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Hawaiian Islands Synoptic Discussion and Guidance

685
FXHW60 PHFO 010629
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST TUE JUN 30 2015

.SYNOPSIS...
A SURFACE RIDGE IN THE VICINITY OF THE ISLANDS WILL KEEP WINDS LIGHT
THROUGH FRIDAY. DAYTIME HEATING WILL PRODUCE LOCAL SEA BREEZES WHICH
WILL CAUSE CLOUDS AND SCATTERED SHOWERS OVER LEEWARD AND INTERIOR
SECTIONS EACH AFTERNOON AND EVENING...FOLLOWED BY LOCAL LAND BREEZES
AND CLEARING SKIES OVERNIGHT. THERE MAY BE A SLIGHT INCREASE IN
AFTERNOON SHOWERS OVER THE NEXT COUPLE OF DAYS AS AN AREA OF
ENHANCED MOISTURE CURRENTLY LOCATED NEAR THE EASTERN BIG ISLAND
PUSHES UP OVER THE REMAINDER OF THE STATE. THE SURFACE RIDGE IS
EXPECTED TO BUILD NORTH OF THE ISLANDS DURING THE UPCOMING HOLIDAY
WEEKEND...WHICH WILL ALLOW THE TRADES TO RETURN.

&&

.DISCUSSION...
THE SOUTHWESTERN END OF THE ROUNDED SUBTROPICAL RIDGE IS LOCATED
JUST NORTH OF THE MAIN HAWAIIAN ISLANDS EARLY THIS EVENING. THE
CLOSE PROXIMITY OF THIS FEATURE TO THE STATE IS KEEPING RELATIVELY
LIGHT WINDS IN PLACE ACROSS THE AREA. THIS IS ALLOWING LOCAL
NIGHTTIME LAND BREEZES TO BECOME ESTABLISHED OVER EACH OF THE
ISLANDS AS NOCTURNAL RADIATIONAL COOLING OCCURS. ALOFT...A
MID-TROPOSPHERIC TROUGH IS EVIDENT NEAR KAUAI IN WATER VAPOR
SATELLITE IMAGERY. THIS TROUGH ALOFT...WHICH APPEARS TO BE MOVING
SLOWLY TOWARD THE EAST NORTHEAST...HAS CAUSED THE LOW-LEVEL
SUBSIDENCE INVERSION BE SLIGHTLY WEAKER THAN NORMAL.

LOOKING AT THE LOW-LEVEL MOISTURE FIELD...THERE IS AN AREA OF
ENHANCED TROPICAL MOISTURE BEING ADVECTED TOWARD THE WINDWARD AND
SOUTHEASTERN BIG ISLAND EARLY THIS EVENING. AS A RESULT...LOW CLOUDS
AND SHOWERS APPEAR TO BE SLIGHTLY ENHANCED THERE. HOWEVER...
SATELLITE DERIVED VALUES OF PRECIPITABLE WATER /PW/ CONTINUE TO SHOW
A ZONE OF RELATIVELY DRY AIR EXTENDING FROM NORTHEAST TO SOUTHWEST
ACROSS OAHU AND MOST OF MAUI COUNTY. THIS SOMEWHAT DRIER AIR HAS
LIMITED CLOUDS AND PREVENTED SHOWER DEVELOPMENT ACROSS MOST OF THOSE
ISLANDS EARLIER THIS AFTERNOON. THERE HAVE BEEN SOME LOW CLOUDS AND
A SHOWER OR TWO NEAR KAUAI...WHICH IS JUST TO THE WEST OF THE ZONE
OF RELATIVELY DRY AIR. THE LOCAL LAND BREEZE CIRCULATIONS...WHICH
ARE CURRENTLY SETTING UP...WILL LIKELY CLEAR MOST LAND AREAS
OVERNIGHT...EXCEPT ALONG THE COASTAL AND UPSLOPE SECTIONS OF THE
WINDWARD AND SOUTHEASTERN BIG ISLAND...WHERE LIGHT EAST WINDS AND
THE AREA OF ENHANCED MOISTURE WILL CONTINUE TO ALLOW LOW CLOUDS AND
SHOWERS TO DEVELOP AND MOVE ONSHORE. SOME OF THESE LOW CLOUDS AND
SHOWERS MAY AFFECT EASTERN MAUI LATER TONIGHT ALSO.

THE FORECAST MODELS SHOW THE MID-TROPOSPHERIC TROUGH SLOWLY MOVING
TOWARD THE EAST NORTHEAST THROUGH LATE WEDNESDAY...AND AWAY FROM THE
AREA BY THURSDAY. IN THE MEANTIME...THIS FEATURE WILL PRODUCE SLIGHT
INSTABILITY IN THE FORM OF WEAK SUBSIDENCE INVERSIONS THROUGH
WEDNESDAY NIGHT. THE GUIDANCE ALSO KEEPS THE SURFACE RIDGE IN THE
VICINITY OF THE ISLANDS THROUGH FRIDAY...SO BACKGROUND WINDS WILL
REMAIN LIGHT OVER MOST AREAS. THIS WILL CONTINUE TO ALLOW AN
ALTERNATING DIURNAL PATTERN OF LOCAL AFTERNOON SEA BREEZES FOLLOWED
BY LOCAL NIGHTTIME LAND BREEZES.

