Area Forecast Discussion for Hawaii

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Issued: Aug 21, 2017 8:43 PM HST

Synopsis
Plenty of moisture combined with an upper disturbance over the region will support heavy showers and thunderstorms through Tuesday. Breezy trade winds and drier conditions will return beginning Wednesday and continue into the upcoming weekend as high pressure strengthens north of the state and the upper disturbance shifts away from the region.

Discussion
Heavy showers and thunderstorms will remain in the forecast through Tuesday due to a combination of deep tropical moisture and a broad upper low impacting the region. The latest water vapor imagery showed this upper low centered over the offshore waters west of Kauai and plenty of middle- to upper-level moisture streaming northward over the state. Although much of the heavy showers and storms that led to flash flooding earlier this afternoon have decreased in coverage, the threat for flash flooding will remain in place through the overnight hours due to saturated grounds and swollen streams (especially over Kauai and Maui). The current forecast package remains on track and no significant updates are anticipated this evening.

Although the upper low west of Kauai is forecast to continue to drift westward and away from the area through the day Tuesday, a sufficient amount of instability (highest over Kauai) and tropical moisture combined with daytime heating will support another active day. Similar to today, the best chance for heavy showers and thunderstorms will remain over Kauai and the Big Island through the afternoon and evening periods. Elsewhere, the bulk of the showers will remain focused over windward and mauka areas as the trades are forecast to gradually begin to strengthen.

Breezy trades and drier conditions are forecast to return across the state beginning Wednesday, as the upper low shifts away from the region and high pressure builds to the north.

Aviation
A surface high far north of the area will maintain moderate low level east northeast winds blowing across the area. A low aloft about 200 nm west northwest of Kauai is making the atmosphere unstable. The low is forecast to move northwest slowly.

Thunderstorms will continue over the area due to the low aloft.The low level winds will continue to carry showery low clouds over northeast sections of all the islands and produce widespread MVFR and isolated IFR conditions. AIRMET Sierra remains in effect for mountain obscuration for Kauai, Oahu, Maui and the Big Island. Mountain obscuration may continue through much of the night.

Conditions will gradually improve on Tuesday as the low aloft moves farther away from the area.

Marine
Trade winds are expected to reach Small Craft Advisory criteria on Tuesday for the normally windier channels between the islands and the coastal waters south of the Big Island.

An upper level low will continue to produce isolated thunderstorms over coastal and offshore waters through Tuesday. The highest risk is for the waters in the west half of the forecast area. More stable and drier conditions are expected on Wednesday and through the second half of the week.

East-facing shores should see a slight increase in surf heights as the trade winds rebuild on Tuesday. A longer period east swell may also arrive during the second half of the week as a result of tropical cyclone activity in the eastern Pacific. South-facing shores may see a slight increase in surf this weekend from a swell train that originated from a Tasman Sea gale. A small background southeast swell is also expected through the forecast period.

Watches/Warnings/Advisories
None.


DISCUSSION...Gibbs

AVIATION...Donaldson

MARINE...Powell