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Hawaiian Islands Synoptic Discussion and Guidance

558
FXHW60 PHFO 020630
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST TUE SEP 1 2015

.SYNOPSIS...
HUMID CONDITIONS AND SHOWERY WEATHER WILL CONTINUE DURING THE NEXT
SEVERAL DAYS...ESPECIALLY IN THE AFTERNOON HOURS. WINDS WILL BECOME
MOSTLY SOUTHERLY THROUGH MUCH OF THE WORK WEEK...THEN BECOME LIGHT
AND VARIABLE DURING THE UPCOMING WEEKEND. THE EXTENDED FORECAST
WILL DEPEND HEAVILY ON THE TRACK OF HURRICANE JIMENA...WHICH SHOULD
REMAIN WELL TO THE NORTHEAST OF THE ISLANDS. A BRIEF RETURN TO DRIER
EASTERLY WINDS IS POSSIBLE BY EARLY NEXT WEEK.

&&

.DISCUSSION...
AT 8 PM HST...TROPICAL STORM IGNACIO WAS LOCATED ABOUT 260 MILES NE
OF OAHU WITH SOME OF ITS RAIN BANDS MOVING ACROSS THE WINDWARD
COASTAL WATERS. EARLIER IN THE EVENING...RADAR AND SATELLITE IMAGERY
SHOWED SOME LINES OF SHOWERS...ASSOCIATED WITH THE OUTER CIRCULATION
OF IGNACIO...BRINGING SOME HEAVY RAIN OVER THE SE SLOPES OF THE BIG
ISLAND...AS WELL AS BRIEF PASSING SHOWERS ACROSS OAHU.

LATEST WATER VAPOR SATELLITE IMAGERY SHOW A HIGH ALOFT TO THE W OF
THE MAIN HAWAIIAN ISLANDS...WHILE A TROUGH REMAINS TO THE NW.
MEANWHILE...A BROAD SFC HIGH IS CENTERED FAR N OF THE STATE.

THE LOW LEVEL FLOW ACROSS THE ISLAND CHAIN REMAINS WEAK AND
GENERALLY FROM THE SW AS IGNACIO KEEPS BLOCKING THE TRADES AND
WEAKENING PRESSURE GRADIENTS S OF ITS CENTER. THE AIRMASS REMAINS
MOIST ENOUGH TO KEEP GENERATING SHOWERS TONIGHT...BUT BOTH MIMIC AND
MODEL ANALYSES SUGGEST A SLIGHTLY DRIER AIRMASS FILTERING INTO THE
ISLAND CHAIN...ESPECIALLY THE SMALLER ISLANDS. THIS IS PROBABLY
RELATED TO SUBSIDENCE DEVELOPING S OF IGNACIO. NEVERTHELESS...
ADDITIONAL SHOWERS AND ISOLATED DOWNPOURS ARE STILL POSSIBLE
TONIGHT...ESPECIALLY OVER THE BIG ISLAND WHERE HIGHER PWATS RESIDE.

OVERALL...THE LOW LEVEL FLOW WILL KEEP CHANGING AS IGNACIO MOVES
FURTHER AWAY FROM THE AREA DURING THE NEXT COUPLE OF DAYS. THIS WILL
BRING PERIODS OF SW OR S WINDS COLLIDING WITH THE MOUNTAINS...WHICH
COULD RESULT IN SOME TERRAIN ANCHORING AND LOCALIZED HEAVY RAIN AT
TIMES. THE WEAK WINDS WILL ALSO ALLOW FOR VERY HUMID CONDITIONS TO
CONTINUE...WHICH WILL TRANSLATE IN ABOVE NORMAL MAX TEMPERATURES...
POTENTIAL FOR HEAT INDEX VALUES IN THE UPPER 90S...AND OVERNIGHT
TEMPERATURES HOVERING THE UPPER 70S. THEREFORE...EXPECT GENERALLY
MUGGY AND WARM CONDITIONS TO PREVAIL.

IGNACIO SHOULD BE LIFTING N AND FURTHER AWAY FROM THE ISLANDS BY
FRI...BUT THE LOW LEVEL FLOW WILL NOT CHANGE SIGNIFICANTLY EVEN
THROUGH SAT....THUS EXPECT THE VERY HUMID CONDITIONS AND SHOWERY
WEATHER TO CONTINUE INTO THE WEEKEND. THE ONLY SIGNIFICANT CHANGE
IN THE FORECAST WILL BE THE DECREASE IN CHANCES FOR HEAVY RAIN
AFTER THU AS THE GLOBAL MODELS SHOW A BROAD RIDGE DEVELOPING ALOFT
AND FILTERING INTO THE AREA FROM THE E...WHICH WILL RESULT IN AN
INCREASE IN AIRMASS STABILITY.

