Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

407 am hst Fri Jun 22 2018

Synopsis

A moderately strong high pressure system is slated to pass north of the islands over the weekend, resulting in a boost in the trade winds to breezy conditions. This ramping up of the trades will be accompanied by an increase in trade showers, where the lee side of the smaller islands will experience some passing showers. Leeward Big Island will have some scattered showers each afternoon and evening for the coming days.

Discussion

The strengthening trades is being brought on by an approaching surface high currently located 1515 miles northwest of Kauai. It has a central pressure of 1028 MB, and is forecast to strengthen to 1035 MB when it passes some 1150 miles north of the islands Saturday and Saturday night. The trades will strengthen to moderate to strong by this evening that will warrant an expansion of the Small Craft Advisory, and possibly a limited Wind Advisory to the notoriously windy areas such as the Kohala Mountains area and Lanai. The trades are slated to tone down slightly, to moderate to locally strong on Sunday night. Another round of breezy trades may resumes on Tuesday night as this same high makes a turn to the south and southwest, back toward the islands.

Satellite imagery shows plenty of open-cell clouds upwind of the islands. They are moving west at 20 to 25 mph, with a cloud top of between 8 and 10k feet. The upper low currently 730 miles east- northeast of the Big island, continues to move west at 20 mph. The ECMWF and GFS models are still in good agreement in the track of this low, where they take it northward after reach 150w longitude Saturday evening. The GFS solution has the marine layer at 10k area wide tonight, and will lift higher to 14k feet over the Big Island by this afternoon. It remains in the 10 to 14k foot range through Saturday night when a mid level ridge comes in and lowers the inversion to 8 to 10k feet. Although we may see a slight drop off in the trade showers, orographic influence, can still maintain a wet trade pattern.

Aviation

Trade winds will be strengthening today as high pressure builds far northeast of the state. VFR will prevail over most sections of the individual isles, with brief MVFR conditions likely in windward and mauka areas in passing showers, with the Big Island seeing the most precipitation.

No AIRMETs are currently posted. However, one for low level turbulence may be needed this morning as trades become stronger.

Marine

A 1029 high about 1300 nm northwest of the main Hawaiian Islands is forecast to move east at 20 kt. The high will build to 1035 mb as it reaches a point about 1000 nm north of the area Saturday night. The pressure gradient near the main Hawaiian Islands is already tight enough to produce locally strong easterly trade winds. A Small Craft Advisory (SCA) is in effect through Saturday for Maalaea Bay, the Kaiwi, Pailolo and Alenuihaha channels, the windward Maui county waters and waters south of the Big Island. Wind speeds may reach near-gale force in Maalaea Bay and the Alenuihaha and Pailolo channels. As the high moves farther east early next week, wind speeds will gradually subside.

Surf will gradually build along east facing shores as trade winds strengthen, but is expected to remain below the advisory threshold. Surf will remain small along other shores with no significant swells through the weekend. A pair of swells from southeast of New Zealand will reach the area next week. The first swell is expected to peak Wednesday and the second on Thursday. Surf along south facing shores may reach advisory levels.

Watches, Warnings, Advisories

Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay, Pailolo and Alenuihaha channels, and waters south of the Big Island.

Small Craft Advisory until 6 PM HST Saturday for Kaiwi Channel, and windward Maui County waters.

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