Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

400 pm hst Wed Oct 21 2020


A humid and somewhat unstable southeast to south flow will persist over Kauai and Oahu for the next couple of days, leading to periods of locally heavy showers. A drier and more stable east to southeast flow will prevail over the eastern half of the state, leading to mainly isolated showers. A brief increase in stability, and possibly easterly winds, is expected during the weekend, followed by a return to humid southerly winds and potentially unsettled weather for portions of the island chain early next week.


Kauai and Oahu remain locked in a humid and somewhat unstable southeast to south flow. This is the result of a highly amplified pattern, featuring deep high pressure between California and Hawaii and a deep north Pacific trough centered just west of the islands. While a few showers are flaring near Kauai this afternoon, Oahu has seen the brunt of the shower and thunderstorm activity so far today. Expect the highest chances for showers and thunderstorms to shift to Kauai overnight.

A weak surface ridge near Maui County is maintaining rather dry and stable conditions for the eastern half of the state. High clouds from the nearby trough have thinned during the day, and the only location with extensive low cloud cover is leeward big Island, where afternoon sea breezes have produced minimal rainfall. Land breezes will develop over Maui County and the Big Island overnight, with little chance for rainfall.

Little change is due Thursday and Friday. Chances for showers, some briefly heavy, will be highest on the western end of the state, favoring Kauai and, to a lesser degree, Oahu. East winds near the Big Island will veer over the island chain and become light southerly near Kauai, leaving a dominant land and sea breeze pattern in place.

The upper level trough north of the state is expected to briefly fill during the weekend. This could lead to stronger easterly winds spreading to Maui County and a decrease in moisture and shower activity near Kauai and Oahu.

Another north Pacific trough will dig toward the state early next week, likely weakening winds again and possibly leading to an increase in showers across portions of the state.


A low pressure complex northwest of the islands will continue to enhance convective activity over the western half of the state. Isolated thunderstorms will remain possible over Oahu through late this evening and over Kauai and Niihau including adjacent waters through Thursday. Brief periods of MVFR conditions are expected from Kauai to Oahu with isolated IFR coverage possible in the heaviest shower bands.

Elsewhere, over Maui County and the Big Island, a drier more stable air mass prevails. Light sea breezes will be replaced by weak land breezes after sunset. Low topped cumulus buildups over leeward and interior Big Island will dissipate overnight, but will likely redevelop tomorrow afternoon.

Bands of high clouds will continue to spread across the area from the southwest.

AIRMET Sierra for mountain obscuration across Oahu will likely remain necessary through at least early evening. Conditions may spread to Kauai overnight.


A surface high far northeast of the area and a surface trough to the northwest will persist through next Monday. Locally fresh easterly trade winds will continue around the Big Island of Hawaii with gentle variable winds elsewhere over the coastal waters. A low aloft will keep the atmosphere unstable, so isolated heavy showers and thunderstorms will continue over the coastal waters and northern offshore waters. The global models show a front moving into the offshore waters from the northwest Monday night. The front may reach the coastal waters by the middle of next week.

A northeast swell producing moderate surf along east facing shores today will slowly subside through Friday. A small and rather short-period south swell will continue through the weekend. Surf along other shores will remain small through Friday. A northwest swell is expected to arrive Saturday night, peak near the advisory threshold Sunday, then subside through early next week.

Peak monthly tides and higher than normal sea levels may produce local coastal flooding around the time of the high tide tomorrow morning. See the Special Weather Statement for more information.

Watches, Warnings, Advisories


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