Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

330 am hst Tue Sep 19 2017


High pressure to the north-northeast of the islands will keep a moderate to breezy trade wind flow in place through the remainder of the work week. The trades will ease to light and locally moderate levels over the weekend into early next week, as the high north of the State weakens. Showers will favor windward and mauka areas through the period, with a stray shower reaching leeward areas from time to time. Showers will be most prevalent during the night and morning hours.


Currently at the surface, a 1030 mb high centered around 1350 miles to the north-northeast of Honolulu, is driving moderate trade winds across the island chain early this morning. Infrared satellite imagery shows clear to partly cloudy skies in most areas, with a few patches of enhanced cloud cover riding into windward areas with the trades. Meanwhile, radar imagery shows scattered windward showers with a stray shower drifting leeward from time to time. Main short term concerns revolve around trends in the trade wind speeds and rain chances.

Today through Friday, High pressure to the north-northeast of the island chain will maintain a moderate to breezy trade wind flow through the remainder of the work week, but we should see a gradual decrease in the trade winds speeds as the weak progresses. Bands of clouds and showers will continue to drift through the island chain, with the showers focusing primarily over windward and mauka areas during the night and morning hours. A stray shower may also drift into leeward areas from time to time due to the strength of the trades.

Friday night through next Monday, High pressure will weaken and shift eastward well to the northeast of the State over the weekend into early next week, as a cold front tracks eastward across the central Pacific well north of the island chain. This will likely result in a decrease in the trade wind speeds across the area, with generally light to locally moderate trades expected. Additionally, some localized land and sea breezes may become more prominent in the more sheltered areas. A windward/mauka favored shower pattern is expected to continue through the period, although we may see an increase in leeward and interior shower development during afternoons and evenings due to the easing of the trades and an upper level trough in the vicinity of the islands.


The Strong High far north of the islands will continue to support breezy trade winds. AIRMET Tango remains in effect.

Ragged clouds and isolated showers carried in by the trade winds will favor the eastern exposures of all islands both day and night. Convective showers are expected to redevelop over interior and leeward portions of the Big Island this afternoon and persist through early evening. Brief MVFR conditions possible in showers statewide, otherwise VFR conditions will prevail all areas.


Strong high pressure far NNE of the islands will support fresh to strong trade winds as it remains nearly stationary through Wednesday. The high will gradually diminish late in the week and over the weekend, with the associated ridge being shunted toward the islands as a front approaches from the NW. This will lead to a gradual decrease in trade wind speeds, especially over the weekend, potentially continuing into early next week. A Small Craft Advisory is posted for marine zones where winds are most accelerated by island terrain, and has been extended into Thursday.

The nearby and upstream fetch of elevated trade winds will support choppy short-period surf along E facing shores the next couple of days. Otherwise, the only significant swell source is a low to the NW that will send a 3' WNW swell toward the islands Thursday and Friday. The recently updated Oahu Surf Discussion (SRDHFO) has additional information on swell sources.

Watches, Warnings, Advisories

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

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