN AT LEAST THROUGH TONIGHT DUE TO
THE PASSAGE OF THESE TROPICAL DISTURBANCES TO THE SOUTH OF THE AREA.

ALOFT...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. THIS WILL MAINTAIN THE ALTERNATING
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. THE AFTERNOON
CONVECTION COULD ALSO FEATURE SOME BRIEF HEAVY DOWNPOURS AGAIN
TODAY...BUT THESE MAY BE A LITTLE LESS LIKELY ON SUNDAY AS A
SLIGHTLY MORE STABLE WEATHER PATTERN BEGINS TO DEVELOP.

A MODERATELY STRONG SURFACE HIGH PASSING FAR NORTH OF THE ISLANDS
WILL INCREASE THE PRESSURE GRADIENT OVER THE AREA EARLY NEXT WEEK.
THIS WILL ALLOW THE LOW-LEVEL TRADE WIND FLOW TO GRADUALLY RETURN
STARTING MONDAY. 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...
WEAK LAND BREEZES THIS MORNING WILL GIVE WAY TO GENTLE SEA BREEZES
THIS AFTERNOON...ALL ISLANDS. MOSTLY CLEAR SKIES PRESENTLY PREVAIL
ACROSS THE SMALLER ISLANDS BUT BY EARLY AFTERNOON CONVECTIVE
DEVELOPMENT WILL BE UNDERWAY AND FOCUSED ON ISLAND INTERIORS. THE
BIG ISLAND REMAINS UNDER THE INFLUENCE OF ENHANCED LOW LEVEL
MOISTURE AND UPPER LEVEL INSTABILITY. ISOL TO TEMPO MVFR CIGS IN
PASSING SHOWERS ARE EXPECTED THROUGH MIDDAY OVER THE SOUTHEASTERN
THIRD OF THE ISLAND. THE REMAINDER OF THE BIG ISLAND WILL SEE
PARTIAL CLEARING LATER THIS MORNING FOLLOWED BY CONVECTIVE BUILD-UPS
TOWARDS MID-AFTERNOON. OTHERWISE PREVAILING VFR ALL AREAS.

OUTLOOK...SEA BREEZES AND SEA BREEZE DRIVEN CONVECTION WILL
CONTINUE INTO EARLY EVENING. AFTER SUNSET SEA BREEZES WILL DIMINISH
AND SKIES WILL BEGIN TO CLEAR. ONLY THE SOUTHEASTERN THIRD OF THE
BIG ISLAND IS EXPECTED TO REMAIN SHOWERY 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. NOTE THAT 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...HOUSTON
AVIATION...BEDAL





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