804  
FXHW52 PHFO 190101  
SRDHFO  
 
COLLABORATIVE NEARSHORE SWELL AND WIND FORECAST FOR OAHU  
NWS/NCEI HONOLULU HI  
301 PM HST WED JUL 18 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 7 ENE 9 6 8 UP 17-21 ENE UP  
07/18 2 SSW 15 3 5 UP  
1 SSW 20 2 4 UP  
 
THU 7 ENE 9 6 8 DOWN MED 17-21 E DOWN  
07/19 2 SSW 17 4 6 SAME MED  
1 SSW 20 2 4 UP LOW  
 
FRI 5 ENE 7 2 4 DOWN LOW 11-16 E SAME  
07/20 2 SSW 17 4 6 SAME LOW  
1 S 20 2 4 UP LOW  
 
SAT 5 ENE 7 2 4 SAME LOW 11-16 E SAME  
07/21 3 SSW 14 4 6 DOWN LOW  
2 S 22 4 7 UP LOW  
 
SUN 5 ENE 7 2 4 SAME LOW 13-19 ENE UP  
07/22 3 S 19 8 10 UP LOW  
 
MON 6 ENE 7 3 4 UP LOW 13-19 ENE SAME  
07/23 3 S 17 6 8 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...  
TRADE WINDSWELL DROPPING AS SOUTHERLY SWELL RISES.  
 
DETAILED...  
MID WEDNESDAY ON EASTERN SHORES HAS BREAKERS FROM 60-90 DEGREES AT  
LEVELS ABOVE THE TRADE WINDSWELL AVERAGE. ELEVATED HEIGHTS ARE  
EXPECTED TO CONTINUE INTO THURSDAY.  
 
A TROPICAL DISTURBANCE SOUTH OF THE ISLAND OF HAWAII 7/18 IS  
TRACKING STEADILY EAST. IN COMBINATION WITH THE SEASONALLY SEMI-  
PERMANENT SURFACE HIGH PRESSURE OVER THE CENTRAL TO NORTHEAST  
PACIFIC, AN AREA OF FRESH TO STRONG BREEZES NORTH OF THE  
DISTURBANCE TO 24N HAS ALLOWED THE ROUGH, ABOVE AVERAGE SURF  
LOCALLY 7/18. ASCAT SATELLITE AT 9 AM HST 7/18 SHOWS STRONG WIND  
NNE OF THE BIG ISLAND. THE PACIOOS/CDIP HILO BUOY IN THE MORNING  
7/18 CLIMBED TO 10 FEET SIGNIFICANT WAVE HEIGHT.  
 
OAHU SHOULD SEE A RISE IN SURF 7/18 IN THE PM WITH THE EVENT  
PEAKING OVERNIGHT. HEIGHTS SHOULD REMAIN ABOVE AVERAGE FROM 60-90  
DEGREES ON THURSDAY.  
 
ASCAT SATELLITE 7/18 SHOWS A LARGE REGION EAST OF 150W NORTH OF  
20N WITH MOSTLY GENTLE TO MODERATE TRADES. THE LOCAL WINDSWELL  
SHOULD DROP SHARPLY LATE THURSDAY INTO FRIDAY TO LEVELS BELOW  
AVERAGE. IT SHOULD HOLD ABOUT THE SAME SATURDAY TO MONDAY FROM  
60-90 DEGREES.  
 
MID WEDNESDAY ON SOUTHERN SHORES HAS RISING, LONG-PERIOD BREAKERS  
FROM 185-200 DEGREES AT LEVELS NEAR THE SUMMER AVERAGE. AN  
INCREASE IS PREDICTED FOR THURSDAY.  
 
A SERIES OF LOW PRESSURES S TO SE OF NEW ZEALAND 7/9-12 IS  
EXPECTED TO MAKE OVERLAPPING EVENTS LOCALLY 7/18-21 FROM THE SSW.  
THE PATTERN OF LOW PRESSURE SYSTEMS SHIFTED EAST 7/13-16, LEADING  
TO NEW OVERLAPPING EVENTS FROM MORE STRAIGHT SOUTH LOCALLY  
7/21-25.  
 
THE FIRST SYSTEM HAD GALES BEHIND A FRONT TO NEAR 45S TO THE SE OF  
NEW ZEALAND 7/9-10 WITH SEAS 20-25 FEET. LONG-PERIOD FORERUNNERS  
ARRIVED LOCALLY 7/17 AND THE EVENT IS PEAKING 7/18 CENTERED NEAR  
190 DEGREES.  
 
