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Hawaiian Islands Synoptic Discussion and Guidance

681
FXHW60 PHFO 310625
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SAT AUG 30 2014

.SYNOPSIS...
LIGHT TRADE WINDS WILL PREVAIL THROUGH THE HOLIDAY WEEKEND...
DELIVERING A FEW CLOUDS AND SHOWERS TO WINDWARD AREAS...MAINLY
DURING NIGHTS AND MORNINGS. THE LIGHT WINDS WILL ALSO ALLOW
AFTERNOON SEA BREEZES TO PRODUCE LOCALIZED CLOUD BUILDUPS AND A FEW
SHOWERS OVER LEEWARD AND INTERIOR AREAS. A MORE TYPICAL TRADE WIND
WEATHER PATTERN IS EXPECTED BY THE MIDDLE OF NEXT WEEK.

&&

.DISCUSSION...
NO SIGNIFICANT CHANGES TO THE FORECAST PHILOSOPHY THIS EVENING. THE
POORLY DEFINED TROUGH HAS CONTINUED TO BECOME LESS DEFINED SINCE
SUNSET. SHOWERS OVER THE KAU SLOPES BEGAN TO DISSIPATE SHORTLY AFTER
SUNSET...BUT ADDITIONAL CLOUDS SOUTHEAST OF THE BIG ISLAND COULD
BRING MORE SHOWERS TO THE AREA OVERNIGHT. NO UPDATES ARE EXPECTED TO
THE FORECAST AT THIS TIME.

PREVIOUS DISCUSSION FOLLOWS.

&&

.PREV DISCUSSION...
ISSUED AT 350 PM

CURRENTLY...ALTHOUGH LIGHT ENE TRADE WINDS PREVAIL ON THE LARGE
SCALE...WINDS ARE LOCALLY GUSTY THIS AFTERNOON AS DAYTIME HEATING
HELPS TO MIX HIGHER MOMENTUM AIR DOWN TO THE SURFACE. THE TRADES ARE
BEING SUPPORTED BY A RELATIVELY WEAK 1025 MB SURFACE HIGH CENTERED
ABOUT 1300 MILES N OF THE ISLANDS. HOWEVER...A WEAK AREA OF LOW
PRESSURE...MARKING THE REMNANT OF WHAT ONCE WAS TROPICAL CYCLONE
LOWELL...IS LOCATED ABOUT 700 MILES N OF THE ISLANDS NEAR 30N
155W...AND LIES BETWEEN THE HIGH AND THE ISLANDS. CLOSER TO HOME...A
POORLY DEFINED N-S ORIENTED TROUGH IS MOVING W ACROSS THE ISLAND
CHAIN...WITH THE AXIS CURRENTLY NEAR MOLOKAI. THE PRESENCE OF
THESE WEAK LOWS IS CONTRIBUTING TO THE RELATIVELY LOOSE PRESSURE
GRADIENT AND REDUCED TRADE WINDS NEAR THE ISLANDS. WHILE ONLY
LIMITED MOISTURE IS ASSOCIATED WITH THE PASSING TROUGH...IT APPEARS
THAT IT DID HELP TO FUEL CLOUDY SKIES OVER PORTIONS OF MAUI COUNTY
THIS AFTERNOON. AN OVERHEAD MID-LEVEL RIDGE IS MAINTAINING A STRONG
YET SOMEWHAT ELEVATED CAP ON THE ISLAND ATMOSPHERE...WITH AFTERNOON
SOUNDINGS INDICATING THE INVERSION BASE IS NEAR 10 KFT.

ALTHOUGH THE INVERSION BASE IS RELATIVELY HIGH...A RELATIVELY DRY
WEATHER PATTERN IS EXPECTED OVER THE NEXT COUPLE OF DAYS. THE
FORECAST CALLS FOR THE NEARBY TROUGH...AND THE LOWELL REMNANT...TO
MOVE GENERALLY WNW WHILE WEAKENING OVER THE NEXT DAY OR TWO...WITH
LOW LEVEL WINDS NEAR THE ISLANDS REMAINING LIGHT...BUT VEERING TO AN
ESE DIRECTION TONIGHT AND SUNDAY. THIS VEERING OF THE LOW LEVEL
WINDS WILL PLACE THE SMALLER ISLANDS IN THE WIND SHADOW OF THE BIG
ISLAND...AND WINDS ARE EXPECTED TO BE MORE VARIABLE IN DIRECTION ON
SUNDAY AS COMPARED TO TODAY. WHILE VOLCANIC EMISSIONS FROM THE BIG
ISLAND ARE NOT EXPECTED TO SPREAD OVER THE SMALLER ISLANDS WITH THIS
FLOW...SURFACE DEW POINTS ARE EXPECTED TO INCREASE SLIGHTLY...AND
WILL BE IN THE LOW 70/S STATEWIDE SUNDAY AND MONDAY. COMBINED WITH
THE LIGHT WINDS...CONDITIONS WILL BE WARM AND MUGGY.