THE ECMWF FORECAST MODEL CONTINUES TO SHOW AN AREA OF SLIGHTLY DRIER
AIR WILL REMAIN OVER PORTIONS OF THE CENTRAL ISLANDS AS IT EDGES
SLOWLY NORTHWESTWARD. HOWEVER...MOST OF THE OTHER MODELS...
INCLUDING THE HIGH-RESOLUTION REGIONAL MODELS...SUGGEST THAT THE
ENHANCED MOISTURE CURRENTLY LOCATED EAST AND SOUTHEAST OF THE BIG
ISLAND WILL CONTINUE TO GRADUALLY ADVECT OVER MOST OF THE SMALLER
MAIN HAWAIIAN ISLANDS WEDNESDAY AND THURSDAY. FOR THE MOMENT...THE
FORECAST KEEPS PORTIONS OF THE ISLANDS OF MAUI COUNTY AND THE BIG
ISLAND WETTER THAN NORMAL THROUGH WEDNESDAY AFTERNOON...WITH
SLIGHTLY DRIER CONDITIONS ON OAHU. THE CURRENT FORECAST ALSO
INDICATES INCREASED RAIN CHANCES OVER THE LEEWARD AND INTERIOR
SECTIONS OF MOST OF THE ISLAND CHAIN THURSDAY AND FRIDAY. WITH
RELATIVELY STABLE CONDITIONS EXPECTED AFTER THE MID-LEVEL TROUGH
MOVES AWAY FROM THE AREA...NO SIGNIFICANT HEAVY RAINFALL IS
EXPECTED...ESPECIALLY THURSDAY AND FRIDAY.

THE LONGER RANGE FORECAST GUIDANCE INDICATES SURFACE HIGH PRESSURE
WILL BUILD NORTH OF THE STATE STARTING THIS WEEKEND...AND CONTINUE
INTO NEXT WEEK. THE INCREASE OF THE PRESSURE GRADIENT OVER THE STATE
WILL GRADUALLY RETURN THE TRADES TO THE ISLANDS...WITH MODERATE
SPEEDS EXPECTED AGAIN BY MONDAY. THIS SHOULD ALSO BRING A RETURN TO
MORE TYPICAL TRADE WIND WEATHER CONDITIONS...WITH LOW CLOUDS AND
TRADE SHOWERS MAINLY AFFECTING WINDWARD AND MAUKA AREAS.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL OVER MOST AREAS...BUT LOCALIZED MVFR
CONDITIONS DUE TO LOW CLOUDS AND SHRA MAY LINGER ACROSS EAST AND
SOUTHEAST SECTIONS OF MAUI/BIG ISLAND FROM TONIGHT INTO WEDNESDAY
MORNING. A SIMILAR DIURNAL CONVECTIVE PATTERN CONSISTING OF
LOCALIZED AFTERNOON SEA BREEZES WITH LEEWARD AND INTERIOR CLOUDS AND
SCATTERED SHRA IS EXPECTED ON WEDNESDAY. NOTE THAT AN INCREASE IN
MOISTURE COMBINED WITH SLIGHTLY UNSTABLE ATMOSPHERIC CONDITIONS MAY
LEAD TO MORE AREAS WITH MVFR DUE TO AN INCREASE IN COVERAGE OF LOW
CLOUDS AND SHRA WEDNESDAY AFTERNOON.

&&

.MARINE...
THE CURRENT FORECAST DOES NOT INDICATE SMALL CRAFT ADVISORY /SCA/
CONDITIONS OVER ANY HAWAIIAN WATERS THROUGH THE UPCOMING HOLIDAY
WEEKEND.

THE WEATHER PATTERN HAS BEEN FAIRLY ACTIVE IN THE SOUTHERN
HEMISPHERE...ESPECIALLY IMMEDIATELY TO THE EAST AND SOUTHEAST OF NEW
ZEALAND. THEREFORE...EXPECT PULSES OF SMALL TO MODERATE SOUTH AND
SOUTHWEST SWELL ENERGY TO AIM TOWARD THE ISLAND CHAIN OVER THE NEXT
SEVERAL DAYS. THIS WILL MAINTAIN SMALL TO MODERATE SURF ALONG SOUTH
FACING SHORES OF THE STATE THROUGH FRIDAY. A SLIGHTLY LARGER
REINFORCING SOUTH SWELL ARRIVING LATE FRIDAY WILL LIKELY PRODUCE A
SLIGHT BUMP IN SURF ALONG SOUTH FACING SHORES DURING THE UPCOMING
HOLIDAY WEEKEND...BUT SURF HEIGHTS ARE EXPECTED TO REMAIN BELOW THE
HIGH SURF ADVISORY CRITERIA.

A CAPTURED FETCH ASSOCIATED WITH THE CIRCULATION AROUND A SURFACE
LOW LOCATED ABOUT MIDWAY BETWEEN THE ALEUTIAN ISLANDS AND HAWAII HAS
BEEN AIMED AT THE MAIN HAWAIIAN ISLAND CHAIN. THIS WILL SEND A SMALL
OUT-OF-SEASON NORTHWEST SWELL...WHICH IS EXPECTED TO REACH THE NORTH
AND WEST FACING SHORES OF MOST ISLANDS WEDNESDAY NIGHT...PEAK
THURSDAY...THEN SLOWLY DIMINISH THROUGH FRIDAY.

&&

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

&&

$$






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