068  
FXHW52 PHFO 260100  
SRDHFO  
 
COLLABORATIVE NEARSHORE SWELL AND WIND FORECAST FOR OAHU  
NWS/NCEI HONOLULU HI  
300 PM HST WED APR 25 2018  
 
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 NNW 10 2 3 DOWN 17-21 ENE SAME  
04/25 8 ENE 8 4 6 SAME  
1 SW 16 2 3 UP  
 
THU 2 NW 16 3 5 UP LOW 11-16 E DOWN  
04/26 6 ENE 8 3 5 DOWN MED  
2 SW 14 3 4 SAME LOW  
 
FRI 5 NNW 13 8 10 UP LOW 9-13 SW UP  
04/27 4 ENE 8 2 4 DOWN LOW  
2 SW 13 2 4 DOWN LOW  
 
SAT 4 NNW 11 6 8 DOWN LOW 11-16 N UP  
04/28 8 NNW 11 6 10 UP LOW  
2 NE 10 2 3 SAME LOW  
2 SW 12 2 3 DOWN LOW  
 
SUN 12 N 12 14 18 UP LOW 7-10 N SAME  
04/29 2 SSW 15 3 5 UP LOW  
 
MON 10 N 12 12 16 DOWN LOW 7-10 N SAME  
04/30 2 SSW 14 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...  
ACTIVE SPRING PATTERN.  
 
DETAILED...  
MID WEDNESDAY ON NORTHERN SHORES HAS DECLINING, SMALL BREAKERS  
FROM 315-340 DEGREES. A NEW EVENT IS DUE LATE THURSDAY.  
 
A SEVERE GALE WITH SEAS TO NEAR 25 FEET NEAR THE ALEUTIANS WEST OF  
THE DATE LINE 4/22 WEAKENED AS THE FETCH BROADENED AND EXPANDED SE  
4/23. THE HEAD OF THE FETCH OF MARGINAL GALES OVER THE 320-340  
DEGREE BAND REACHED ABOUT 1600 NM AWAY LATE 4/23. THE FETCH TURNED  
MORE WEST TO EAST AIMING WELL NE OF HAWAII 4/24.  
 
THE WAVE WATCH III MODEL, WW3, BRINGS IN THE SWELL EARLY THURSDAY  
WITH A STEADY RISE. HOWEVER, THIS SOURCE PATTERN LOOKS SIMILAR TO  
THE PATTERN THAT MADE THE EVENT LOCALLY A WEEK AGO 4/19-21 THAT  
WAS BIASED HIGH IN WW3. SO THIS FORECAST IS GOING FOR SLOWER AND  
LOWER THAN WW3 FOR THE ONSET THURSDAY.  
 
THE EVENT SHOULD PEAK FRIDAY AFTERNOON NEAR THE WW3 ESTIMATE  
CENTERED NEAR 330 DEGREES. IT IS PREDICTED TO DROP ON SATURDAY AS  
A NEW EVENT BUILDS.  
 
MODELS HAVE TENDED TO BETTER AGREEMENT ON THE STRENGTH AND  
LOCATION OF A CUT-OFF, UPPER LEVEL LOW PRESSURE AND ASSOCIATED  
SURFACE LOW CENTER JUST NNW TO N OF HAWAII FOR FRIDAY. THE LOW  
CENTER NEAR 33N, 160W IS EXPECTED TO SLOWLY MOVE NE 4/27-29. GALES  
ARE EXPECTED OVER A COMPACT AREA WITH STRONG TO NEAR GALE WINDS  
NOSING TO WITHIN 300 NM OF OAHU OVER THE 330-360 DEGREE FETCH. THE  
PATTERN IS EXPECTED TO CHANGE LITTLE OVER THE WEEKEND, THEN LIFT  
OUT TO THE NE EARLY NEXT WEEK AS IT RAPIDLY WEAKENS.  
 
WITH THE PROXIMITY, ABOVE AVERAGE SURF IS EXPECTED. PROXIMITY  
WOULD ALSO GIVE OVERLAPPING SHORT- AND MODERATE-PERIOD WAVES  
MAKING FOR AN IRREGULAR BREAKING PATTERN.  
 
