High Surf in Hawaii, September 22 to 24, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST SAT SEP 22 2012

.SYNOPSIS...
STRENGTHENING TRADE WINDS WILL BRING AN INCREASE IN MOISTURE TONIGHT
AND SUNDAY. RAINFALL WILL BE FOCUSED ACROSS WINDWARD AND MAUKA AREAS
THOUGH BRIEF SHOWERS WILL PASS OVER LEEWARD AREAS BEGINNING TONIGHT.
A SLIGHTLY DRIER WEATHER PATTERN WILL SETTLE OVER THE ISLANDS ON
MONDAY AFTERNOON AS TRADE WINDS DROP SLIGHTLY BUT REMAIN AT MODERATE
STRENGTH.

&&

.DISCUSSION...
INCREASING SHOWERS ARE IN STORE TONIGHT. CONDITIONS ARE RATHER DRY
OVER THE ISLANDS THIS AFTERNOON...WITH PRECIPITABLE WATER VALUES
RUNNING AROUND 70 TO 80 PERCENT OF NORMAL AND FEW LOW CLOUDS IN THE
TRADE WIND FLOW. HOWEVER...A FIELD OF SHOWER-BEARING OPEN-CELLED
CUMULUS CLOUDS WILL MOVE IN FROM THE EAST THIS EVENING BRINGING A
BOOST IN SHOWERS. PRECIPITABLE WATER /PW/ VALUES IN THE CLOUD FIELD
ARE RUNNING AROUND SEPTEMBER NORMALS OF 1.4 INCHES OR SO...AND MID
LEVEL RIDGING OVER THE STATE WILL PRODUCE STABLE CONDITIONS. AS A
RESULT...SHOWERS WILL BE OF MODERATE INTENSITY AT BEST AND FOCUSED
OVER WINDWARD SLOPES WITH SOME BRIEF LIGHT RAINFALL SPREADING TO
LEEWARD SECTIONS OF THE SMALLER ISLANDS AT TIMES.

TRADE WINDS CONTINUE TO GRADUALLY STRENGTHEN OVER THE NEXT DAY OR
SO. THE PRESSURE GRADIENT BETWEEN THE BUOYS NORTH AND SOUTH OF THE
ISLANDS HAS INCREASED BY ABOUT 2.0 MB IN THE PAST 24 HOURS...AND
WINDS HAVE INCREASED. EXPECT WINDS INCREASE A SLIGHTLY OVER THE NEXT
24 TO 36 HOURS AS A FRONT SEVERAL HUNDRED MILES NORTH OF THE STATE
WASHES OUT AND A SMALL HIGH PRESSURE CELL PASSES ALONG 35N.

A FURTHER UPTICK IN SHOWERS IS EXPECTED SUNDAY NIGHT. PW VALUES
INCREASE TO 1.6 INCHES ALONG THE TRAILING EDGE OF THE ABOVE
MENTIONED CLOUD FIELD ABOUT 475 MILES EAST OF THE BIG ISLAND. AS
THIS FEATURE MOVES THROUGH SUNDAY NIGHT WE WILL SEE A BIT MORE
SHOWERS. THE ATMOSPHERE WILL REMAIN STABLE...AND WHILE SHOWERS WILL
REMAIN FOCUSED OVER WINDWARD SLOPES...STRONGER TRADE WINDS WILL
CARRY MORE SHOWERS TO LEEWARD AREAS. SHOWER COVERAGE WILL DIMINISH
BY MONDAY AFTERNOON...WITH THE GFS MODEL PREDICTING PW VALUES
DROPPING BELOW NORMAL AGAIN.

TRADE WINDS WILL DROP SLIGHTLY LATE MONDAY AND TUESDAY BUT WILL
REMAIN AT MODERATE STRENGTH THROUGH THE WORK WEEK. THE SUBTROPICAL
RIDGE DRIVING THE TRADES WILL START OUT NEAR 35N ON MONDAY AND WILL
THEN SLOWLY CREEP SOUTHWARD TO 30N BY FRIDAY. THE RIDGE WILL BE
PUSHED FARTHER SOUTH DURING THE WEEKEND...POSSIBLY CAUSING WINDS TO
SHIFT OUT OF THE EAST SOUTHEAST.

IN ADDITION...THE AUTUMNAL EQUINOX WAS AT 1449 UTC...OR 449 AM HST
THIS MORNING. THIS MARKS THE OFFICIAL END OF SUMMER IN THE NORTHERN
HEMISPHERE AND THE START OF AUTUMN.

