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Hawaiian Islands Synoptic Discussion and Guidance

923
FXHW60 PHFO 271430
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST THU AUG 27 2015

.SYNOPSIS...
AFTER LITTLE SHOWER ACTIVITY AND DIMINISHED CLOUD COVER EARLY THIS
MORNING...LOCALLY HEAVY SHOWERS AND ISOLATED THUNDERSTORMS ARE ONCE
AGAIN EXPECTED TO DEVELOP LATE IN THE MORNING AND THE AFTERNOON WITH
FLASH FLOODING POSSIBLE. WEAK TRADE WINDS WILL BEGIN TO BUILD ON
FRIDAY...AND ALTHOUGH SHOWERS WILL FAVOR WINDWARD AND MAUKA AREAS...
AFTERNOON CLOUDS AND SPOTTY SHOWERS CAN BE EXPECTED OVER LEEWARD
AREAS. HURRICANE IGNACIO WILL APPROACH THE STATE LATE IN THE WEEKEND
AND MAY THREATEN THE ISLANDS DURING THE FIRST PART OF NEXT WEEK.

&&

.DISCUSSION...
ALTHOUGH SHOWER ACTIVITY HAS BEEN MINIMAL THROUGH MOST OF THE
NIGHT...MOIST AND UNSTABLE CONDITIONS PERSIST. THE 12Z SOUNDINGS
SHOW VERY MOIST CONDITIONS UP TO 20000 FT WITH VERY HIGH
PRECIPITABLE WATER VALUES RANGING FROM ABOUT 1.8 INCHES AT HILO TO
NEARLY 2 INCHES AT LIHUE. THERE IS ALSO NO SUBSIDENCE INVERSION DUE
TO A NEARLY STATIONARY MID TO UPPER LEVEL TROUGH OVER AND NORTH OF
THE ISLANDS. THE TRIGGER FOR HEAVY SHOWERS AND THUNDERSTORMS HAS
MAINLY BEEN INTERACTION WITH ISLAND TERRAIN. WITH BACKGROUND WINDS
LIGHT AND VARIABLE THIS MORNING DUE TO A TROUGH STALLED TO THE
NORTH...EXPECT LITTLE SHOWER ACTIVITY AND MINIMAL CLOUD COVER EARLY
THIS MORNING...ALONG WITH VERY HUMID CONDITIONS AS DEW POINTS
LINGER IN THE MID 70S.

THIS AFTERNOON SHOULD MORE OR LESS BE A REPEAT OF YESTERDAY. THE
SURFACE TROUGH STALLED A COUPLE HUNDRED MILES NORTH OF THE ISLANDS
WILL KEEP WINDS LIGHT AND VARIABLE...ESPECIALLY ACROSS THE WESTERN
END OF THE STATE...AND CONDITIONS WILL REMAIN MOIST AND UNSTABLE.
EXPECT ANOTHER ROUND OF HEAVY SHOWERS AND THUNDERSTORMS TO GET
UNDERWAY AS SEA BREEZES KICK IN LATE THIS MORNING THEN PERSIST
THROUGH THE AFTERNOON. A FLASH FLOOD WATCH REMAINS IN EFFECT THROUGH
THE AFTERNOON...AND WITH THE GROUND ALREADY SATURATED...FLOODING
COULD HAPPEN QUICKLY. SHOWERS WILL ONCE AGAIN TAPER OFF DURING THE
EVENING.

ON FRIDAY LIGHT TRADE WINDS WILL GRADUALLY REBUILD AS THE SURFACE
TROUGH TO THE NORTH FILLS...ALLOWING HIGH PRESSURE TO THE NORTH TO
EXERT A GREATER INFLUENCE. SHOWER WILL FAVOR WINDWARD SLOPES...BUT
WITH MID TO UPPER LEVEL TROUGHING HANGING AROUND OVER AND JUST NORTH
OF THE STATE...CLOUDS AND SPOTTY AFTERNOON SHOWERS WILL BE POSSIBLE
ACROSS LEEWARD AREAS.

FORECAST UNCERTAINTY INCREASES SIGNIFICANTLY ON MONDAY AND
TUESDAY...HINGING ON THE EVENTUAL TRACK OF HURRICANE IGNACIO.
HURRICANE IGNACIO WILL APPROACH THE STATE LATE IN THE WEEKEND AND
MAY THREATEN THE ISLANDS EARLY NEXT WEEK. WITH FORECAST TRACK ERRORS
OUT AT THAT TIME RANGE MORE THAN A COUPLE HUNDRED MILES...IT IS TOO
EARLY TO TELL WHAT THE DIRECT IMPACTS WILL BE. EVERYONE IN THE STATE
SHOULD MONITOR THE PROGRESS OF IGNACIO DURING THE NEXT SEVERAL
DAYS...AND THE NATIONAL WEATHER SERVICE URGES INDIVIDUALS TO HAVE AN
ACTION PLAN IN CASE IGNACIO...OR ANY HURRICANE...THREATENS.

&&

.AVIATION...
VFR CONDITIONS ARE EXPECTED TO PREVAIL ACROSS THE ISLANDS THROUGH
AT LEAST THE EARLY MORNING HOURS. ANTICIPATE A BUILD UP OF CLOUDS
AND SHOWERS WITH DAYTIME HEATING...THAT COULD BRING MVFR AND IFR
CONDITIONS. NO AIRMETS AT THIS TIME...BUT THAT COULD CHANGE
DEPENDING ON THE AMOUNT OF BUILDS UPS TODAY. ANTICIPATE CLEARING
AGAIN THIS EVENING/TONIGHT.

&&

.MARINE...
ASIDE FROM A LIGHT CHANCE OF THUNDERSTORMS TODAY...THE MAIN CONCERN
WILL BE A MODERATE WEST TO WEST-NORTHWEST SWELL DURING THE NEXT
SEVERAL DAYS. THE HANALEI BUOY HAS BEEN PICKING UP AN INCREASE IN
LONG PERIOD ENERGY OVERNIGHT. WHILE THE IMPACTS OF THIS SWELL WILL
BE MODEST ON MOST ISLANDS...WE WILL HAVE TO MONITOR THE BUOYS
TODAY...SINCE ADVISORY LEVEL SURF COULD DEVELOP ALONG WEST FACING
SHORES OF THE BIG ISLAND AS EARLY AS TONIGHT AND LAST THROUGH
SATURDAY.

UNCERTAINTY IS VERY HIGH REGARDING THE POTENTIAL IMPACTS ON THE
STATE FROM HURRICANE IGNACIO EARLY NEXT WEEK...BUT WE ARE CONFIDENT
THAT AN EAST-SOUTHEAST WILL ARRIVE ON SATURDAY. THIS SWELL WILL
MAINLY IMPACT THE BIG ISLAND AND MAUI...WHERE ADVISORY LEVEL SURF IS
EXPECTED ALONG EAST FACING SHORES ON SATURDAY THEN LIKELY RISE TO
WARNING LEVELS ON SUNDAY. SOME OF THIS SWELL ENERGY WILL EVENTUALLY
IMPACT THE REST OF THE STATE.

WINDS AND SEAS WILL REMAIN BELOW SMALL CRAFT ADVISORY LEVELS THROUGH
SATURDAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH IN EFFECT THROUGH 6 PM HST THIS AFTERNOON FOR
NIIHAU KAUAI OAHU MOLOKAI LANAI MAUI AND THE BIG ISLAND.

&&

$$

DISCUSSION AND MARINE...WROE
AVIATION...MBALLARD