Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

935 am hst Wed apr 21 2021

Synopsis

Light winds will continue through today as troughing to the west of the islands disrupts normal trade wind flow. Trade winds will begin to return Thursday, then last through the rest of the forecast period, as high pressure to our north reasserts itself.

Discussion

Troughing to the west of the islands keeps local wind flow rather weak and shifted to southeasterlies, maintaining a Big Island blocking pattern. The resulting light environmental flow across the smaller islands will allow development of local land and sea breezes today and tonight. Overnight soundings show PW around 1.5 inches, providing enough moisture for interior showers this afternoon as sea breezes develop. Interior clouds and showers will last into this evening before clearing overnight.

Models show trade winds will begin to return Thursday as the troughing west of the islands retreats westward and weakens in the face of strengthening high pressure to our north and northeast. This will shift the local cloud and shower pattern back to a trade wind setup, with both favoring windward areas. Trade flow will continue through the rest of the forecast period, becoming locally breezy by Thursday night into Friday.

An upper trough expected to develop across the islands this weekend will introduce cooler temperatures aloft and may destabilize our airmass enough to enhance trade showers into early next week. Light snow may occur on the highest Big Island summits late Saturday. Models show this upper trough lifting out by the middle of next week.

Aviation

Vfr conditions expected today as troughing weakens northwest of the state. Trade winds will gradually build in from the east through Thursday, but leeward sections will have sea breezes develop this afternoon that will bring cloud build ups and few showers to these areas. Brief/isolated mountain obscurations may occur with the cloud build ups. Expect decreasing clouds tonight for most areas, with the exception of the windward side of the Big Island which will see incoming clouds and light showers as the trades push back in. No AIRMETs are in effect at this time, which should carry through at least midnight tonight.

Marine

A trough northwest of Kauai will gradually dissipate through Friday, allowing light trades to spread westward by Thursday. A surface high is forecast to pass north of the islands Thursday through Friday. This will result in a boost in the trade winds with speeds reaching locally strong for waters around Maui County and south of the Big Island through Sunday, then decreasing Sunday night into Monday. A Small Craft Advisory for those nearshore waters will be issued as early as tonight.

A small to moderate northwest swell arriving today will lead to a bump in surf along north and west facing shores. This swell will max out Thursday then decline through Friday. A small to moderate, long-period west-northwest swell is slated to arrive next Monday night and peak Tuesday, followed by a slow decline. A continuous stream of small, but long period swell from the south and southwest will maintain small surf along south facing shores throughout the forecast period. Surf along the east facing shores will be on the rise Friday and Saturday when the returning trades turn breezy. Expect surf to build to small to moderate heights, then diminish Monday as the trade winds weaken.

Watches, Warnings, Advisories

None.

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