Heavy Rain in Hawaii, September 21 to 24, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1010 AM HST MON SEP 21 2015

.SYNOPSIS...
MODERATE TO LOCALLY BREEZY TRADE WINDS WILL PREVAIL OVER THE NEXT
WEEK. A FAIRLY WET WEATHER PATTERN WILL CONTINUE THE NEXT COUPLE OF
DAYS...THANKS TO A NEARBY DISTURBANCE IN THE UPPER LEVELS OF THE
ATMOSPHERE. WHILE CLOUDS AND SHOWERS WILL PRIMARILY AFFECT WINDWARD
COASTS AND SLOPES...WINDS WILL BE SUFFICIENTLY STRONG TO PUSH
SHOWERS TO LEEWARD AREAS AT TIMES.

&&

.DISCUSSION...
SURFACE HIGH PRESSURE CENTERED FAR NNE OF THE ISLANDS WILL BE
REINFORCED BY A SECOND HIGH CURRENTLY CENTERED NEAR THE ALEUTIAN
ISLANDS OVER THE NEXT COUPLE OF DAYS...WITH THIS SECOND HIGH
BUILDING SE TOWARD THE END OF THE WEEK INTO NEXT WEEKEND. NET EFFECT
NEAR THE ISLANDS WILL BE THE MAINTENANCE OF A FAIRLY TIGHT LOW-LEVEL
PRESSURE GRADIENT...AND PERSISTENT MODERATE TO LOCALLY FRESH TRADE
WIND FLOW OVER THE AREA.

THE PERSISTENT TRADE WINDS WILL DELIVER PERIODS OF CLOUDS AND
SHOWERS TO THE ISLANDS THROUGH THE UPCOMING WEEK...WITH A
PARTICULARLY SHOWERY AREA AFFECTING KAUAI AND OAHU TODAY. TRADES
WILL BE STRONG ENOUGH AND SHOWERS TALL ENOUGH TO EASILY SPREAD
LEEWARD AT TIMES ON THE SMALLER ISLANDS...WHILE HEATING OF THE KONA
SLOPES WILL BRING AFTERNOON SHOWERS TO LEEWARD BIG ISLAND.

A CLOSED LOW ALOFT SEVERAL HUNDRED MILES NE OF THE ISLANDS WILL
TRACK NE AWAY FROM THE AREA OVER THE NEXT COUPLE OF DAYS...BUT AN
ASSOCIATED TROUGH ALOFT WILL LINGER OVER THE AREA INTO NEXT WEEKEND.
THE PRESENCE OF THE TROUGH WILL KEEP THE TRADE FLOW ON THE WETTER
SIDE...AND THE PROXIMITY OF THE LOW IS THE REASON WHY A FLASH FLOOD
WATCH IS IN EFFECT FOR MAUI COUNTY AND THE BIG ISLAND TODAY. CURRENT
TRENDS INDICATE THAT THE THREAT OF WIDESPREAD FLASH FLOODING IS
DIMINISHING OVER THE AREA AS UPSTREAM MOISTURE HAS DIMINISHED AND
SHOWERS ARE MOVING ALONG SMARTLY IN THE TRADE FLOW. HOWEVER...
LEEWARD SLOPES COULD STILL SEE HEAVY DOWNPOURS...AND POSSIBLY A
THUNDERSTORM...THIS AFTERNOON.

&&

.AVIATION...
HIGH PRESSURE WELL NORTH OF THE ISLANDS WILL KEEP A BREEZY EAST-
NORTHEAST TRADE WIND FLOW IN PLACE THROUGH TONIGHT...WITH CLOUDS AND
SHOWERS FOCUSED PRIMARILY ACROSS NORTH THRU EAST SECTIONS OF THE
ISLANDS. MVFR CIGS/VSBYS WILL BE POSSIBLE IN PASSING SHRA...WITH
PHTO AND PHLI THE MOST LIKELY TO BE IMPACTED BY REDUCED CIGS/VSBYS.
ADDITIONALLY...PHNY WILL LIKELY SEE MVFR CIGS THROUGH EARLY THIS
EVENING.

AIRMET SIERRA IS IN EFFECT FOR MOUNTAIN OBSCURATION ACROSS PORTIONS
OF OAHU...MAUI AND THE BIG ISLAND. AIRMET SIERRA FOR MOUNTAIN
OBSCURATION MAY BE NEEDED FOR THE OTHER ISLANDS LATER TODAY.

&&

.MARINE...
A SMALL CRAFT ADVISORY /SCA/ IS POSTED THROUGH TONIGHT FOR ALL
HAWAIIAN WATERS. WIND SPEEDS ARE EXPECTED TO BE CLOSE TO ADVISORY
LEVELS...AT LEAST IN THE WINDIER MARINE ZONES...FOR MUCH OF THE
UPCOMING WEEK. WILL WAIT AND SEE IF LATEST OBSERVATIONS AND GUIDANCE
SUPPORT MAINTAINING THE SCA FOR ALL ZONES.

A LONG-PERIOD S SWELL CURRENTLY MOVING THROUGH THE AREA HAS PROMPTED
A HIGH SURF ADVISORY FOR S FACING SHORES THROUGH TODAY. THE SWELL IS
EXPECTED TO PEAK TODAY AND THEN GRADUALLY DIMINISH. WHETHER OR NOT
AN EXTENSION IN TIME IS NECESSARY WILL BE DETERMINED BY SURF AND
BUOY OBSERVATIONS THAT ARRIVE DURING THE DAY. THE PERSISTENT TRADE
WINDS WILL DRIVE SHORT-PERIOD WIND WAVES TOWARD THE ISLANDS FROM THE
EAST...SUPPORTING MODERATE SURF ALONG E FACING SHORES INTO NEXT
WEEKEND. NO OTHER SIGNIFICANT SWELLS ARE EXPECTED.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 PM HST THIS EVENING FOR SOUTH FACING
SHORES OF ALL HAWAIIAN ISLANDS.

FLASH FLOOD WATCH UNTIL 6 PM HST THIS EVENING FOR MOLOKAI...LANAI...
KAHOOLAWE...MAUI...AND THE BIG ISLAND.

SMALL CRAFT ADVISORY UNTIL 6 AM HST TUESDAY FOR ALL HAWAIIAN
WATERS.

&&

DISCUSSION/MARINE...BIRCHARD
AVIATION...JELSEMA