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Hawaiian Islands Synoptic Discussion and Guidance

851
FXHW60 PHFO 270225
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST TUE MAY 26 2015

.SYNOPSIS...
WINDS ARE EXPECTED TO BE SUFFICIENTLY LIGHT TO ALLOW LOCALIZED LAND
AND SEA BREEZES THROUGH THE REST OF THE WEEK. INCREASING MOISTURE
WILL LEAD TO AN INCREASINGLY CLOUDY AND SHOWERY WEATHER PATTERN...
PARTICULARLY DURING THE AFTERNOONS AND EARLY EVENINGS.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS A BROAD UPPER LEVEL TROUGH OVER THE
EASTERN HALF OF THE STATE...ASSOCIATED WITH A LOW FAR TO THE
NORTHEAST. ANOTHER SHORTWAVE IS APPROACHING KAUAI. AT THE
SURFACE...A COMPLEX LOW IS LOCATED FAR NORTHEAST OF THE STATE...
WITH A TROUGH THAT EXTENDS TO NEAR THE BIG ISLAND. WITHIN THE
TROUGH...A SMALL SCALE LOW CAN BE SEEN IN THE CLOUD FIELD OVER THE
OFFSHORE WATERS SOUTH OF THE BIG ISLAND. 00Z RAOBS SHOW INVERSIONS
RANGING FROM 7KFT AT LIHUE TO 11KFT AT HILO...WITH PRECIPITABLE
WATERS JUST UNDER 1.2 INCHES AT BOTH LOCATIONS. LATE MORNING TOTAL
PRECIPITABLE WATER IMAGERY SHOWS A BAND OF HIGHER MOISTURE NEAR
KAUAI/OAHU EXTENDING A COUPLE HUNDRED MILES NORTH...AND ANOTHER AREA
OF EVEN HIGHER MOISTURE SOUTH AND EAST OF THE BIG ISLAND.

CLOUDS HAVE FILLED IN ACROSS THE STATE WITH MOSTLY CLOUDY CONDITIONS
FOR MOST AREAS...AIDED BY THE MOISTURE ADVECTION AND THE APPROACH
OF THE SHORTWAVE NEAR KAUAI. CLOUDS BASES HAVE REMAINED RELATIVELY
HIGH THOUGH...AND SHOWERS WERE STILL ON THE LIGHT SIDE TODAY. MOST
RAINFALL REPORTS HAVE BEEN A FEW HUNDREDTHS OVER THE PAST 12
HOURS...WITH ONLY TWO GAGES ON OAHU PICKING UP OVER A TENTH. SOME OF
THE MORE WIDESPREAD MID-LEVEL CLOUDS SHOULD THIN OUT OVERNIGHT
BEHIND THE SHORTWAVE...AND SOME OF THE THICKER LEEWARD CLOUDS THAT
BUILT UP ALONG SEA BREEZES WILL BEGIN TO FADE BY SUNSET. LAND
BREEZES ARE ONCE AGAIN EXPECTED AS WELL. HOWEVER...THE COMBINATION
OF ADDITIONAL MOISTURE APPROACHING FROM NORTH OF KAUAI AND A
STRONGER NORTHEASTERLY GRADIENT OVER THE WESTERN HALF OF THE STATE
MAY HELP TO KEEP SOME CLOUDS AND SHOWERS ACROSS WINDWARD SECTIONS
THROUGH THE NIGHT. THE CLOUD-LEVEL LOW SOUTH OF THE BIG ISLAND
EMBEDDED WITHIN THE SURFACE TROUGH WILL HELP TO ENHANCE CLOUDS AND
SHOWERS ACROSS WINDWARD AND SOUTHEAST BIG ISLAND...KEEPING THEM
CLOUDY OVERNIGHT.

