Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

1010 am hst Sat jul 20 2019


A broad area of high pressure far north of the islands will continue to feed the islands with locally breezy trade winds through at least the middle of next week. There could be a boost in enhanced trade wind showers between Monday and Tuesday, especially the eastern end of the island chain, in association with a low level disturbance passing south of the islands and the interaction with an upper level low northwest of the islands.


A well anchored surface high north of the islands will be the continuing source for trade winds for the area through at least the middle of next week. These trade winds will be mainly be in the moderate to locally strong range.

There has been some late changes with what the models have instore for the islands in the next 72 hours or so. From what was a benign, uneventful, trade wind pattern, models are now suggesting a disturbance to pass south of the islands between Monday and early Tuesday time frame. The disturbance will be accompanied by tropical moisture that will sweep across the area from east to west. Further more, an upper low at 500 mb is located 355 miles north-northwest of Kauai now. It is forecast to deepen slightly in the next 36 hours while remaining in that general vicinity through early Monday morning, before moving further away from the islands. By Tuesday afternoon the upper low is expected to be 700 miles west-northwest of Kauai. Both the GFS, ECMWF, and NAM, are in pretty good agreement with this scenario, although the EC is a few hours behind the GFS with the onset of the weather. Current grids have thunderstorms slated for the Big Island Monday morning, as per GFS solution.

Comparing the latest satellite imagery with the models, there is a presence of low level low 460 miles south of South Point, fitting the models solution. The models weakens this low to a strong trough as it pushes west- northwestward through Sunday, then northwest through Monday night, running parallel to the island chain. The upper low northwest of Kauai, with its southerly winds aloft, will be helping steer the trough's course. Perhaps the question mark is how much tropical moisture will be accompanying this trough. We hope to have a clearer picture by this evening.

Current grids have thunderstorms for the Big Island beginning Monday morning, and keeping the smaller islands out of this unsettled weather for now. With the upper low being weaker and some 900 miles away, the weather will return to regular trades by Tuesday night or Wednesday.

Currently, satellite imagery shows broken low clouds coverage upwind of the islands, especially upwind of Maui and the Hamakua coast. Some dissipation is expected with these clouds, but not all. So expect some passing showers for the rest of the day, and perhaps this evening for Maui.


A high will remain nearly stationary far north of the main Hawaiian islands and maintain locally strong east winds over the area. AIRMET TANGO is in effect for moderate turbulence below 7,000ft and this AIRMET will likely continue into Sunday.

The east winds will carry scattered to broken Low clouds over windward areas. Later today clouds and showers will develop over the southwest slopes of the Big Island and Maui. We expect VFR conditions to prevail and we do not expect to have to issue AIRMET SIERRA today.


Fresh to locally strong easterly trades are forecast to continue through next week as high pressure remains positioned north-northeast of the area. The stronger winds will remain across the typically windier locations from Maui County to the Big Island due to terrain accelerations. The current Small Craft Advisory remains in place for these areas through Sunday and will likely need to be extended into early next week.

Rough surf will continue along east facing shores each day due to strong onshore winds. Heights are forecast to slightly trend down through the latter half of the weekend into early next week due to the upstream trades trending down.

Surf along south facing shores will remain small through the weekend, then trend up through the first half of the upcoming week due to a recent system passing through/near the Tasman Sea. Heights should remain below advisory levels as this south- southwest swell moves through and peaks Monday through Tuesday. In addition to this long-period source, a combination of small south to southeast swells will be enough to keep the surf from going flat along southern exposures through midweek.

A south-southeast (150-160 deg) long-period swell associated with a compact gale that has developed southeast of the Tuamotus will be possible late next week with a peak around Thursday night into Friday. Surf will rise along exposed shores but should remain below advisory levels.

See the latest Oahu Surf Discussion (SRDHFO) for additional details on surf and swell.

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.

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