Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

821 pm hst Tue Jun 15 2021


Moderate trade winds will prevail through Friday, but will ease a little over the weekend, and may become even lighter next week. The trade winds will deliver an area of showery low clouds to Kauai and Oahu tonight and Wednesday, primarily dampening windward areas, and bringing a slight chance of thunderstorms to Kauai. Otherwise, limited moisture will result in just a few showers over windward areas. Periods of high clouds, associated with a disturbance aloft, can be expected the next couple of days.


Currently at the surface, a 1029 mb high is centered around 1400 miles northeast of Honolulu, and is driving moderate trade winds across the island chain this evening. Infrared satellite imagery shows a mix of high and low clouds resulting in mostly cloudy conditions across the state. Meanwhile aloft, water vapor imagery shows a broad mid/upper level low several hundred miles northwest of Kauai. Radar imagery shows scattered showers moving into windward areas of Kauai and Oahu, as well as over leeward Big Island. A few decaying showers are reaching leeward areas of Kauai and Oahu, with very little rainfall noted elsewhere across the state. Main short term concerns revolve around thunder chances the next couple days.

High pressure northeast of the state will lift slowly northward through Friday, keeping moderate trade winds in place across the island chain. A couple fronts approaching and stalling northwest of the islands Saturday through next Tuesday, will suppress the trades down into the light to locally moderate range. The lighter winds will allowing for some localized land and sea breezes to develop during this time as well.

As for the remaining weather details, the remnants of an old front will pass through Kauai and Oahu tonight and Wednesday, bringing an increase in shower activity primarily to windward and mauka areas. We will need to keep a slight chance of thunderstorms in the forecast for Kauai late tonight through Wednesday evening due to a disturbance aloft moving overhead while the old frontal remnants move through. Meanwhile across Maui County and the Big Island, a fairly dry trade wind pattern will prevail, although showers may increase a bit over windward Big Island during the day on Wednesday.

A more typical trade wind pattern will resume Wednesday night and then continue through early next week, with clouds and showers favoring windward and mauka areas. Showers will be most likely at night and in the early morning, with a few showers spilling leeward at times as well. A few showers may also develop each afternoon over leeward areas in the lightened trade wind regime Saturday through next Tuesday. Some lingering instability may allow for a thunderstorm or two to pop on the Big Island Thursday afternoon as well.


Moderate easterly trades will persist tonight and concentrate most shower activity along the windward coasts and slopes. In addition, a moisture boundary moving south towards the islands will bring an increase in shower activity for Oahu and Kauai. Relatively unstable air aloft will lead to a slight chance of thunderstorms over Kauai and surrounding waters on Wednesday.

No AIRMETs are currently in effect.


High pressure north of the islands will maintain moderate to locally strong trade winds through the week. Showers will increase over the northern waters tonight into Wednesday and may produce a few isolated thunderstorms around the waters of Kauai. Small Craft Advisory is in effect through Thursday afternoon for the windier waters near Maui and the Big Island. Trade winds are expected to decrease over the weekend as a front develops well northwest of the state.

Surf along south facing shores will remain small through Wednesday, then slowly rise and approach High Surf Advisory levels over the weekend. The current small short-period north swell will decline Wednesday. Surf along north facing shores will be minimal through the rest of the week, but a small northwest swell is possible early next week. Trade winds upstream of the islands will produce small and choppy surf along east facing shores.

Watches, Warnings, Advisories

Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.

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