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Hawaiian Islands Synoptic Discussion and Guidance

742
FXHW60 PHFO 010628
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SUN AUG 31 2014

.SYNOPSIS...
HIGH PRESSURE FAR NORTH OF THE ISLANDS WILL MOVE TO THE EAST
THROUGH MIDWEEK. SURFACE TROUGHS NEAR AND UPSTREAM OF THE ISLANDS
WILL HELP TO DISRUPT THE TRADE WIND FLOW THROUGH LABOR DAY. EXPECT
LIGHT EAST-SOUTHEAST WINDS TO PREVAIL THROUGH LABOR DAY...FOLLOWED
BY LIGHT TO MODERATE TRADE WINDS FOR THE REMAINDER OF THE WEEK. THE
LIGHT WINDS WILL ALLOW AFTERNOON SEA BREEZES TO PRODUCE LOCALIZED
CLOUD BUILDUPS AND A FEW SHOWERS OVER LEEWARD AND INTERIOR AREAS. AS
TRADE WINDS STRENGTHEN SLIGHTLY TOWARDS THE MIDDLE OF THE WEEK...THE
FOCUS FOR LIMITED CLOUDS AND SHOWERS WILL SHIFT TO WINDWARD SLOPES
AND COASTS. TRADE SHOWERS MAY INCREASE BY FRIDAY AS UPSTREAM
MOISTURE INCREASES.

&&

.DISCUSSION...
GENERAL FORECAST PHILOSOPHY REMAINS THE SAME THIS EVENING...WITH NO
ANTICIPATED UPDATES EXPECTED TO THE FORECAST IN THE SHORT TERM.

PREVIOUS DISCUSSION FOLLOWS.

&&

.PREV DISCUSSION...
ISSUED AT 345 PM

LATEST SATELLITE AND RADAR ANIMATIONS SHOW THE LOW-LEVEL FLOW NEAR
THE ISLANDS IS FROM THE ESE...EFFECTIVELY PLACING THE SMALLER
ISLANDS IN THE WIND SHADOW OF THE BIG ISLAND. A RELATIVELY WEAK 1026
MB SURFACE HIGH CENTERED ABOUT 1500 MILES NNE OF THE ISLANDS IS
DRIVING LIGHT TO MODERATE TRADE WINDS OVER THE TROPICAL CENTRAL N
PACIFIC...BUT WEAK LOW LEVEL TROUGHS NEAR THE ISLANDS HAVE WEAKENED
AND VEERED THE LOW LEVEL FLOW. WITH THE LIGHT WINDS...DAYTIME
HEATING OF THE ISLANDS LED TO SEA BREEZES WHICH DROVE CLOUD AND
SHOWER FORMATION OVER THE ISLANDS. ENHANCED MOISTURE NEAR THE BIG
ISLAND IS ASSOCIATED WITH ONE OF THESE PASSING TROUGHS...WHICH ADDED
SOME FUEL TO THE AFTERNOON CONVECTIVE CLOUDS/SHOWERS THERE. WATER
VAPOR IMAGERY SHOWS RIDGING ALOFT CENTERED OVER THE ISLANDS...WITH
VERY DRY UPPER LEVELS...YET BALLOON SOUNDINGS CONTINUE TO SHOW AN
ELEVATED INVERSION BASE NEAR 10 KFT...AND ABUNDANT MOISTURE BELOW
THIS CAP.

OVERNIGHT...SEA BREEZE-DRIVEN CLOUDS AND SHOWERS WILL PERSIST INTO
THE EARLY EVENING BEFORE DISSIPATING SHORTLY THEREAFTER. WITH THE
LOW LEVEL FLOW CONTINUING TO FAVOR AN ESE DIRECTION...A FEW SHOWERS
WILL MOVE ASHORE OVER E AND SE SHORES AND SLOPES...BUT A MOSTLY DRY
NIGHT IS EXPECTED AS THE BULK OF THE MOISTURE NEAR THE BIG ISLAND
MOVES ON A TRAJECTORY TO KEEP IT AWAY FROM THE OTHER ISLANDS. LIGHT
WINDS ALLOWED A GROUND-BASED INVERSION TO DEVELOP IN MANY AREAS LAST
NIGHT...WHICH IS USUALLY NOT TOO MUCH OF AN ISSUE...BUT SMOKE/ODORS
ASSOCIATED WITH A RELATIVELY NEW LAVA FLOW IN THE PUNA REGION BECAME
TRAPPED NEAR THE SURFACE BEFORE THE INVERSION BROKE BY MID-MORNING
TODAY. THE EXPECTATION IS THAT THIS WILL OCCUR AGAIN TONIGHT...BUT
TRADE WINDS SHOULD BE STRONG ENOUGH BY TUESDAY TO PREVENT THIS FROM
OCCURRING THEREAFTER.

