Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

325 pm hst Fri Sep 30 2022

Synopsis

Moderate trade winds will decline tonight and Saturday as an approaching front weakens the surface ridge north of the islands. Showers will remain focused over windward slopes, and daytime sea breezes will produce afternoon clouds and a few showers over leeward areas. The front will drift southward and bring an increase in rainfall to Kauai, Oahu, and Maui County on Sunday. The stalled and gradually dissipating front will keep high shower chances over the central portion of the island chain Monday into Wednesday as trade winds increase to moderate strength.

Discussion

Moderate trade winds will begin a decline tonight. An advancing cold front about 325 miles north of Kauai is eroding a surface ridge currently sitting about 350 miles northeast of the Big Island. As this ridge wanes tonight, moderate trades while begin a gradual decline, while a mid level ridge overhead maintains stable conditions. Aside from a few evening showers over leeward Big Island and Kauai, expect typical trade wind weather with limited showers confined to windward slopes.

As the front approaches on Saturday, trade winds will decline, and afternoon leeward sea breezes will become more extensive. Modest showers will favor windward slopes, and daytime sea breezes will lead to afternoon clouds and a few showers over leeward areas. The mid level ridge will weaken, but overall, stable conditions will hang on.

The front will drift southward over Kauai and Oahu Sunday morning and will likely reach Maui County by evening. A brief shot of locally breezy trades will fill in behind the front around Kauai, and possibly Oahu, but otherwise, gentle to moderate trade winds will prevail. Rainfall along the front will be focused across windward slopes, but showers will periodically move over leeward areas. A mid level to upper level trough moving over the state will produce enough instability to produce some heavy showers along the front, and an isolated thunderstorm will be possible.

The front will likely stall and slowly dissipate across the central portion of the island chain Monday through Wednesday. As a result, the highest rainfall chances will remain across windward areas of Oahu and Maui County, and the lingering mid to upper level trough will maintain some chance for heavy showers along the old frontal boundary. The surface high north of the state will weaken, favoring gentle to moderate trade winds.

Late next week, the GFS and ECMWF models show that the large scale pattern over Pacific will favor deep troughing to the north and northeast of the state, leading to suppressed trade winds and no organized weather systems affecting the islands.

Aviation

The current moderate trade winds will decrease some tonight with light to moderate trade winds expected for Saturday. Mostly dry conditions are expected this evening with a slight increase of showers expected toward the morning hours. VFR conditions are expected at most terminals with brief MVFR ceilings at times mainly over windward and mountain areas.

No AIRMETs are in effect.

Marine

Gentle to light east trade winds will hold through Saturday as a front approaches from the north. This front will sag southward into the local northern waters late Saturday night and Sunday. This will introduce stronger moderate to locally fresh trades along and north of the boundary. The front will stall out and dissipate over or near the island chain early next week. Gentle to moderate winds will prevail through mid week.

A small, medium period north northwest swell building in today will peak on Saturday then lower Sunday into early next week. At the same time, a slightly larger north swell will arrive tonight and then peak this weekend. A moderate, longer period north northeast swell is scheduled to arrive late Sunday. This swell may peak surf heights to near advisory levels along north-facing shores Monday. Another small, medium period northwest swell appears to move through the islands Monday through the middle of next week. Mainly background swell energy from the south and southeast will keep south-facing shore surf small the next few days. A small, long period south swell is expected to fill in Sunday into Monday with a slow and gradual decline through late next week. East-facing shore surf will remain small the next several days due weaker trade winds over and upstream of the islands.

Watches, Warnings, Advisories

None.

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