Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

1006 am hst Mon Mar 27 2017

Synopsis

A ridge will remain north of the state through Thursday night and will drive moderate east to southeast flow across the state. A trough aloft will dig east of the island chain Tuesday night and will bring more windward showers to the state during the second half of the work week. A prefrontal band of showers may impact the state Thursday night and Friday, but the front is not expected to reach the state. A strong high will set up northeast of the state this weekend and bring windy conditions.

Discussion

A 1031 mb surface high lies about 1750 miles northeast of Honolulu, with a surface ridge extending southwest from the high and passing 375 miles north of Honolulu. The ridge will remain nearly in place through Thursday night and drive moderate east to east southeast flow across the state through then.

A ridge aloft will remain over the main Hawaiian islands through tomorrow night. The ridge will keep the atmosphere dry and stable, so rainfall will remain light until then. Models show an increase in precipitation arriving early Wednesday morning as an upper trough digs in east of the state on Tuesday night. The trough will enhance the moisture tracking in from upstream and will bring relatively wetter trade wind weather during the second half of the week. Winds will focus most of the precipitation over windward and southeast facing slopes. Leeward areas may see a few passing showers at times.

The ridge to our north will slide to the east Thursday night as a cold front approaches from the west. The front will stall out Friday night and a building high to the north over us will force the front away from us before impact. However, a prefrontal band of showers will form out of the lingering moisture brought in with the trade winds and set up over the state Thursday night and Friday.

The aforementioned high pressure system to the north is forecast to strengthen to about 1038 mb this weekend. Subtropical highs at this strength typically produce some windy conditions for us. The atmosphere will stabilize this weekend with typical windward and mauka showers moving in with the trades.

Aviation

A stable trade wind flow will produce isolated MVFR ceilings along windward terrain. VFR will dominate elsewhere. An inversion will remain in place between 5000 to 7000 ft with moderate E to SE winds up to 20 kt at low levels. No AIRMETs in effect.

Marine

The current west-northwest swell continues to bring advisory level surf to various north and west facing shores exposed to the swell. In addition, resulting seas in excess of 10 ft are contributing to a Small Craft Advisory (SCA) for most coastal waters. The swell is expected to slowly ease into tomorrow, and then hold steady into Wednesday as a slight reinforcement arrives.

In addition to the elevated seas, winds remain elevated in the typically windier locations near Maui and the Big Island. Winds will slowly drop below SCA levels mid week.

Surf along east facing shores will remain small and choppy through the week due to moderate trade winds. The above mentioned west-northwest swell is wrapping around the islands and is aiding a small south-southwest long period swell is bumping surf along south facing shores. A long period south swell is expected to bring a bump to south facing shores again Thursday through Saturday.

A new, potentially larger, west northwest swell is expected to arrive Thursday night, peak Friday and then slowly ease over the weekend. The swell is being generated by a strong low off the coast of Japan with hurricane force winds. Will be monitoring model runs as the system evolves.

Watches, Warnings, Advisories

High Surf Advisory until 600 PM HST today for north and west facing shores of Niihau, Kauai, Oahu and Molokai, north facing shores of Maui, and west facing shores of the Big Island.

Small Craft Advisory through 600 PM HST today for all Hawaiian waters except Oahu leeward waters and Maalaea Bay.

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