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Marine section from the Area Forecast Discussion
issued at: Mar 28, 2017 8:25 PM HST

just over 1500 nm northwest of Kauai near the Date Line (around 40N) and a ridge axis extending westward from a 1030 mb high centered off the coast of California to just north of the islands. A very strong pressure gradient was depicted south of this low northwest of the state, that was supported by an ASCAT pass earlier today where a large area of strong west-northwest gales (focused within the 290 to 310 directional band relative to the islands) were shown out to around 500 nm south-southwest from the low (leading edge of the fetch was around 1300 nm northwest of the islands). According to the latest altimeter passes, seas associated with this system have dropped slightly from yesterday, but still upwards within the 28 to 33 ft range over the heart of the fetch region focused toward the islands. Despite some small differences between solutions, the latest model guidance is remaining in decent agreement with these latest satellite trends and shows this system continuing on an east-northeast track crossing the Date Line Tuesday night into Wednesday around 40N.

A large west-northwest swell (290-310 deg) associated with this system discussed above will result across the region, that should reach the islands Thursday night, peak through the day Friday and Friday night, then slowly ease over the weekend. Warning-level surf and advisory-level seas will be expected Thursday night into the weekend before trending down over exposed waters and beaches. For timing, the ECMWF-Wave, GFS-WAVEWATCH III and ensemble mean solutions all remain in close agreement and depict seas ramping up quickly around midnight Thursday night to Small Craft Advisory levels, peaking within the 10 to 14 ft range Friday, then slowly easing through the weekend. A few of the ensemble solutions are coming in slightly higher (by a couple of feet) at the northwest buoys and converge at a peak near 15 ft Thursday night. If the swell ends up coming in larger than predicted or later, warning- level surf could continue into Saturday before dropping to advisory levels along exposed north and west facing shores. Surf should dip below advisory levels Sunday.

Local winds and seas will likely remain below advisory levels through Friday as the ridge of high pressure north of the state slightly weakens in response to a cold front that is forecast to approach the region later in the week, then stall and weaken north and northwest of the area into the upcoming weekend. Advisory- level trade winds will return across portions of the waters over the weekend as high pressure builds north of the state behind this front.

Prior to the arrival of the large swell expected Thursday night through the weekend, surf along north and west facing shores will hold at levels below the advisory criteria Wednesday through Thursday due to a lingering northwest swell from earlier this week. Will continue to monitor buoy observations overnight for an expected small reinforcement out of the northwest (320 deg).

Surf along east facing shores will remain small and choppy through the week due to moderate to fresh onshore winds persisting. Surf will begin to build over the weekend as the winds strengthen locally and upstream of the islands and may near or reach advisory levels along east facing shores by the end of the weekend and into early next week.

A slight increase in surf along south facing shores will be possible Thursday through Saturday due to a long-period swell (190 deg) associated with recent activity across the southern Pacific. In addition to this southerly swell, wrap from the previously discussed large west-northwest swell will add to the mix by Friday at select spots along exposed southern facing beaches.

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Routine forecast are issued for the Hawaiian Coastal Waters, the Offshore Waters, and High Seas. Coastal Waters are within 40 nautical miles of the main Hawaiian islands. The Offshore area extends from this 40 nautical mile mark to 240 nautical miles of the main Hawaiian islands. The High Seas forecasts are issued for an area that covers approximately 15,000,000 square miles, extends as far north as 30N, and as far south as 25S, and between 120W south of the equator and 140W north of the equator to 160E.
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