Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

412 am hst Sun may 19 2024


A late season Kona will continue to move north and weaken as a ridge of high pressure over the state shift north today and Monday. Expect a gradual drying trend and the return of easterly trade winds Monday into the second half of the week.


The Flood Watch for Kauai has been cancelled. The heaviest rainfall activity has moved north of Kauai and expected to continue to move north and dissipate. A few small moderate to heavy showers did move through the area overnight producing rainfall totals a little over a quarter of an inch. Oahu received a little more rainfall than Kauai early on in the night but then the light to moderate showers moved over Maui County. Latest satellite imagery shows a mix of high and low clouds across the state which is going to make for a cloudy start to the morning. Radar imagery shows mid to high reflectivity returns, indicating some expansive light to locally moderate stratiform rain over much of the state. Again rainfall totals so far have remained modest and conditions are anticipated to clear from east to west as the day progress.

Gentle to locally breezy east to southeast winds are expected to persist through today. Lighter and more variable winds over the western half the state will allow for land seas breeze pattern. The axis of above normal moisture is also expected to shift northwest and an upper level short wave trough will exit to the east during this time. Expect more stabilizing trends over the western half of the state today. A slight chance of thunderstorms is still possible for the upper slopes of the Big Island this afternoon as the upper level trough exits east.

Lingering moisture and some upper level instability from a couple more short wave trough may enhance trade wind showers Tuesday into the second half of the week as the ridge settles north of the state. A more typical trade wind pattern may return late in the week.


A trough west of Kauai will allow scattered to numerous showers to persist through mid-morning, mainly for areas west of the Big Island. Relatively low ceilings between 1500 and 2000 feet AGL will lead to MVFR and IFR conditions. By late this morning, as the trough pulls westward and further away from the state, conditions are forecast to improve. Scattered showers will favor islands west of Maui and their surrounding waters with isolated activity expected elsewhere. A slight chance of diurnally driven thunderstorms will exist over the Big Island this afternoon.

Weak to moderate east southeast background flow will lead to a land/sea breeze pattern on Kauai, Oahu, and leeward sections of Maui, Molokai, and the Big Island.

AIRMET TANGO remains in effect for moderate upper level turbulence from FL250-400 as deep upper level troughing remains west of the area. AIRMET SIERRA remains in effect for mountain obscurations over most islands except Big Island due reduced visibility from ceilings near 2000 ft. These conditions should improve by this afternoon. Periods of light to moderate rime icing is expected from Kauai to Maui in layer 130-FL220 through at least mid-morning. Therefore, AIRMET ZULU is in effect.


Locally strong east-southeast winds will persist through Monday, before transitioning to easterly trades Tuesday through next weekend. A Small Craft Advisory (SCA) remains in effect through 6 PM this evening for the windier waters around Maui County and the Big Island. This SCA may need to be extended in time later today.

A pair of overlapping south swell will keep south shore surf near low end advisory levels today. The swell will hold near or just below advisory levels tonight and Monday, then gradually lower through the middle of the week. South shore surf will fall back below the summertime average Thursday through next weekend, with only small south and south-southwest swells rolling through.

The current very small northwest swell will gradually lower through Monday, with north shore surf going flat Tuesday through next weekend.

Surf along east facing shores will remain well below average through early next week. An upward trend is expected Tuesday through next weekend as trade winds strengthen over and upstream of the islands.

Watches, Warnings, Advisories

High Surf Advisory until 6 PM HST this evening for Niihau-Kauai Leeward-Waianae Coast-Kahoolawe-Maui Leeward West-Kona-Kauai South-East Honolulu-Honolulu Metro-Ewa Plain-Molokai Southeast- Molokai Leeward South-Lanai Leeward-Lanai South-Maui Central Valley South-South Maui/Upcountry-South Haleakala-Big Island South-Big Island Southeast.

Small Craft Advisory until 6 PM HST this evening for Maui County Windward Waters-Alenuihaha Channel-Big Island Windward Waters- Big Island Leeward Waters.

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