AS THE SHORTWAVE NEAR KAUAI PASSES THROUGH THE STATE...IT WILL HELP
TO REINFORCE THE LONGWAVE TROUGH OVER THE STATE...WHERE IT WILL
LINGER FOR MUCH OF THE WEEK. AT THE SURFACE...A WEAK RIDGE WILL TRY
TO EDGE IN NORTH OF THE STATE FROM A HIGH TO THE NORTHWEST...BUT THE
WEAK TROUGH WILL LINGER NEAR AND TO THE EAST OF THE BIG ISLAND. THE
BACKGROUND FLOW STRENGTHENS...AT LEAST OVER THE WEST OF THE
STATE...LEADING TO MORE LOCALIZED LAND AND SEA BREEZES. SEA BREEZES
SHOULD REMAIN MORE WIDESPREAD OVER THE EASTERN HALF OF THE STATE.
FOR BOTH AREAS...INCREASED CONVERGENCE ALONG THE SEA BREEZES WILL
LEAD TO CLOUD/SHOWER BUILDUPS ACROSS INTERIOR/LEEWARD SECTIONS
DURING THE AFTERNOON. THE COMBINATION OF FALLING HEIGHTS UNDER THE
UPPER LEVEL TROUGH AND INCREASING LOW-LEVEL MOISTURE WILL HELP TO
WEAKEN AND LIFT THE INVERSION...ALLOWING FOR GREATER MOISTURE DEPTH.
CROSS SECTIONS SHOW MOISTURE UP TO 700MB BY THE END OF THE WEEK...
AND POTENTIALLY CONTINUING THROUGH THE WEEKEND. THE CURRENT FORECAST
REFLECTS THE ISLAND-SCALE LAND AND SEA BREEZE PATTERN...WITH
INCREASING RAINFALL CHANCES AND AMOUNTS THROUGH THE WEEK DUE TO THE
INCREASE IN MOISTURE AND INVERSION HEIGHT.

OVER THE WEEKEND...A WEAK SURFACE HIGH BUILDS NORTH OF THE STATE.
THE SURFACE PATTERN REMAINS RATHER COMPLEX TO OUR NORTH...BUT
FEATURES A SLIGHT STRENGTHENING OF THE GRADIENT OVER THE ISLANDS.
WHILE WE GET BACK TO A LIGHT TRADE WIND BACKGROUND FLOW...WINDS
APPEAR TO REMAIN LIGHT ENOUGH FOR LOCALIZED SEA BREEZES ACROSS
SHELTERED LEEWARD AREAS. THEREFORE WE TRANSITION TO HYBRID RAINFALL
PATTERN...WITH CLOUDS AND SHOWERS FOCUSED ACROSS WINDWARD AREAS AT
NIGHT AND MORNING AND ACROSS LEEWARD AREAS DURING THE AFTERNOON AND
EVENING.

&&

.AVIATION...
A WEAK PRESSURE GRADIENT ACROSS HAWAII HAS RESULTING IN SEA BREEZES
AND CLOUD BUILD UPS OVER ISLAND INTERIORS THIS AFTERNOON. THESE
CLOUDS ARE PRODUCING TEMPO MVFR CIGS AND MTN OBSC ACROSS KAUAI AND
OAHU. EXPECT SOME OF THE CLOUDS TO DISSIPATE AFTER SUNSET AS THE
LAND BREEZES SET IN...AND THE AIRMET DROPPED AFTER 05Z. LOCALIZED
AFTERNOON SEA BREEZES AND CLOUDS WILL LIKELY OCCUR ON
WEDNESDAY...AND IT IS POSSIBLE THAT ANOTHER AIRMET FOR MTN OBSC MAY
BE NECESSARY DURING THE AFTERNOON.

&&

.MARINE...
THE CURRENT SOUTH SWELL WILL CONTINUE TO DIMINISH...BUT A SERIES OF
SOUTH SWELLS WILL KEEP SMALL TO MODERATE SURF ALONG SOUTH FACING
SHORES. THE LATEST SURFACE ANALYSIS SHOWS A STRONG FORCE LOW EAST OF
NEW ZEALAND WITH A LONG FETCH POINTED TOWARD THE STATE. LATEST
WAVEWATCH GUIDANCE SHOWS THIS SWELL REACHING US SATURDAY NIGHT AND
PEAKING ON MONDAY...WITH SURF LIKELY REACHING ADVISORY LEVELS FOR
SOUTH FACING SHORES.

&&

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

&&

$$

DISCUSSION/MARINE...BRAVENDER
AVIATION...FOSTER



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