Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

850 pm hst Fri Sep 29 2023


Moderate to locally breezy trade winds will deliver passing clouds and showers through the weekend, mainly to windward and mauka areas. Weak instability associated with a low aloft may bring an isolated thunderstorm through Saturday. Light winds and mostly dry weather are expected next week, with limited showers focused over interior areas during the afternoon.


N Pacific surface analysis continues to feature a slow-moving trade- wind-supporting 1030 mb high centered about 1200 miles NNE of the islands, and a pair of strong extratropical lows to the distant N and NW of the area. Afternoon sounding data indicates that a low aloft several hundred miles NW of Kauai has destabilized the island atmosphere, with a weak/absent subsidence inversion noted in the Lihue and Hilo soundings respectively. Although prevailing moisture depth is confined to the lowest 10-12 kft of the atmosphere, an area of deeper moisture over windward waters about 30 miles E of Oahu did manage to produce a few brief lightning strikes earlier this evening. Infrared satellite imagery briefly indicated isolated cloud tops near 40 kft in this area, while also depicting statewide scattered to broken high clouds that added some color to this evening's sunset. Radar shows a cluster of heavier showers associated with the earlier lightning strikes over Oahu's windward waters, with another cluster of heavier showers/towering cumulus moving over windward Big Island. Elsewhere, isolated showers are noted.

Little overall change to the current weather regime is anticipated over the next couple of days, with bigger changes expected next week as the trade wind flow breaks down. Until then, low-level moisture will arrive on a moderate to locally breezy trade wind flow that will be gustiest in the afternoons. An increase in PWAT tonight and Saturday will ensure that windward areas receive some showers, while also bringing a slight chance of thunderstorms (CB). The ongoing forecast does indicate the potential for isolated CB over the Big Island slopes Saturday afternoon, and over waters around Kauai and Oahu, but some fine tuning of the forecast will likely be required as the approaching moisture slug moves W over the chain through Saturday.

A trend toward decreasing PWAT is expected on Sunday, although mid- level instability will likely linger until Monday. From Monday onward, trade wind speeds will diminish as the low to the distant NW moves E and weakens the surface ridge N of the islands. The associated high will move E, and as a weakening front associated with the low draws closer to the islands, the surface ridge will move over the islands for most of next week.

With a mid-level ridge building over the area from the W, a mostly dry, light wind pattern is anticipated. Some interior and upslope clouds may develop in response to daytime heating, but little in the way of rainfall is expected, with mostly clear nights and mornings. Depending on the evolution of the parent low N of the islands, the weakening front may move over Kauai over the second half of next week, with deterministic ECMWF guidance continuing this theme over the past 24 hours. Thereafter, significant differences are noted in the reliable guidance, with GFS favoring a weaker low and continued dry with light winds, while ECMWF favors a stronger low driving weak fronts across the area, with the potential for moderate NW to N winds. Either way, the recent persistent trade wind flow will be absent.


Surface high pressure well northeast of the islands will allow moderate to occasionally breezy trades to persist through the remainder of tonight. Thus, expect scattered showers to mainly affect windward locations with isolated activity elsewhere. There will be brief periods of MVFR ceilings and visibility associated with any heavier showers.

AIRMET Tango remains in effect for tempo moderate turbulence below 9000 feet for areas downwind (south through west) of island terrain. This AIRMET will likely remain in effect through most of the weekend before conditions improve early next week.


Moderate to locally strong easterly trade winds will persist through the weekend. The ongoing Small Craft Advisory for the windier waters surrounding Maui County and Big Island will likely need an extension in time through Sunday. The impetus to these recently robust trades, surface high pressure centered near 40N150W creating a tight pressure gradient down across the regional waters, will travel east as a cold front moves into the central North Pacific early next week. This will kill the upstream pressure gradient and considerably weaken winds to just light and variable by Tuesday. The near and offshore waters will experience these light to gentle breezes through next weekend.

A pair of northern Pacific systems moving south of the Aleutian Islands and into the Gulf of Alaska the next several days will produce a couple of north to northwest swells that will reach the islands Sunday and fill in Monday and then again Tuesday into Wednesday. The will result in heightened north and west-facing shore surf through next week. The first medium period near 4 foot northwest (330 degree) swell is timed to fill in during the day on Sunday. This swell should peak Sunday night and Monday surf to or above head high and then slowly fall through Tuesday. A slightly larger, medium period more northwest-turning-north swell (310 becoming 350 degree) swell is expected to reach the islands and fill in from late Tuesday through Wednesday. This swell will reinforce head to slightly higher surf heights through the middle of the week.

East-facing short period chop will significantly fall next week as east trades transform to more light to gentle breezes. South-facing shore surf will remain around waist high through early next week. There may be a slight bump in southeast swell (mixed into the background south swell) this weekend that is originating from a small fetch region 1,000 miles southeast of the island chain. This may lift surf from chest to near head high along more southeastern exposures from Sunday into mid week.

Watches, Warnings, Advisories

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

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