Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

1005 am hst Fri Sep 24 2021


Locally breezy trade winds will bring passing clouds and showers to mainly windward and mountain areas through the weekend and into next week. A weak surface trough passing south of the islands will boost rainfall across windward Big Island late Saturday and Sunday.


A surface ridge to the N of the islands will maintain trade winds through the weekend and into next week, with locally breezy conditions prevailing. The island atmosphere will remain stable, but this will not prevent the trade winds from occasionally delivering showery low clouds that will dampen primarily windward and mauka areas from time to time. One such cloud cluster is moving in on Kauai, with another cluster a little ways E of windward Big Island and Maui that will likely arrive by this evening.

A more significant area of moisture, associated with a surface trough, is about 900 miles SE of the Big Island. Model guidance continues to indicate this trough and associated moisture moving toward the islands over the next 2 days, with the trough passing S of the islands Saturday night and Sunday, potentially leading to an increase in showers over the Big Island (and maybe windward Maui). Otherwise, guidance is not indicating any organized moisture sources into next week. Trade winds will likely receive a boost in strength as the trough passes as well.


Clouds and showers associated with a weak trough embedded within locally breezy trades reached Oahu during the predawn hours prompting issuance of AIRMET Sierra for mountain obscuration. Conditions have begun to improve and this AIRMET is expected to expire as scheduled around noon HST. Trades will undergo modest strengthening and clouds and showers will increase in coverage over windward and mauka areas this evening through the first half of tonight as aforementioned trough axis advances through the island chain. AIRMET Sierra may be needed for portions of the island chain during this time.

AIRMET Tango for lee turbulence has been issued and will remain in effect through at least tonight and possibly through Saturday.


A ridge of high pressure far north of the state will generally maintain moderate to locally strong trades through early next week. A surface trough is expected to move into the southeast offshore waters Saturday and pass south of the Big Island Saturday night. As this trough passes south of the state, the pressure gradient will tighten and will result in a slight increase in wind speeds Saturday night through Sunday. Widespread fresh to strong winds are expected during this time and we could see the Small Craft Advisory (SCA) expanded into other areas. Currently our SCA is only in effect for the windier waters around Maui County and the Big Island.

Buoy observations indicate the new long-period north- northwest swell is larger than initially expected and will bring high surf to exposed north and west facing shores. The swell is likely near its peak around Kauai this morning and should continue to build across the rest of the island chain through this afternoon. A High Surf Advisory is in effect for north and west facing shores of Kauai and north facing shores of Oahu, Molokai and Maui. For north facing shores next week, small surf is expected with only small swells from the north and north-northwest. No significant swells are expected along south and east facing shores.

Watches, Warnings, Advisories

High Surf Advisory until 6 AM HST Saturday for north and west facing shores of Kauai and Niihau and for north facing shores of Oahu Molokai and Maui.

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

Small Craft Advisory until 6 AM HST Saturday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters.

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