Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

1000 am hst Sat aug 19 2017


Light to moderate trade winds will prevail through early next week as a ridge of high pressure north of the State weakens. The lighter winds will allow local sea breezes to be more prominent across leeward areas, with an increase in clouds and showers each afternoon and evening. Meanwhile, low clouds carried by the trades will still continue to affect windward and mauka areas at times. Tropical moisture from the east will bring an increase in shower coverage and muggy conditions beginning tonight and continuing through Tuesday. Expect drier and windier conditions to return Wednesday through late next week as high pressure strengthens north of the island chain.


Currently at the surface, a 1030 mb high is centered around 1850 miles northeast of Honolulu, with a ridge axis extending southwest from the high to a location around 800 miles north of Kauai. The resulting gradient is driving light to moderate trade winds across the island chain this morning. Visible satellite imagery shows clear to partly cloudy conditions in place across much of the state, however a band of low clouds and showers is bringing mostly cloudy conditions to windward sections of Maui County and the Big Island. Radar imagery shows trade wind showers moving into windward areas, with a few showers occasionally spilling over into leeward areas. Shower coverage is greatest over windward sections of Maui County and the Big Island. Main short term concern revolves around rain chances over the next few days.

Rest of today, The high pressure ridge to the north of the islands will keep light to moderate trade winds in place across the State, with localized sea breezes developing in the more sheltered leeward areas. Clouds and showers will favor windward and mauka areas, with a few showers drifting into leeward areas, primarily this morning. Showers are expected to be most prevalent over windward sections of Maui County and the Big Island this morning, with daytime heating then decreasing shower coverage during the afternoon hours.

Tonight through Tuesday night, Light to moderate trade winds will continue through early next week, with some slight strengthening of the trades expected late Tuesday and Tuesday night as high pressure strengthens to the north of the State. Aloft, an upper level low will drift southwestward and closer to the islands tonight through Sunday night, then pivot off to the west and further away from the state early next week. This will act to de-stabilize the airmass over the islands a bit, particularly over the western end of the State. At the same time, some deeper moisture will begin to work its way into the area from the east tonight, with precipitable water values climbing into the 1.5 to 2.0 inch range Sunday through Tuesday night.

As for sensible weather details, we should begin to see an increase in trade wind showers beginning tonight, with periods of showery weather expected Sunday through early next week, particularly in windward areas. In leeward areas, we should also see an increase in showers, with some localized downpours possible each afternoon into the early evening in association with sea breeze enhancement. Additionally, due to the more unstable airmass over the islands, isolated thunderstorms will be possible over leeward and interior sections of the Big Island each afternoon into the evening. Isolated thunderstorms may also affect the waters north of Oahu and around and including the island of Kauai Sunday night through Monday evening as the upper level low makes its closest approach. Conditions will also become quite humid Sunday through early next week, as the trades lighten up and dewpoints climb into the lower and middle 70s across the area.

Wednesday through Friday, High pressure will strengthen north of the State, bringing moderate to breezy trade winds back to the island chain through the end of the work week. Drier more stable conditions are also expected, with more comfortable trade wind weather expected. Showers will continue to favor windward and mauka areas, with a stray shower reaching leeward areas from time to time due to the strength of the trades.


Vfr conditions prevail at most TAF sites with occasional MVFR conditions being reported at PHTO and PHLI. Occasional MVFR conditions are also occurring over windward slopes for most of the state as moisture riding in on the trades interact with island terrain. AIRMET Sierra is currently in effect for N through E sections of Maui and The Big Island as satellite imagery shows an area of enhanced moisture moving into those areas. Conditions may improve somewhat later today.


Winds and seas are expected to remain below Small Craft Advisory levels through early next week as a trough aloft helps weaken the pressure gradient. The trough may make showers more active, and could even produce some thunderstorms Sunday night through Monday night. Wind speeds will increase again later next week.

Surf along all shores is expected to remain well below the High Surf Advisory threshold through the middle of next week.

The peak high tides for the month will occur over the next few days. Water levels are running as much as a foot above the predicted levels due to an eddy moving westward through Hawaiian waters. Coastal flooding is possible around the times of the high tides through early next week. See the Special Weather Statement for more details.

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