Area Forecast Discussion for Hawaii

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Issued: Aug 28, 2014 8:30 PM HST

Synopsis
Trade winds will weaken, becoming light by early next week. Land breezes will encourage cloud buildups and a few showers in island interiors by day, while land breezes will produce clearing at night. The trade winds may start to return by the middle of next week, though they will remain on the light side of average.

Discussion
Surface observations around the main Hawaiian islands reported trade wind speeds on the light side of average, though still not quite as light as guidance suggested earlier. The general trend toward lower speeds did appear to hold, though the forecast may need a bit of nudge upward if it outpaces reality through too much of the night.

Precipitation across the state was sparse, or at least mostly missing the rain gauges, but where they did occur some of the showers dropped over one quarter of an inch in three hours. Although a mid-level ridge prevailed to the southeast of the islands, a broad trough was present to the northwest through northeast. Most likely it was this feature that encouraged active towering cumuli to develop near 30°N, and contributed to shower development here by raising the inversion. Also, though precipitable water over the state and vicinity was on the dry side of average, satellite estimates showed a moister plume approaching Maui county from the east. Guidance predicted similar moisture surges to reach the state periodically into next week, though with time the different solutions ended up following different periods, complicating a deterministic forecast. With trade winds very weak by Monday, diurnal showers produced by sea breezes may dominate weather from day to day, with the moisture surges only amplifying the magnitude of the daily event.

In any case, ridging aloft will expand over the islands from the southeast during the weekend, so by next week subsidence will start to crush down the inversion. However, a front north of 30°N will keep the surface ridge anchored near the islands well into next week, so the trade winds will recover only very slowly.

Marine
Easterly swell is still forecast to build over the Hawaiian coastal waters tonight and Friday, but confidence regarding the resulting surf heights along east-facing shores was lower than earlier. Advisory-level surf also will affect south-facing shores through Friday. Interaction between the long-period east and south swells may produce strong currents in some areas. The surf from both swells will decline slowly through the weekend.

With trade wind speeds more likely to diminish than to increase through the first half of the forecast period, it is doubtful that they will approach the criterion for a small craft advisory. However, the long-period easterly and southerly swell could produce locally large and confused seas.

Watches/Warnings/Advisories
High surf advisory until 6 am HST Saturday for Niihau, Kauai windward, Kauai leeward, Oahu south shore, Waianae coast, Oahu Koolau, olomana, Molokai, Lanai makai, Kahoolawe, Maui windward west, Maui leeward west, Maui central valley, windward Haleakala, leeward Haleakala, Kona, south Big Island, Big Island north and east.


RYSHKO