Flash Flooding and Heavy Rain in Hawaii, September 15 to 21, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
350 PM HST TUE SEP 15 2015

.SYNOPSIS...
MODERATE TO LOCALLY STRONG TRADE WINDS WILL PREVAIL THROUGH NEXT
WEEKEND...BRINGING PERIODS OF CLOUDS AND SHOWERS TO THE ISLANDS.
FEWER SHOWERS ARE EXPECTED OVER THE SMALLER ISLANDS ON WEDNESDAY...
AFTER A BATCH OF MOISTURE GRADUALLY MOVES WEST OVERNIGHT. A
DIFFERENT BATCH OF MOISTURE IS EXPECTED TO MOVE OVER WINDWARD BIG
ISLAND TONIGHT THROUGH WEDNESDAY...KEEPING CONDITIONS WET. ALTHOUGH
SHOWERS WILL TEND TO FAVOR WINDWARD AREAS...THE WINDS WILL BE STRONG
ENOUGH TO PUSH THEM TO LEEWARD AREAS AT TIMES...WHILE DAYTIME
HEATING WILL TRIGGER AFTERNOON SHOWERS OVER THE BIG ISLAND LEEWARD
SLOPES AND COAST. ANOTHER INCREASE IN SHOWERS IS POSSIBLE THIS
WEEKEND AS AN UPPER LEVEL DISTURBANCE MOVES TOWARD THE ISLANDS FROM
THE NORTHEAST.

&&

.DISCUSSION...
LITTLE SIGNIFICANT CHANGE TO THE ONGOING FORECAST PHILOSOPHY...
WITH TRADE WIND WEATHER GENERALLY EXPECTED TO PREVAIL
THROUGH THE FORECAST PERIOD. SURFACE HIGH PRESSURE CENTERED FAR
NORTH OF THE ISLANDS WILL BUILD SOUTH...AND THEN SOUTHEAST...THROUGH
THE WEEKEND. THIS WILL BE THE PRIMARY FEATURE DRIVING WHAT IS
EXPECTED TO BE A MODERATE TO LOCALLY STRONG LOW-LEVEL TRADE WIND
FLOW OVER THE ISLANDS. WINDS WILL INITIALLY FAVOR AN ENE
DIRECTION...THEN EVENTUALLY TURN TO THE E AND ESE THROUGH THE
WEEKEND AS THE HIGH MOVES TO A POSITION NE OF THE ISLANDS...AND
DEVELOPING LOW PRESSURE POTENTIALLY DEEPENS TO THE SW AND W OF THE
AREA.

A COUPLE OF ENHANCED MOISTURE AREAS WILL AFFECT THE SHORT TERM
FORECAST...WITH ONE PARTICULARLY SHOWERY AREA MOVING THROUGH THE
SMALLER ISLANDS BECOMING A LITTLE LESS DEFINED SINCE THIS MORNING.
THIS MOISTURE IS EXPECTED TO PUSH W OF THE SMALLER ISLANDS THROUGH
TONIGHT...WITH MOST MODELS DEPICTING MUCH DRIER WEATHER MOVING OVER
THE SMALLER ISLANDS ON WEDNESDAY. WHILE WINDWARD AREAS WILL ABSORB
MOST OF THE RAINFALL...SOME OF THE SHOWERS WILL CONTINUE TO SPREAD
OVER LEEWARD SECTIONS.

ALTHOUGH A DRYING TREND IS EXPECTED OVER THE SMALLER ISLANDS BY
WEDNESDAY...AN AREA OF INCREASED MOISTURE LURKING E OF WINDWARD BIG
ISLAND IS EXPECTED TO FUEL SHOWERS OVER THE BIG ISLAND TONIGHT
THROUGH WEDNESDAY. A TROUGH AXIS ALOFT EXTENDING OVER THE AREA FROM
THE NE WILL MAINTAIN AN UNSTABLE AIR MASS...WITH THUNDERSTORMS
OBSERVED OVER THE BIG ISLAND SLOPES AS OF PACKAGE TIME. GIVEN THIS...
ISOLATED THUNDERSTORMS HAVE BEEN ADDED TO THE BIG ISLAND FORECAST
THROUGH THIS EVENING. TRADE WIND WEATHER WILL OTHERWISE PERSIST INTO
THE WEEKEND...BRINGING PASSING CLOUDS AND SHOWERS TO WINDWARD
AREAS...WITH NO ORGANIZED SHOWER AREAS EXPECTED.