A RELATIVELY DRY WEATHER PATTERN IS EXPECTED OVER THE NEXT COUPLE OF
DAYS. WINDS WILL SLOWLY BACK TO A MORE TYPICAL TRADE WIND
DIRECTION...AND STRENGTHEN SLIGHTLY...ON MONDAY AND TUESDAY AS THE
TROUGHS NEAR THE ISLANDS WEAKEN FURTHER AND MOVE NW...AND THE
SURFACE RIDGE TO THE N REBOUNDS SOMEWHAT. UNTIL THE TRADES FIRM
UP...WEATHER OVER LEEWARD AREAS WILL BE DRIVEN BY LAND AND SEA
BREEZES...WITH A MOSTLY CLEAR OVERNIGHT/MORNING...AND INCREASING
LATE MORNING CLOUDS LEADING TO A FEW AFTERNOON SHOWERS THAT
GRADUALLY CLEAR IN THE EVENING.

AS TRADE WINDS STRENGTHEN SOMEWHAT TUESDAY AND REMAIN LIGHT TO
MODERATE THROUGH 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 THROUGH THURSDAY...THUS
RAINFALL TOTALS WILL BE ON THE LOW END. INCREASED MOISTURE LOOSELY
ASSOCIATED WITH THE REMNANT OF MARIE PASSING FAR NE OF THE ISLANDS
MAY INCREASE WINDWARD SHOWERS FROM THURSDAY NIGHT THROUGH SATURDAY...
WITH THIS SOLUTION SHARED BY BOTH THE GFS AND ECMWF.

&&

.AVIATION...
PERIODS OF MVFR CONDITIONS WILL IMPACT PHTO TERMINAL OVERNIGHT AS
SHOWERS AND CLOUDS MOVE INTO THE BIG ISLAND FROM THE SOUTHEAST...AND
AN AIRMET FOR MOUNTAIN OBSCURATION MAY BE NEEDED FROM HILO SOUTHWARD
ALONG THE PUNA AND KAU COASTS.

VFR CONDITIONS WILL PREVAIL ELSEWHERE TONIGHT AND MONDAY. CLOUDS AND
WIDELY SCATTERED SHOWERS SHOULD DEVELOP ONCE AGAIN MONDAY AFTERNOON
ALONG SEABREEZES ACROSS INTERIOR AND LEEWARD AREAS...WITH ISOLATED
MVFR CONDITIONS POSSIBLE.

&&

.MARINE...
ISSUED AT 345 PM

A MODEST REINFORCEMENT TO THE CURRENT S SWELL IS EXPECTED INTO
MONDAY...BUT IS NOT READILY APPARENT IN LATEST BUOY/SURF
OBSERVATIONS...WITH MOST OBSERVATIONS SHOWING LITTLE CHANGE OVER
THE PAST 24 HOURS OR SO. SURF ALONG S FACING SHORES IS EXPECTED TO
REMAIN BELOW ADVISORY LEVELS INTO TUESDAY...BUT A VERY LONG PERIOD
/20-25 SECOND/ S SWELL ARRIVING TUESDAY AND BUILDING THROUGH
WEDNESDAY WILL LIKELY PRODUCE ADVISORY LEVEL SURF ALONG S FACING
SHORES INTO THURSDAY. LONG PERIOD SWELL FROM THE E CONTINUES TO BE
OBSERVED...BUT SHOULD CONTINUE TO TREND DOWN THROUGH MONDAY. WINDS
AND SEAS WILL REMAIN BELOW SMALL CRAFT ADVISORY THRESHOLDS FOR AT
LEAST THE NEXT SEVERAL DAYS.

&&

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

&&

$$

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



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