Area Forecast Discussion for Hawaii

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Issued: Sep 03, 2015 8:30 PM HST

A humid and unstable air mass will support spotty, but locally very heavy, showers and thunderstorms into Friday. Weak trade winds and somewhat more stable conditions will spread from the east on Friday night and Saturday, though the weather will remain hotter and more humid than usual. Prevailing winds will diminish into next week, leading to stagnant conditions and increased afternoon showers again.

Although precipitable water remained down slightly from the insane values earlier in the week, especially over the Big Island, slight convergence in the weak southeast surface flow around the state continued to encourage deep convection. A shallow mid-level trough extending southeast over the islands from tropical cyclone Ignacio also contributed.

Much of the convection shifted offshore during the late afternoon, but it will be some hours yet before land breezes become strong enough to enforce this change at all reliably. In particular, the convective plume extending northwest from Molokai threatened to move over Oahu.

On Friday, a shift in the pattern of tropical cyclones driving regional weather may bring a noticeable change from the humid and stagnant conditions. Although tropical cyclone Jimena will remain far enough away not to affect the islands directly, a swath of northeast winds will extend past the north edge of the storm and over the state. Guidance predicted fairly modest wind speeds, but they stand a good chance of being stronger than the current prevailing flow. Most solutions also predicted a slightly drier air mass to advect in on the winds. Convergence along the leading edge of the new air mass may encourage a slight burst of showers at first, but by Saturday subsidence around Jimena will help to stabilize conditions.

This period of trade winds will end by Sunday night once Jimena moves far enough north, leaving feeble northerlies in place. Stagnant flow into next week will permit evaporation to moisten the air mass again, and diurnal effects will prevail. Conditions during the latter part of next week depended heavily on the precise track of Jimena.

An area of low level convergence drifting westward over the central and western part of the state, .combined with an upper level trough and copious tropical moisture, will continue to generate scattered heavy showers and isolated thunderstorms overnight and Friday. Areas of MVFR with localized IFR, along with lightning, turb and icing will be possible with any thunderstorms.

Airmet sierra for mtn obsc is currently posted for the entire state. This airmet will likely be dropped for some islands during the evening, but additional airmets will likely be needed at times late tonight and Friday.

Incoming east swell from hurricane Jimena continued to require a high surf warning for east-facing shores of most islands. East-shore surf will remain near warning levels through the weekend, and likely into early next week.

A small craft advisory also remained in effect through Friday afternoon for hazardous seas produced by the swells from Ignacio and Jimena. Winds are expected to remain below the advisory threshold this week.

A long-period swell from the south-southwest will produce advisory-level surf along south-facing shores this weekend. Please refer to the latest collaborative nearshore swell and wind forecast /srdhfo/ for more details on the swells.

Flash flood watch through Friday afternoon for all Hawaii islands.

High surf advisory from 6 am Friday to 6 pm HST Saturday for Niihau, Kauai leeward, Oahu south shore, Waianae coast, Lanai makai, Kahoolawe, Maui leeward west, leeward Haleakala, Kona.

High surf warning until 6 pm HST Saturday for Kauai windward, Oahu Koolau, olomana, Molokai, Maui windward west, Maui central valley, windward Haleakala, south Big Island, Big Island north and east.

Small craft advisory until 6 pm HST Friday for Kauai northwest waters, Kauai windward waters, Kauai channel, Oahu windward waters, Kaiwi Channel, Maui county windward waters, Pailolo channel, Alenuihaha channel, Big Island windward waters, Big Island southeast waters.