Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

843 pm hst Mon Mar 19 2018


Trade winds will increase to locally breezy levels by midweek, then ease toward the weekend as high pressure strengthens to our north before getting edged out by a front approaching from the west. Trade winds will push clouds and showers across the islands, focusing rainfall mainly across windward and mauka areas, with only light amounts expected. Showers will increase late in the work week and over the weekend as a weak surface low forms along a trough across the islands, with locally heavy rainfall possible.


An area of layered clouds has moved east of the smaller islands this evening and will likely pass east of the Big island within the next hour or so. Isolated thunderstorms which formed over the Big island this afternoon, thanks to a passing upper level shortwave impulse, have collapsed. The brief bout of wintry weather at the summits has also ended with the passing of the layered clouds. As drier conditions move in this evening, the islands will see a continued trade wind weather pattern, with scant rainfall focused mainly across windward and mauka areas.

High pressure far north of the main Hawaiian islands will shift southeastward through much of the work week, maintaining trade flow across local waters at least through midweek. The gradient between this high pressure and equatorial low pressure will be sufficiently steep to boost winds to locally breezy levels by midweek. This trade flow will push low clouds and showers across the islands, focusing rainfall across windward and mauka areas. Leeward areas may get wet as well from time to time.

Models are in reasonably good agreement showing an upper shortwave trough digging to the west of the state Friday and Friday night, with a surface low developing near or over the islands along a lingering surface trough over the weekend. The surface low is then expected to lift northeast and away from the state early next week.

Deep tropical moisture is expected to begin advecting over the western islands on Friday, then shift slowly eastward Friday night and over the weekend, resulting in a wet conditions statewide. Conditions should begin to improve early next week as the surface low pulls northeast and away from the state, dragging the deepest moisture east of the island chain. There is a potential for heavy rainfall and flash flooding over the weekend, but it remains too early to tell how much rainfall will occur or where flooding, if any, would threaten. This will continue to be monitored closely as the week progresses. The wind forecast remains problematic as it will depend greatly on the location, track and intensity of the surface low through the weekend.


A short-wave trough aloft near the Big Island will move east and weaken, ending the threat for thunderstorms over the area.

A surface ridge NE of the area will maintain low-level ENE winds around the main Hawaiian Islands. The trailing end of an old frontal band has moved S of the islands. Clouds and showers have decreased, but the low level flow will continue to push enough clouds over NE sections of the islands and produce tempo MVFR ceilings. AIRMET SIERR for MTN OBSC will likely remain up through the night for NE sections of most islands.


Moderate to Fresh trades will hold through Tuesday before increasing into the fresh to strong range Tuesday night through midweek as high pressure builds southeast over the northeast Pacific and the gradient tightens locally. Increasing winds combined with a large northeast swell expected will lead to small craft advisory conditions across most waters Tuesday night through Thursday. Winds and seas are forecast to trend down Friday into the weekend as weak low pressure develops in the vicinity of the islands and the swell lowers.

Surf along north and east facing shores will begin to rise Tuesday night, peak Wednesday through Thursday, then slowly trend down into the weekend as a large northeast (020-040 deg) swell moves through. Recent satellite data showed a good sized area of gale- to storm- force winds focused at the state with seas in the 20 to 25 ft range around 1000 nm northeast of the islands. Surf will likely reach warning levels along north and east facing shores around the peak Wednesday into Thursday, with advisory level surf potentially lingering into Saturday for east facing shores as this sources lowers.

Surf along south facing shores will remain small through the rest of the week, but could see an upward trend Friday into the weekend due to a recent uptick in activity over Hawaii's swell window across the southern Pacific.

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