Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

344 am hst Sat apr 1 2023


Considerable high cloud cover will keep skies mostly cloudy through the weekend. Rather dry weather will continue over the eastern end of the state today and tonight, while scattered showers continue and a few thunderstorms remain possible over the western islands. An afternoon thunderstorm may develop on the Big Island as well. A disturbance aloft will develop over the state on Sunday, increasing showers and bringing the potential for a few thunderstorms to the entire island chain. Moderate to breezy trade winds and more typical trade wind weather will then return Monday and linger through late next week.


Currently at the surface, a trough of low pressure is located 175 miles northwest of Kauai, while a 1027 mb high is centered around 1500 miles north-northeast of Honolulu. Light winds prevail over the western islands, while moderate trades persist over the eastern end of the state. Infrared satellite imagery shows mostly cloudy skies across the state. Radar imagery shows scattered showers over Kauai, a few showers showers affecting the Big Island, and very little if any shower activity over Oahu and Maui County at the moment. Main short term focus revolves around trade wind trends and thunder chances during the next couple days.

The trough of low pressure will remain nearly stationary northwest of Kauai today and tonight, with little change to the winds expected. The trough will then slowly shift west and further away from the islands Sunday and Sunday night as high pressure builds to the distant northeast, allowing moderate to locally breezy trade winds to build back across the entire island chain. A ridge of high pressure will then settle in several hundred miles to the north and northeast of the state next week, keeping moderate to breezy trade winds in place across the island chain.

As for the remaining weather details, little change in the overall weather pattern is expected through tonight, with shower coverage remaining the highest over Kauai. Shower coverage will be less over Oahu during this time, but not as dry as over Maui County and the Big Island where fairly dry weather will continue. The airmass will remain unstable over the western end of the state, so a slight chance of thunderstorms will need to be included in the forecast from Oahu westward. A thunderstorm or two could pop over the Big Island slopes this afternoon as well. A disturbance aloft will develop over the state Sunday, bringing the potential for some heavier showers and even a few thunderstorms to the entire island chain. Considerable cloudiness will continue statewide through the weekend. The most unstable airmass appears to slide south of the state by daybreak Monday, allowing a more typical trade wind pattern featuring mainly windward and mauka showers to settle in and linger through the end of the work week.


The active pattern with showers and the potential for isolated thunderstorms will continue this weekend due to an upper disturbance and surface trough lingering to the northwest. Low- level flow will remain out of the east-southeast, which will translate to the localized land/sea breeze regime holding for Oahu and Kauai. Although the bulk of the shower activity will remain focused over the western end of the state this morning, much of the state could get into the mix later today through the end of the weekend as deep moisture pools northward ahead of the upper trough.

AIRMET Sierra remains in effect for mountain obscuration for Kauai and Oahu due to low clouds and showers. This may be lifted for Oahu later this morning if conditions continue to improve.

AIRMET Tango remains in effect for tempo moderate turbulence in the layer FL200-FL400.


Weak ridging over the state, with a stationary trough west of Kauai, is resulting in generally moderate areawide southeasterlies that are veering more southerly around Kauai. The trough is forecast to remain nearly stationary through the day before it drifts slowly westward into early next week. This will allow trades to return over the near and offshore waters through the remainder of the week. Winds across the eastern end of the state will remain strong enough to continue a Small Craft Advisory (SCA) for the windier areas around Big Island. Upper level troughing northwest of the area, along with convergence ahead of the surface trough, will create enough instability to support thunderstorm activity over the coastal waters from Oahu westward through the weekend. Thunderstorm activity may expand into more eastern waters Sunday as a new low aloft develops over the state.

The ongoing small northwest (320 degree) swell will slowly decline through Saturday. A small northeast swell will fill in tonight, peak Saturday and then level out through early next week. There are no other significant North Pacific swells that will affect the islands through late next week. Moderate to fresh trades upstream of the state will continue to produce choppy surf conditions along eastern exposures through next week. Small background energy from different southerly directions will continue to hold small surf in along south-facing shores for the foreseeable future.

Watches, Warnings, Advisories

Small Craft Advisory until 6 AM HST Sunday for Maui County Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel- Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters.

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