ALTHOUGH THE WEAKENING REMNANT OF FORMER TROPICAL CYCLONE MARIE WILL
BE PASSING FAR NE OF THE ISLANDS DURING THE UPCOMING WEEK...FORECAST
MODELS DEPICT SUFFICIENT SURFACE RIDGING BUILDING N OF THE ISLANDS
TO SUPPORT A MODERATE TRADE WIND FLOW...INITIALLY STRENGTHENING AND
BACKING TO THE E ON MONDAY...BECOMING ENE TUESDAY...AND REMAINING
THAT WAY THROUGH THE END OF THE WEEK.

WHAT WE LIKE TO CALL A HYBRID TRADE WIND PATTERN IS EXPECTED
THROUGH THE REMAINDER OF THE HOLIDAY WEEKEND. IN THIS REGIME...
LIGHT TRADE WINDS MANAGE TO DELIVER A FEW SHOWERS TO WINDWARD
AREAS...MAINLY NIGHTS AND MORNINGS...WHILE WEATHER OVER LEEWARD
AREAS WILL BE DRIVEN BY THE DIURNAL CYCLE OF LAND AND SEA BREEZES.
THIS MEANS MOSTLY CLEAR NIGHTS AND MORNINGS...WITH INCREASING LATE
MORNING CLOUDS LEADING TO A FEW AFTERNOON SHOWERS THAT GRADUALLY
CLEAR IN THE EVENING. AS TRADE WINDS STRENGTHEN LATER IN THE
WEEK...LEEWARD AREAS WILL BE MOSTLY DRY...WITH BRIEF SHOWERS OVER
WINDWARD AREAS. THE EXCEPTION IS LEEWARD BIG ISLAND...WHERE CLOUDS
AND A FEW SHOWERS WILL FAVOR AFTERNOON AND EVENING HOURS BEFORE
CLEARING LATE AT NIGHT. MID-LEVEL RIDGING PERSISTING IN THE AREA
WILL KEEP THE ISLAND ATMOSPHERE STABLE...AND NO SIGNIFICANT MOISTURE
SOURCES ARE EXPECTED...THUS RAINFALL TOTALS WILL BE ON THE LOW END.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL ACROSS THE STATE TONIGHT AND SUNDAY.
BRIEF PERIODS OF MVFR WILL BE POSSIBLE WITH PASSING SHOWERS
ACROSS WINDWARD SECTIONS OVERNIGHT INTO EARLY SUNDAY MORNING...
AND ACROSS LEEWARD/INTERIOR AREAS AS WIDELY SCATTERED SHOWERS
DEVELOP ALONG SEABREEZES SUNDAY AFTERNOON.

NO AIRMETS ARE IN EFFECT AT THIS TIME. THERE IS A SMALL CHANCE THAT
AIRMET SIERRA FOR MOUNTAIN OBSCURATION COULD BECOME NECESSARY IF A
MORE PERSISTENT OR WIDESPREAD AREA OF SHOWERS/CLOUDS DEVELOPS.


&&

.MARINE...
THE LATEST BUOY DATA CONTINUES TO SHOW A SLOW DECREASING TREND IN
SWELL AND RESULTING SURF HEIGHTS ALONG SOUTH FACING SHORES.
HOWEVER...SURF WILL REMAIN ELEVATED ALONG S FACING SHORES FOR AT
LEAST THE NEXT SEVERAL DAYS. ALTHOUGH A REINFORCING SWELL IS
EXPECTED SUNDAY...RESULTANT SURF IS EXPECTED TO REMAIN BELOW
ADVISORY LEVELS THROUGH MONDAY.

A VERY LONG PERIOD /20-25 SECOND/ SOUTH SWELL ARRIVING TUESDAY AND
BUILDING THROUGH WEDNESDAY WILL LIKELY PRODUCE ADVISORY LEVEL
SURF INTO THURSDAY.

LONG PERIOD SWELL FROM THE EAST IS STILL BEING OBSERVED AT NEAR
SHORE BUOYS...BUT THE MAGNITUDE OF THE SWELL IS LOW ENOUGH THAT SURF
ALONG EAST FACING SHORES REMAINS BELOW ADVISORY LEVELS.

WINDS AND SEAS WILL REMAIN BELOW SMALL CRAFT ADVISORY THRESHOLDS FOR
AT LEAST THE NEXT SEVERAL DAYS...WITH AN OUTSIDE CHANCE THAT WINDY
MARINE ZONES AROUND THE MAUI AND THE BIG ISLAND REACH CRITERIA AS
TRADES INCREASE AROUND MID-WEEK.

&&

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

&&

$$

DISCUSSION/MARINE...M BALLARD
PREV DISCUSSION...BIRCHARD
AVIATION...JACOBSON




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