DRIER EASTERLY FLOW MAY DEVELOP EARLY NEXT WEEK...BUT CONFIDENCE
REMAINS LOW IN THE EXTENDED. THIS IS MAINLY DUE TO UNCERTAINTY
REGARDING THE EVOLUTION OF HURRICANE JIMENA...WHICH IS FORECAST TO
GRADUALLY TRACK FAR E AND NE OF THE ALOHA STATE.

&&

.AVIATION...

SEE TC SIGMET SERIES TANGO FOR TROPICAL CYCLONE IGNACIO.

BANDS OF SHOWERS PASSING OVER KAUAI AND OAHU WILL PERSIST THROUGH
THE NIGHT AND MOVE INTO MAUI COUNTY ON WEDNESDAY. CONVECTIVE SHOWER
ACTIVITY ON THE BIG ISLAND WILL TAPER OFF BY MORNING ONLY TO
REDEVELOP TOMORROW AFTERNOON. ISOL MVFR CONDITIONS ARE POSSIBLE IN
PASSING SHOWERS OTHERWISE VFR CONDITIONS WILL PREVAIL ALL AREAS.

AIRMET SIERRA FOR MOUNTAIN OBSCURATION EAST SIDE BIG ISLAND WILL
LIKELY BE CANCELLED LATER THIS EVENING...BUT MAY BE NECESSARY AGAIN
ON TOMORROW AFTERNOON.

THE OVERNIGHT THUNDERSTORM THREAT IS LIKELY TO REMAIN
MINIMAL...EXCEPT IN THE FAR NORTHERN COASTAL WATERS NEAR IGNACIO.
HOWEVER...THE THREAT OF THUNDERSTORMS WILL INCREASE SIGNIFICANTLY
TOMORROW AFTERNOON...ESPECIALLY FOR MAUI AND THE BIG ISLAND.

&&

.MARINE...
A HIGH SURF WARNING REMAINS IN EFFECT FOR MOST EAST FACING SHORES
OF THE ISLANDS THROUGH THURSDAY AS SWELLS FROM IGNACIO AND JIMENA
CONTINUE TO MOVE ACROSS THE WINDWARD HAWAIIAN WATERS. SOME OF THE
INCOMING EASTERLY SWELLS FROM JIMENA COULD ALSO IMPACT SOUTHEAST
SECTIONS OF THE BIG ISLAND AND MAUI. WAVEWATCH GUIDANCE SHOWS THIS
SWELL BUILDING THROUGH TONIGHT...PEAKING WEDNESDAY/WEDNESDAY NIGHT
AND REMAINING STEADY THROUGH THURSDAY. LATE IN THE WEEK...WE SHOULD
SEE THE DIRECTION START TO BACK TO A MORE EAST-NORTHEAST DIRECTION
AS JIMENA GAINS MORE LATITUDE...AND EVEN SEE AN INCREASE IN HEIGHT
AS IT MOVES CLOSER TO THE STATE. THE HIGH SURF WARNING COULD BE
EXTENDED THROUGH THE WEEKEND OR LONGER.

A SMALL CRAFT ADVISORY IS ALSO IN EFFECT THROUGH THURSDAY FOR
HAZARDOUS SEAS DUE TO THE SWELLS FROM IGNACIO AND JIMENA. THIS
ADVISORY MAY ALSO NEED TO BE EXTENDED THROUGH THE WEEKEND OR LONGER
AS SEAS REMAIN ELEVATED. WINDS ARE EXPECTED TO REMAIN BELOW ADVISORY
LEVELS THIS WEEK.

LONG-PERIOD SWELLS FROM THE SOUTH-SOUTHWEST MAY PRODUCE
ADVISORY-LEVEL SURF ALONG SOUTH-FACING SHORES THIS WEEKEND. PLEASE
REFER TO THE LATEST COLLABORATIVE NEARSHORE SWELL AND WIND
FORECAST /SRDHFO/ FOR MORE DETAILS ON THE SWELLS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF WARNING UNTIL 6 PM HST THURSDAY FOR EAST FACING SHORES OF
KAUAI...OAHU...MOLOKAI...MAUI...AND THE BIG ISLAND.

SMALL CRAFT ADVISORY UNTIL 6 PM HST THURSDAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI CHANNEL-OAHU WINDWARD WATERS-
KAIWI CHANNEL-MAUI COUNTY WINDWARD WATERS-PAILOLO CHANNEL-ALENUIHAHA
CHANNEL-BIG ISLAND WINDWARD WATERS-BIG ISLAND SOUTHEAST WATERS.

&&

$$

DISCUSSION/MARINE...REYNES
AVIATION...BEDAL