High Surf in Hawaii, September 21 to 22, 2015

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

.SYNOPSIS...
FRESH TRADES WILL KEEP BRINGING MAINLY WINDWARD AND MAUKA SHOWERS
THROUGH TONIGHT WITH THE MOST ABUNDANT AND HEAVIEST RAINFALL
EXPECTED OVER THE EASTERN HALF OF THE ISLAND CHAIN. SOME SHOWERS
MAY REACH LEEWARD LOCATIONS OF THE SMALLER ISLANDS AT TIMES. THE
WET TRADE WIND PATTERN MAY CONTINUE DURING THE SECOND HALF OF THE
WORK WEEK AS ANOTHER AREA OF ENHANCED MOISTURE APPROACHES THE
ALOHA STATE FROM THE EAST.

&&

.DISCUSSION...
A BROAD SURFACE HIGH PRESSURE SYSTEM REMAINS CENTERED FAR NORTH OF
THE AREA WHICH IS DRIVING FRESH TRADE WINDS. RADAR HAS BEEN
SHOWING PERSISTENT RAIN PUSHING IN WITH THE TRADES ACROSS MAUI AND
THE BIG ISLAND OVERNIGHT. THE 12Z SOUNDING FROM HILO SHOWED PWATS
OF 2.2 INCHES...ALONG WITH A SATURATED VERTICAL PROFILE THROUGH 20
THOUSAND FEET. THESE SHOWERS ARE BEING ENHANCED BY AN UPPER LEVEL
LOW...EMBEDDED WITHIN A TROUGH...TO THE EAST OF THE STATE. THESE
SHOWERS ARE DROPPING HEAVY RAIN AS THEY INTERACT WITH THE ISLAND
TERRAIN ACROSS THE WINDWARD AND SOUTHEASTERN SLOPES OVER THE BIG
ISLAND AND MAUI. INSTANTANEOUS RAINFALL RATES OBSERVED FROM RADAR
SHOWS 1-2 INCHES PER HOUR.

MEANWHILE...NUMERICAL MODELS SHOW THE PERSISTENT RAIN COMING IN
WITH THE TRADE WINDS TO IMPACT WINDWARD MAUI AND BIG ISLAND
THROUGH THIS EVENING. LOCALLY HEAVY SHOWERS ARE IN THE FORECAST
FOR MAUI COUNTY AND BIG ISLAND THROUGH THEN...ALONG WITH A FLASH
FLOOD WATCH THROUGH 6 PM TODAY. A SLIGHT CHANCE OF THUNDERSTORMS
IS ALSO IN THE FORECAST FOR THIS AFTERNOON OVER MAUI AND THE BIG
ISLAND.

SHOWERS ARE LIKELY TO PICK UP FOR THE WINDWARD SIDES OF
KAUAI...OAHU AND MOLOKAI TODAY AS WINDS CARRY THIS MOISTURE
WESTWARD. HOWEVER...THE BULK OF THE RAINFALL WILL STAY OVER THE
EASTERN HALF OF THE ISLAND CHAIN ACCORDING TO MODELS. LEEWARD
SIDES OF THE SMALLER ISLANDS ARE LIKELY TO SEE SOME PASSING
SHOWERS OVER THE NEXT 24 HOURS.

FOR TUESDAY INTO THE SECOND HALF OF THE WORK WEEK...MODELS SHOW
THE AFOREMENTIONED UPPER LOW MIGRATING TO THE NORTHEAST AND
FURTHER AWAY FROM THE AREA...ALLOWING FOR THE SURFACE HIGH FAR
NORTH OF THE ISLANDS TO STRENGTHEN AND THE TRADE WINDS BLOWING.
THE MOVEMENT OF THE LOW AWAY FROM THE STATE WILL RESULT IN A MORE
STABLE ATMOSPHERE...WHILE RELATIVELY DRIER AIR MAKES IT/S WAY
TOWARDS HAWAII. THUS...SHOWERS WILL BECOME DIFFUSE AND LIGHTER
TUESDAY AND WEDNESDAY...BUT WILL CONTINUE TO FAVOR THE WINDWARD
AND MAUKA AREAS. SHOWER ACTIVITY MAY PICK LATE IN THE WEEK AND
THIS WEEKEND AS A ADDITIONAL MOISTURE BEGINS TO FUNNEL IN WITH
THE TRADE WINDS.

&&

.AVIATION...
AIRMET SIERRA IS IN EFFECT FOR MOUNTAIN OBSCURATION ACROSS NE
SECTIONS OF MAUI AND THE BIG ISLAND. CLD AND SHRA N OF A
TROPICAL DISTURBANCE WILL PRODUCE FREQUENT MVFR COND AND ISOL
IFR COND OVER NE SECTIONS OF THE BIG ISLAND OF HAWAII AND MAUI
THROUGH MONDAY INCLUDING PHTO AND PHHN. VFR COND PREVAIL OVER
OAHU AND KAUAI THIS EVENING BUT MVFR COND COULD SPREAD TO THOSE
ISLANDS ON MONDAY. A SFC HIGH FAR N OF THE AREA WILL MAINTAIN
LOCALLY BREEZY ENE TRADE WINDS THROUGH MONDAY.

&&

.MARINE...
SCATTEROMETER SHOWED A BOOST IN THE TRADE WINDS AND HAS PROMPTED
EXPANSION OF THE SMALL CRAFT ADVISORY /SCA/ AREA TO ALL COASTAL
WATERS THROUGH TONIGHT. WINDS ACROSS THE STATE WILL REMAIN
BORDERLINE SCA CONDITIONS THROUGH THE WEEK...AND SOME OR ALL ZONES
MAY BE EXTENDED BEYOND TONIGHT.

A NEW LONG PERIOD SOUTH SOUTHWEST SWELL WILL PEAK THIS MORNING.
A HIGH SURF ADVISORY IS NOW IN EFFECT FOR SOUTH FACING SHORES AS
THE BARBER/S POINT BUOY HAS BEEN SHOWING EVIDENCE OF LIKELY HIGH
SURF. SURF ALONG SOUTH FACING SHORES SHOULD FALL BELOW ADVISORY
CRITERIA BY THIS EVENING.

THE TRADE WINDS WILL MAINTAIN ROUGH SURF ALONG EAST FACING
SHORES. THE SURF ALONG EAST FACING SHORES IS EXPECTED TO
APPROACH...BUT REMAIN BELOW THE ADVISORY THRESHOLD TODAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 PM HST THIS EVENING FOR SOUTH FACING
SHORES OF NIIHAU...KAUAI...OAHU...MOLOKAI...LANAI...KAHOOLAWE...MAUI
AND BIG ISLAND.

FLASH FLOOD WATCH THROUGH THIS AFTERNOON FOR
MOLOKAI...LANAI...KAHOOLAWE...MAUI...BIG ISLAND.

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

&&

DISCUSSION/MARINE...FOSTER
AVIATION...DONALDSON