THE SECOND PATTERN 7/10-11 WAS SIMILAR, WITH A LOW TRACKING EAST  
ALONG 60S AND GALES BEHIND A FRONT PUSHING TO 45S TO THE SE OF NEW  
ZEALAND. SINCE IT WAS ACTING UPON EXISTING SEAS, SURF POTENTIAL  
IN HAWAII IS GREATER. NOAA SOUTHERN BUOY 51002 SHOWS AN INCREASE  
IN THE 16-19 SECOND BAND 7/18. THIS EVENT SHOULD BE FILLED IN ON  
THURSDAY BRINGING BREAKERS A NOTCH ABOVE THE SUMMER AVERAGE FROM  
190 DEGREES.  
 
THE THIRD LOW PRESSURE 7/11-12 WAS MUCH STRONGER WITH STORM-FORCE  
WINDS AND SEAS WITHIN 30-40 FEET. HOWEVER, IT WAS CLOSER TO  
ANTARCTIC AND MORE ZONAL, OR WEST TO EAST IN THE FETCH. JASON  
ALTIMETER SHOWED SEAS HIGHER THAN PREDICTED BY WAVE WATCH III 7/12  
AT BOTH 12 AND 18Z FOR THE PORTION CLOSEST TO THE HAWAII GREAT  
CIRCLE ROUTE ALONG 195 DEGREES. THIS SYSTEM SHOULD HAVE LONG-  
PERIOD FORERUNNERS ARRIVING 7/19 WITH THE PEAK ABOVE AVERAGE ON  
7/20. IT SHOULD SLOWLY DECLINE FROM 185-200 DEGREES INTO THE  
WEEKEND AS NEW SWELL ARRIVE.  
 
THE JET STREAM TROUGH EAST OF NEW ZEALAND PROGRESSED EAST  
7/13-17. A PAIR OF LOW PRESSURES GAINED STRENGTH AT THE EASTERN  
EDGE OF THE HAWAII SWELL WINDOW.  
 
THE FIRST ONE 7/13-14 HAD SEVERE GALES AND BROADENED AS THE AIM  
BECAME MORE MERIDIONAL JUST EAST OF THE HAWAII SWELL WINDOW. THE  
FETCH WAS BROAD WITH SEAS NEAR 25 FEET. IT WAS CLOSE ENOUGH FOR  
ANGULAR SPREADING TO BRING SWELL LOCALLY. LONG-PERIOD FORERUNNERS  
ARE DUE LATE ON 7/20 LOCALLY FROM 180-190 DEGREES. IT SHOULD PEAK  
ON SATURDAY ABOVE AVERAGE.  
 
THE SECOND 7/15-17 WAS MUCH STRONGER, TO HURRICANE-FORCE. THE LOW  
CENTER TRACKED STEADILY EAST MAKING FOR A BRIEF PERIOD IN THE  
HAWAII SWELL WINDOW. BUT THE LARGE FETCH AND SEAS TO 40 FEET  
SHOULD ALLOW AMPLE SWELL TO REACH HAWAII THROUGH ANGULAR  
SPREADING.  
 
EXTRA-LONG WAVE PERIODS WITHIN 20-25 SECONDS ARE EXPECTED TO FILL  
IN LOCALLY 7/21 FROM 175-185 DEGREES. THE EVENT SHOULD CLIMB ABOVE  
AVERAGE SATURDAY NIGHT, PEAK SUNDAY, THEN SLOWLY DROP ON MONDAY.  
 
INTO THE LONG RANGE, THE LAST S EVENT IN THE SERIES SHOULD HOLD  
NEAR TO A NOTCH ABOVE THE AVERAGE INTO 7/24, DROP NEAR THE AVERAGE  
7/25, AND FADE TO BACKGROUND LEVELS BY 7/26 FROM 160-180 DEGREES.  
 
MODELS SUGGEST BREAKERS FROM WINDSWELL INCREASING TO NEAR THE  
AVERAGE 7/24-25.  
 
LONG RANGE FORECASTS ARE SUBJECT TO LOW CONFIDENCE.  
 
THIS COLLABORATIVE FORECAST WILL RESUME ON FRIDAY, JULY 20.  
 
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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