539  
FXHW52 PHFO 210039  
SRDHFO  
 
COLLABORATIVE NEARSHORE SWELL AND WIND FORECAST FOR OAHU  
NWS/NCEI HONOLULU HI  
231 PM HST WED SEP 20 2017  
 
THIS COLLABORATIVE FORECAST WILL BE UPDATED MONDAY THROUGH FRIDAY AT  
300 PM WHEN PAT CALDWELL IS AVAILABLE.  
 
FORECAST SWL DMNT DMNT H H HGT WIND WIND SPD  
DATE HGT DIR PD 1/3 1/10 TEND PROB SPD DIR TEND  
 
1 PM 2 WNW 14 3 5 UP 11-16 E DOWN  
09/20 7 E 8 4 6 SAME  
2 SE 9 2 3 SAME  
 
THU 3 WNW 12 3 5 SAME LOW 11-16 E SAME  
09/21 6 E 8 3 5 DOWN LOW  
2 SE 9 2 3 SAME LOW  
 
FRI 3 NW 12 3 5 SAME LOW 11-16 E DOWN  
09/22 5 ENE 8 3 4 DOWN LOW  
2 SE 9 2 3 SAME LOW  
 
SAT 3 NW 11 3 5 DOWN LOW 9-13 E DOWN  
09/23 4 ENE 8 2 4 DOWN LOW  
2 SE 9 2 3 SAME LOW  
 
SUN 3 NNW 11 3 5 SAME LOW 9-13 E SAME  
09/24 3 NE 8 2 3 SAME LOW  
2 SW 15 3 4 UP LOW  
 
MON 2 NNW 11 3 4 DOWN LOW 9-13 E SAME  
09/25 3 NE 8 2 3 DOWN LOW  
2 SW 13 3 4 DOWN LOW  
 
LEGEND:  
SWL HGT OPEN OCEAN SWELL HEIGHT MEASURED FROM TROUGH TO CREST  
IN FEET LOCATED 20 NAUTICAL MILES OFFSHORE  
DMNT DIR DOMINANT DIRECTION TYPICALLY +/-10 DEGREES IN 16 COMPASS  
POINTS  
DMNT PD DOMINANT PERIOD IN SECONDS  
H1/3 SIGNIFICANT WAVE HEIGHT IN THE SURF ZONE  
H1/10 AVERAGE HEIGHT IN THE HIGHEST ONE-TENTH WAVES IN THE SURF  
ZONE  
HGT TEND HEIGHT TENDENCY OF SWELL (VALID VALUES: UP/DOWN/SAME)  
PROB PROBABILITY OF OCCURRENCE (VALID VALUES: HIGH/MED/LOW)  
WIND SPD OPEN WATER WIND SPEED MEASURED IN KNOTS LOCATED  
20 NAUTICAL MILES OFFSHORE  
WIND DIR WIND DIRECTION IN 16 COMPASS POINTS  
SPD TEND WIND SPEED TENDENCY (VALID VALUES: UP/DOWN/SAME)  
 
SURF HEIGHTS WILL VARY BETWEEN DIFFERENT BEACHES AND AT THE SAME  
BEACH AT DIFFERENT BREAK AREAS.  
 
DISCUSSION:  
SUMMARY...  
BELOW AVERAGE, SHORTER-PERIOD SURF.  
 
DETAILED...  
MID WEDNESDAY ON NORTHERN SHORES HAS BREAKERS FROM 305-315 DEGREES  
BELOW THE SEPTEMBER AVERAGE. SIMILAR SURF IS PREDICTED FOR THURSDAY.  
 
A COMPACT GALE MOVED ENE FROM THE KURIL ISLANDS 9/16 AND OCCLUDED  
NEAR 45N, 165E 9/17. NOAA NW HAWAII BUOYS PICKED UP THE LOW,  
MODERATE-PERIOD SWELL OF 14-15 SECONDS LATE 9/19. THE MID DAY  
9/20 DATA SHOW THOSE WAVE PERIOD BANDS DROPPING IN ENERGY AS A  
RISE BEGINS IN THE 11-13 SECOND BANDS. THE REST OF THIS EVENT  
SHOULD BE OF SHORTER-WAVE PERIODS. THE PACIOOS WAIMEA BUOY DID  
SHOW IN INCREASE EARLY 9/20 WITH AN INCREASE IN BREAKERS FOR WNW  
EXPOSURES.  
 
ONCE THE SYSTEM BECAME OCCLUDED, THE SURFACE WINDS WEAKENED AS THE  
LOW CENTER MOVED TO THE SSE. A LONG FETCH FROM NORTH TO SOUTH TO  
THE WEST OF THE DATELINE AIMED TOWARD THE MARSHALL ISLANDS WITH A  
SHORT FETCH OF NEAR GALES AIMED AT HAWAII. THE LATTER FETCH AREA  
NOSED TO NEAR 1500 NM AWAY 9/18.  
 
SHORTER-PERIOD SWELL OF 11-13 SECONDS SHOULD KEEP SMALL BREAKERS  
FROM 305-315 DEGREES ON THURSDAY INTO FRIDAY.  
 
