Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

927 am hst Fri aug 17 2018


A moderate and somewhat unstable trade wind flow will continue. Showers will be focused over windward slopes, with afternoon clouds and showers, some briefly heavy, developing over leeward areas. An increase in humidity and enhanced shower activity is due this weekend, as area of tropical moisture passes overhead in the trade wind flow. Breezy and drier trade winds will return on Monday. Hurricane Lane will be crossing the region near the islands during the middle of next week, though it remains too early to determine how it will affect island weather.


A moderate and somewhat unstable trade wind flow remains in place. The trades are being driven by surface high pressure centered far northeast of the state. Weak troughs passing north of the islands are disrupting the local pressure gradient over the state and keeping the trades slightly weakened. A persistent upper level trough parked over the region is making the atmosphere somewhat unstable, with the overnight soundings and recent aircraft data showing no inversion. Small pockets of moisture moving along the trade winds have been periodically enhanced, mostly recently over windward East Maui where a brief thunderstorm flared this morning. Otherwise, modest showers have been confined to windward slopes this morning, mainly across the eastern end of the state.

The somewhat unstable trade wind flow will continue today. Low level moisture will be slightly higher from Oahu to the Big Island, resulting in a greater amount of windward shower activity on those islands. Expect another round of afternoon convection across leeward and interior Big Island, where a few thunderstorms are expected. On the other islands, spotty showers, some briefly heavy, could develop across leeward terrain during the afternoon. Conditions will also be somewhat humid as dew points creep up into the low to mid 70s.

Humidity and enhanced shower activity will increase during the weekend. A pocket of tropical moisture, currently located more than 300 miles east of the Big Island, will be move overhead in the moderate trade wind flow. This should drive dew points up into the mid 70s, leading to a noticeable increase in humidity. Showers will continue to favor windward slopes, while day time sea breezes over sheltered leeward areas produce afternoon clouds and spotty showers. With precipitable water values reaching as high as 2 inches and the lingering upper level trough providing instability, some showers could be heavy, both over windward slopes and across leeward terrain during the afternoon hours.

Drier and more stable trade wind weather is expected on Monday. The area of moisture will move off to the west a ridge builds overhead. The result will be a rather dry and stable trade wind flow with modest showers confined to windward slopes.

Forecast uncertainty ramps up heading into the middle of next week as Hurricane Lane passes near or just south of the islands. It remains too early to forecast the exact location of Lane and whether it will directly impact portions the state.


High pressure far northeast of the islands will maintain light to moderate trade winds across the state into Saturday. Winds are not strong enough to warrant an AIRMET for low level moderate turbulence although there may be light turbulence for Molokai, Lanai and Maui especially in south and west sections downstream of the mountains.

Most of the area will have VFR conditions with the exception of windward areas, especially on Maui and the Big Island where there will be a better chance for shower activity and periods of MVFR conditions. During the early hours of Saturday and into Saturday morning expect an even better chance for heavier rain along the windward slopes of the Big Island which may lead to sustained MVFR conditions requiring an AIRMET for mountain obscuration.

There are currently no AIRMETs in effect.


Light to moderate trade winds are expected to continue through this weekend, but remain below Small Craft Advisory levels. Periods of heavier showers are expected this weekend, particularly over the windward waters, as remnant tropical moisture moves in from the east. Hurricane Lane, currently about 1330 nm east- southeast of Hilo, Hawaii, may bring tropical storm conditions to waters south of the Big Island next week.

No significant swells are expected through the weekend, so expect surf to remain small on all shores. A small, north-northwest swell is expected to arrive next week Tuesday. We will continue to monitor the development of Hurricane Lane, currently located in the eastern Pacific, which may bring an easterly swell to the coastal waters as early as Sunday night of next week.

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

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