THE EVENT IS PREDICTED TO RISE RAPIDLY SATURDAY AFTERNOON TO  
LEVELS ABOVE AVERAGE, PEAK LATE SUNDAY, AND SLOWLY DROP MONDAY  
FROM 330-360 DEGREES.  
 
MID WEDNESDAY ON EASTERN SHORES HAS ROUGH BREAKERS FROM 50-90  
DEGREES ABOVE THE TRADE WINDSWELL AVERAGE. HEIGHTS ARE EXPECTED TO  
DROP NEAR THE AVERAGE ON THURSDAY.  
 
THE UNFOLDING LOW PRESSURE PATTERN TO THE NNW TO N OF HAWAII IS  
EXPECTED TO CEASE THE TRADES BY FRIDAY. SEE THE LATEST NWS STATE  
FORECAST DISCUSSION REGARDING THE LOCAL WINDS AND SKIES ASSOCIATED  
WITH THIS PATTERN.  
 
UPSTREAM TRADES 4/23-26 SHOULD KEEP SMALL BREAKERS FRIDAY INTO  
SATURDAY OUT OF 40-90 DEGREES. EAST-COMPONENT WINDSWELL SHOULD BE  
AT A MINIMUM 4/29-30.  
 
MORE NORTHERLY EXPOSURES OF THE EAST SIDE SHOULD TREND  
AFOREMENTIONED 4/28-30.  
 
MID WEDNESDAY ON SOUTHERN SHORES HAS LOW, LONG-PERIOD SWELL FROM  
208-220 DEGREES MAKING FOR BACKGROUND LEVEL SURF. IT COULD PICK UP  
SLIGHTLY FOR THURSDAY.  
 
SEVERE GALES IN THE SOUTHERN TASMAN 4/15-17 IS THE SOURCE FOR THE  
NEW EVENT PICKING UP LOCALLY 4/25. THE LONG TRAVEL DISTANCE AND  
SHADOWING BY NEW ZEALAND AND ISLANDS OF THE SW PACIFIC LIMIT SURF  
FROM THIS DIRECTION.  
 
GALES IN A MORE FAVORABLE LOCATION WITH A FAVORABLE TRACK FOR  
PRODUCING SURF IN HAWAII UNFOLDED TO THE SE TO E OF NEW ZEALAND  
4/21-23. THE LIMITING FACTOR WAS MARGINAL GALE STATUS OF THE OCEAN  
SURFACE WINDS. IT SHOULD BRING AN EVENT WITHIN BACKGROUND TO THE  
SUMMER AVERAGE BUILDING LATE SATURDAY, PEAKING LATE SUNDAY AND  
DROPPING MONDAY FROM 185-200 DEGREES.  
 
INTO LONG RANGE, A STRONGER LOW PRESSURE TO SEVERE GALES SE OF NEW  
ZEALAND 4/24 WAS SHORT-LIVED AND DID NOT TRACK AS FAR TO THE NE AS  
THE PREVIOUS SYSTEM. LOW, LONG-PERIOD SURF FROM THIS SOURCE SHOULD  
PICK UP TUESDAY 5/1 AT BACKGROUND LEVELS AND PEAK ON 5/2 JUST  
BELOW THE SUMMER AVERAGE FROM 180-200 DEGREES.  
 
FOR NORTH EXPOSURES, THE EXPECT N EVENT FOR 4/28-30 SHOULD TREND  
DOWN 5/1 BELOW AVERAGE THEN LINGER FOR A FEW MORE DAYS OF SHORT  
WAVE PERIODS OUT OF 000-020 DEGREES.  
 
MODELS SUGGEST AN INCREASE IN TRADE WINDSWELL OUT OF 40-90 DEGREES  
5/2-4.  
 
LONG RANGE MODELS ARE SUBJECT TO HIGH UNCERTAINTY.  
 
THIS COLLABORATIVE FORECAST WILL RESUME ON FRIDAY, APRIL 27.  
 
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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