THE LOW CENTER NEAR 35N ON THE DATELINE BEGAN MOVING NNE 9/19 WITH  
THE FETCH NOSING CLOSE TO NEAR 175W OR ABOUT 1200 NM AWAY 9/20.  
THIS COULD KEEP THE SURF ABOUT THE SAME FRIDAY WITH A SECONDARY  
MAXIMUM FRIDAY NIGHT FROM 310-320 DEGREES, SLOWLY TRENDING DOWN  
SATURDAY.  
 
A NEW LOW CENTER FORMED FURTHER NORTH 9/19 NEAR 45N ON THE  
DATELINE WITH A NE TRACK INTO 9/20. STRONGEST WINDS AIMED NORTH TO  
SOUTH TO THE WEST OF HAWAII. THE SHORTER FETCH AIMED AT HAWAII  
COULD ADD SMALL, SHORTER-PERIOD SWELL SUNDAY FROM 315-330 DEGREES.  
IT SHOULD DROP ON MONDAY.  
 
MID WEDNESDAY ON EASTERN SHORES HAS BREAKERS FROM 70-90 DEGREES  
NEAR THE TRADE WINDSWELL AVERAGE. SIMILAR SURF IS LIKELY THURSDAY.  
 
A LARGE FETCH OF MODERATE TO FRESH TRADES WITH POCKETS TO STRONG  
TO THE E TO NE OF HAWAII OUT 1000 NM 9/17-19 SET UP A LONG  
CONVEYOR BELT OF WINDSWELL AIMED AT HAWAII. THIS SHOULD KEEP  
ACTIVE SURF NEAR THE AVERAGE THURSDAY WITH A DOWNWARD TREND INTO  
FRIDAY.  
 
AN UPPER LEVEL LOW PRESSURE NEAR 25N TO THE IMMEDIATE ENE OF  
HAWAII 9/20 HAS STARTED TO WEAKEN THE SURFACE PRESSURE GRADIENT  
AND THUS OCEAN SURFACE WIND SPEEDS FOR THE TRADES CLOSER TO  
HAWAII ON THE SW SIDE OF THE SURFACE HIGH PRESSURE CENTER AT 40N,  
150W. ASCAT SATELLITE 9/20 SHOWS A RIBBON OF FRESH TO STRONG  
TRADES NORTH OF THE INFLUENCE OF THE UPPER LOW NEAR 30N, 145W.  
REMOTE TRADES BEYOND 800 NM ENE TO NE OF HAWAII ARE EXPECTED TO  
REMAIN ABOUT THE SAME A FEW MORE DAYS. THE REMOTE TRADES COULD  
KEEP SMALL BREAKERS FROM WINDSWELL 9/23-25 FROM 40-70 DEGREES.  
 
MID WEDNESDAY ON SOUTHERN SHORES HAS SHORT-PERIOD BREAKERS FROM  
SOUTHERN HEMISPHERE TRADE WINDS AS THE DOMINANT BACKGROUND SURF.  
MORE OF THE SAME IS LIKELY THROUGH SATURDAY.  
 
A COMPACT SEVERE GALE IN THE SOUTHERN TASMAN SEA 9/15-16 WEAKENED  
AS IT MOVED ENE. THE PACIOOS AMERICAN SAMOA BUOY 9/19-20 SHOWS  
LOW, MODERATE- TO LONG-PERIOD SWELL FILLING IN FROM 205-220  
DEGREES. THIS EVENT SHOULD ARRIVE LOCALLY LATE SATURDAY, PEAK  
SUNDAY NEAR TO A NOTCH BELOW THE SUMMER AVERAGE, AND DROP ON  
MONDAY. IT SHOULD NOT BE AS HIGH AS THE RECENT TASMAN EVENT  
LOCALLY OF 9/16-18.  
 
INTO THE LONG RANGE...THE TASMAN SEA HAS A COMPACT GALE 9/19-20 IN  
ABOUT THE SAME PLACE WEST OF NEW ZEALAND. IT SHOULD BE THE  
DOMINANT BACKGROUND SURF 9/26-28 FROM 208-220 DEGREES.  
 
MODELS SHOW A ZONAL JET STREAM 9/22-26 OVER THE CENTRAL NORTH  
PACIFIC WITH WEAK SURFACE LOW PRESSURE SYSTEMS MOVING EAST ALONG  
40N. THIS PATTERN SHOULD RESULT IN BELOW SEPTEMBER AVERAGE SURF  
LOCALLY 9/26-28 FROM WNW TO N.  
 
TRADE WINDSWELL SHOULD BE AT A MINIMUM 9/26-28.  
 
LONG RANGE FORECASTS ARE SUBJECT TO HIGH UNCERTAINTY.  
 
THIS COLLABORATIVE FORECAST WILL RESUME ON FRIDAY, SEPTEMBER 22.  
 
THIS FORECAST WAS PRODUCED THROUGH THE COLLABORATIVE EFFORTS OF  
NWS AND NCEI. PLEASE SEND SUGGESTIONS TO W-HFO.WEBMASTER@NOAA.GOV  
OR CALL THE WARNING COORDINATION METEOROLOGIST AT 808-973-5275.  
 
ADDITIONAL RESOURCES:  
SEE HTTP://WWW.PRH.NOAA.GOV/HNL/PAGES/MARINE.PHP  
 

 
 
NWS FORECASTER AND NCEI PAT CALDWELL  
 
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