Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

850 pm hst Sun Oct 24 2021


Moderate to breezy easterly trade winds will continue into Monday night, with clouds and showers favoring windward and mauka locations overnight through the morning periods. Trade winds will trend down early Tuesday through midweek, potentially enough for a localized land and sea breeze regime to become established. Expect the light winds to stick around through the weekend, which will shift the cloud and shower coverage from the typical windward areas to leeward and interior locations through the afternoon hours. Mostly dry conditions, however, should limit rainfall accumulations through most of the period.


Guidance remains in good agreement through the start of the week and depicts the breezy trades holding for one more day before trending down early Tuesday as a weak upper low and its attendant surface reflection (weak trough) drift into the area from the east. Showers will favor windward and mauka locations through Monday night as pockets of moisture move through, then transition to leeward areas through the afternoon periods Tuesday through midweek where localized sea breezes form. Rainfall accumulations are expected to remain on the light side due to the mostly dry pattern/strong subsidence inversion. A band of moisture associated with the tail-end of a front will drift southward toward the state through Monday, but should stall and diminish just north of Kauai Tuesday through Wednesday.

For the second half of the week and upcoming weekend, a progressive pattern evolving over the northern Pacific featuring a series of fronts passing to the north will keep the subtropical ridge over the region weak. This will translate to the trades completely shutting down by or come Friday, with a land and sea breeze regime prevailing statewide going into the weekend. Plenty of dry air sticking around should continue to limit rainfall chances/accumulations through Friday. Thereafter, there is a chance for increasing moisture as the tail-end of another front approaches and potentially moves into the area by the end of the weekend. If this evolves, a few more interior/leeward showers could materialize through the afternoon periods.


High pressure north of the state will keep breezy trade winds in place through Monday. A band of low clouds and showers associated with old frontal remnants will move through tonight, keeping windward areas wet. AIRMET Sierra is now in effect for windward sections of most islands. Some improvement is possible late tonight, particularly across Oahu, Molokai and Maui, with all windward areas expected to see improvement by mid morning Monday.

AIRMET Tango is also in effect for moderate low level turbulence over and downwind of the terrain of all islands. This AIRMET will likely continue through Monday.


High pressure approximately 1,000 miles north of Hawaii is the main driver to these continued moderate to locally strong trade winds. This afternoon's ASCAT satellite pass verified that winds have fallen below Small Craft Advisory (SCA) criteria within the Kaiwi Channel and Oahu Leeward Waters. The SCA is still in effect through Monday afternoon for the typically windier waters around Maui County and south of Big Island. High pressure north of the islands will progressively advance east the next couple of days as a series of North Pacific storms clip the northern Central Pacific waters. These systems will significantly weaken the pressure gradient over and upstream of the islands. This will produce weakened trades from Tuesday through the remainder of the week.

A new medium period north swell is arriving this evening as this weekend's north swell continues its decline. This swell should push surf heights back up to slightly over head high to double overhead, but not reach high surf advisory levels, along many north facing shores Monday and Tuesday. East facing shores with northern exposures will also be affected by this latest swell energy. A series of North Pacific gale to severe gale storms will initiate a couple of new north swells that will be focused along the island's northwest to north great circle paths. These swells should reach the local waters at mid week and this weekend. East facing shore surf will remain small and choppy through Monday. Weakening trades will lower these wind waves from Tuesday through the middle of the week. As previously mentioned, the lone exception will be along east facing shores that will be more exposed to incoming north swell energy. Surf along south facing shores will trend up by mid week with the arrival of a late season south to southwest swell. Otherwise, south surf will be maintained by very low background south swell and short period, southeast trade wind wave energy.

Watches, Warnings, Advisories

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

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