Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

859 pm hst Fri jul 19 2024


Moderate trade winds will continue over the next few days, with breezy to locally windy trades returning by early next week. A weak disturbance riding in on the trades will increase clouds and shower activity tonight into Saturday, with another weak disturbance moving across the islands next Tuesday. No significant rainfall is expected from either disturbance, but it should provide some beneficial rain for windward and mauka areas.


A weak trough just east of the Big Island will move across the state tonight into Saturday. Clouds and embedded showers continue to move into windward Big Island this evening, with localized areas of moderate to heavy rainfall from Upolu down to Puna. Another batch of moderate to heavy showers are currently off the Kau Coast and will move onshore over the next several hours. Otherwise, there are only a few lingering showers over windward areas of the smaller islands this evening.

The latest CIMSS MIMIC PWATs show values between 1.5-1.75 inches associated with this trough under a relatively stable atmosphere. This setup will support areas of moderate rainfall totals through Saturday across windward Big Island and East Maui. Otherwise, the rest of the state can expect a slight uptick in windward clouds and showers on Saturday.

Drier conditions return Sunday with breezy trades expected by Monday. Breezy to locally windy trade will then continue through much of the upcoming week as high pressure strengthens far northeast of the state.

Another moisture plume, associated with the deeper area of clouds and showers aligned with a weak trough currently near 133 degrees West, is expected to move across the state on Tuesday, with model PWATs around 1.8 inches. This system should bring more widespread clouds and showers to windward and mauka areas of all islands. With the stronger trades in place expect some of these showers to spillover into leeward areas at times. Expect a return to more typical trade wind weather conditions behind the disturbance with strong trades continuing through the end of the upcoming week.


Moderate to locally breezy trades will continue into the weekend. A passing disturbance is expected to bring an increase in windward SHRA to the state tonight through Saturday, especially from Oahu eastward. Low cigs should favor windward and mauka areas with brief periods of MVFR possible. Otherwise, VFR should prevail.

No AIRMETs are currently in effect, though AIRMET Sierra will likely be needed later tonight for mtn obsc as windward showers increase.


A high pressure system and weak trough far north of the Hawaiian Islands will keep moderate easterly trade winds blowing across the region into Saturday. Trade winds will begin to strengthen into the fresh to locally strong range a little earlier than expected, starting as early as Saturday afternoon as the high center, currently northwest of the state, drifts east and strengthens. Trade wind speeds will be strong enough to support Small Craft Advisory conditions over the windier waters and channels near Maui and the Big Island from Saturday evening through much of next week.

Background south swell energy pulses are forecast to continue with surf remaining below average through much of next week. Small, choppy surf is expected for east facing shores for the next several days due to continued trade winds. Surf then becomes increasingly rough along east facing shores as locally strong trades return early next week.

Spring tides will peak this weekend, and local tide gauges indicate water levels are already running above guidance. Water levels are currently running at least 1 foot higher than the Mean Higher High Water (MHHW) threshold at Hilo, Kahului, and Honolulu and have exceeded the 1 foot threshold at Honolulu. The Coastal Flood Statement for isolated minor flooding has been extended through Monday evening.

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