678 FXHW52 PHFO 200117 SRDHFO Collaborative Nearshore Swell and Wind Forecast for Oahu NWS/NCEI Honolulu HI 317 PM HST Fri Jul 19 2019 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 E 8 4 6 SAME 19-23 E SAME 07/19 SAT 7 E 8 4 6 DOWN MED 13-19 E DOWN 07/20 SUN 6 E 8 3 5 SAME LOW 13-19 E SAME 07/21 MON 6 E 8 3 5 SAME LOW 13-19 E SAME 07/22 TUE 6 E 8 3 5 SAME LOW 13-19 E SAME 07/23 2 SSW 15 3 5 UP LOW 2 SE 13 2 4 UP LOW WED 6 E 8 3 5 SAME LOW 13-19 E SAME 07/24 2 SW 14 3 5 DOWN LOW 2 SE 13 2 4 SAME 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... Steady E windswell through the period with average or less SW and SE surf next week. DETAILED... Mid Friday on eastern shores has breakers from 70-90 degrees at levels near to a notch above the average. Similar surf is predicted for Saturday with a slow downward trend. The hiatus of significant high pressure cells N to NE of Hawaii from late April to mid July has kept local trades and windswell below average. The new high cell this week climbs the trades and windswell locally back to climatology for the season. This 1031 high pressure cell set up near 40N 150W 7/17 and has held about the same into 7/19. The high pressure cell is expected to remain N to NE of Hawaii through the period. See the latest NWS State Forecast Discussion regarding the local winds and skies in context with the elusive tropical weather influence in the vicinity within 7/20-24. Breakers from windswell out of 70-90 degrees should hold near the average through the period 7/20-24. Select north and south shore locations receive refracting and diffracting easterly windswell, giving way to tiny to small breakers at select spots. Mid Friday on southern shores has breakers at a seasonal minimum, which is quite a swing of the pendulum from the seasonal maximum 7/14-15. Similar low surf is predicted for Saturday. Wave models keep various sources of swell less than 2 feet within 10-18s wave periods and 140-220 degrees directions into Monday. This is the summer background sources from shorter-period southern hemispheric trades and austral mid latitude low pressure cells not well situated in the Hawaii swell window of 7/11-14. The Tasman Sea has had back to back low pressure systems starting 7/12 and is expected to continue to 7/21. Most have been too weak of magnitude or too narrow of fetch, or both. The best potential for surf in Hawaii from the Tasman unfolded 7/14-15. It should give surf locally out of 208-220 degrees Tuesday 7/23 into 7/24. Heights could build to near the seasonal average at most. The SE Pacific has had various systems aiming swell at Hawaii centered near 160 degrees, which is within the Tuamotu Islands shadow. A subtropical source SSE of the Tuamotu's 7/15-17 could fill in Tuesday 7/23 and hold about the same into 7/24 from 150-170 degrees. Into the long range, the Tuamotu subtropical feature intensified 7/19 as it tracked SSE. The surf potential from the stronger winds is offset by the system moving away from Hawaii. This system is acting upon existing seas from a low pressure north of the Antarctic ice sheet south of Easter Island 7/16-17. It could give surf within background to near the average for the weekend of 7/27 from 150-170 degrees. Trade windswell suggested to hold near the average 7/25-26. Long range forecasts are subject to high uncertainty. This collaborative forecast will resume on Monday, July 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 https://www.weather.gov/hfo/marine $$ NWS Forecaster and NCEI Pat Caldwell