&&

.AVIATION...
VFR CONDITIONS PREVAIL AT ALL TERMINALS AND THESE CONDITIONS ARE
EXPECTED TO CONTINUE THROUGH THE EVENING HOURS. SOME BRIEF MVFR
CONDITIONS MAY OCCUR AT TIMES OVER WINDWARD SLOPES DUE TO LOW CLOUDS
AND SHRA. AN AREA OF ENHANCED MOISTURE WILL REACH THE BIG ISLAND
EARLY TONIGHT AND SPREAD UP THE CHAIN OVERNIGHT AND ON INTO SUNDAY.
THIS WILL LIKELY CAUSE MORE WIDESPREAD MVFR CONDITIONS TO OCCUR OVER
WINDWARD AND MOUNTAIN AREAS WITH SOME LOCAL IFR CONDITIONS POSSIBLE
ESPECIALLY EAST MAUI AND WINDWARD BIG ISLAND.

&&

.MARINE...
THE FIRST SIZEABLE NORTHWEST SWELL OF THE SEASON IS BUILDING AT BUOY
51101. IN FACT...THE 3 PM OBSERVATION CAME IN AT 10.5 FT AT 16
SECONDS...WHICH IS OVER 2 FEET LARGER THAN WAVEWATCH III PREDICTIONS
FOR THIS TIME. THIS IS NOT MUCH OF A SURPRISE AS THE LOW THAT
GENERATED THE SWELL FEATURED A CAPTURED FETCH AND RECENT JASON
PASSES SHOWED SEAS RUNNING A COUPLE OF FEET HIGHER THAN MODEL
FORECASTS. WE UPPED THE NORTH SHORE SURF HEIGHTS A COUPLE OF FEET
AND HAVE ADDED THE WEST SHORE OF OAHU TO THE HIGH SURF ADVISORY.
EXPECT THE HIGH SURF TO PEAK ON SUNDAY AND GRADUALLY DECLINE SUNDAY
NIGHT THROUGH TUESDAY.

TRADES WILL SLOWLY INCREASE THROUGH THE DAY ON SUNDAY. THE PRESSURE
GRADIENT BETWEEN THE BUOYS NORTH AND SOUTH OF THE ISLANDS HAS
INCREASED BY 2.0 IN THE PAST 24 HOURS...EVEN THOUGH A MIDDAY ASCAT
PASS FAILED TO FIND ANY 25 KT WINDS. REGARDLESS...TRADES WILL
CONTINUE TO INCREASE...AND THE SMALL CRAFT ADVISORY /SCA/ HAS BEEN
EXPANDED TO COVER THE TYPICALLY WINDY WATERS AND CHANNELS AROUND THE
BIG ISLAND AND MAUI.

IN ADDITION...THE LARGE BUILDING NW SWELL WILL HELP TO PRODUCE
HAZARDOUS SEAS OF 10 FT OR MORE ACROSS MOST AREAS LATER
TONIGHT...AND THE SCA HAS BEEN EXPANDED TO ALL WATERS THROUGH
SUNDAY NIGHT FOR THE COMBINATION OF WINDS AND SEAS.

THE SOUTH SWELL THAT IS CURRENTLY IMPACTING THE STATE WILL GRADUALLY
FADE THIS WEEKEND. A SERIES OF MODERATE SIZED SOUTHWEST TO SOUTH
SOUTHWEST SWELLS WILL AFFECT SOUTH FACING SHORES OF THE STATE NEXT
WEEK. THE LARGEST OF THESE IS EXPECTED TO ARRIVE NEXT WEDNESDAY
NIGHT...PEAK ON THURSDAY...THEN GRADUALLY DIMINISH ON FRIDAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY FOR MAALAEA BAY...THE WATERS SOUTH OF THE BIG
ISLAND...AND THE PAILOLO AND ALENUIHAHA CHANNEL THROUGH 600 AM HST
SUNDAY.

HIGH SURF ADVISORY FOR NORTH FACING SHORES OF KAUAI...OAHU...
MOLOKAI...MAUI...AND NIIHAU AND FOR WEST FACING SHORES OF KAUAI AND
OAHU THROUGH 6 AM HST MONDAY MORNING.

&&

DISCUSSION...WROE
AVIATION...BURKE
MARINE...WROE