WINDS MAY TURN TOWARD THE ESE LATE THIS WEEKEND AS THE HIGH
RELOCATES TO A POSITION NE OF THE ISLANDS...AND A LOW POTENTIALLY
DEVELOPS W OF THE AREA. AT THE SAME TIME...A CLOSED LOW ALOFT WILL
MOVE TOWARD THE ISLANDS FROM THE NE. WHILE THE BULK OF THE
INSTABILITY ASSOCIATED WITH THE LOW IS EXPECTED TO REMAIN OFFSHORE
TO THE NE OF THE ISLANDS...THE CLOSE PROXIMITY OF THE LOW WILL
LIKELY LEAD TO AN INCREASE IN TRADE WIND SHOWERS...SOME OF WHICH MAY
BECOME HEAVY.

&&

.AVIATION...
MODERATE TO LOCALLY GUSTY TRADE WINDS WILL CONTINUE THROUGH THE NEXT
FEW PERIODS. LOW LEVEL CLOUDS AND SHOWERS RIDING IN THE THE TRADES
WILL PRODUCE PROLONGED PERIODS OF MVFR TO POSSIBLE ISOL IFR
CONDITIONS...MAINLY ALONG WINDWARD COASTS THIS EVENING AND TONIGHT.
A DECREASE IN LOW LEVEL MOISTURE IS FORECAST OVER THE SMALLER
ISLANDS FOR WEDNESDAY.

THUNDERSTORMS WERE ABLE TO GET GOING THIS AFTERNOON OVER THE KAU AND
SOUTH KONA DISTRICTS ON THE BIG ISLAND. THESE SHOULD START TO
DIMINISH THIS EVENING. A VERY WEAK TO NO INVERSION IS EXPECTED TO
REMAIN OVER THE NEXT FEW DAYS.

AIRMET SIERRA FOR MOUNTAIN OBSC IS POSTED OVER NORTH THROUGH EAST
SECTIONS ON THE SMALLER ISLANDS AND ALL OF THE BIG ISLAND. WINDWARD
AREA WILL LIKELY CARRY THIS AIRMET INTO THE NIGHT...WHILE LEEWARD
BIG ISLAND AREA SHOULD START TO SEE IMPROVEMENT THIS EVENING.

AIRMET TANGO FOR LOW LEVEL TURB HAS BEEN CANCELLED. ALTHOUGH WINDS
REMAIN ELEVATED...THE LACK OF AN INVERSION AND NO REPORTS FROM
AIRLINE DISPATCHERS IS THE REASONING BEHIND THIS.

&&

.MARINE...
A SMALL CRAFT ADVISORY /SCA/ IS IN EFFECT THROUGH LATE
THIS AFTERNOON FOR ALL HAWAIIAN WATERS...EXCEPT THE WINDWARD BIG
ISLAND WATERS...DUE TO STRONG TRADE WINDS. WHILE THE SCA CURRENTLY
WILL EXPIRE FOR MOST AREAS THIS EVENING...IT HAS BEEN EXTENDED
THROUGH WEDNESDAY FOR THE WINDIER ZONES AROUND MAUI AND THE BIG
ISLAND. THESE ZONES MAY REQUIRE A FURTHER EXTENSION IN TIME AS WINDS
WILL BE CLOSE TO CRITERIA.

A MODERATE SSW SWELL IS EXPECTED TO CONTINUE THROUGH THURSDAY...
MAINTAINING MODERATE SURF ALONG S FACING SHORES. A SLIGHTLY LARGER
SSW SWELL ON FRIDAY IS EXPECTED TO CAUSE SURF ALONG S FACING SHORES
TO APPROACH THE HIGH SURF ADVISORY THRESHOLD BEFORE DIMINISHING OVER
THE WEEKEND. TRADE WINDS WILL GENERATE MODERATE BUT CHOPPY SURF
ALONG E FACING SHORES OVER THE NEXT SEVERAL DAYS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 PM HST WEDNESDAY FOR MAALAEA BAY-PAILOLO
CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG ISLAND
SOUTHEAST WATERS.

SMALL CRAFT ADVISORY UNTIL 6 PM HST THIS EVENING FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL-OAHU
WINDWARD WATERS-OAHU LEEWARD WATERS-KAIWI CHANNEL-MAUI COUNTY
WINDWARD WATERS-MAUI COUNTY LEEWARD WATERS.

&&

DISCUSSION/MARINE...BIRCHARD
AVIATION...EATON