Area Forecast Discussion for Hawaii

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Issued: Oct 01, 2014 8:40 PM HST

Synopsis
Light trade winds Thursday and Friday will yield to light and variable winds over the weekend, and into next week. The light trades will deliver a few showers to windward coasts and slopes, while also allowing afternoon clouds and showers to develop over interior and leeward areas. The light and variable winds will lead to clear nights and mornings, and cloudy afternoons and evenings, with clouds and showers favoring interior and upslope areas. A more stable air mass developing over the islands should limit shower intensity. Muggy conditions will continue.

Discussion
Heavy downpours that affected parts of the state this afternoon have all dissipated, and skies over the islands are generally clearing, especially over leeward areas. Latest surface analysis depicts a surface ridge just N of Kauai, extending SW from a high centered about 1900 miles NE of the islands. With the ridge in this position, a light E to SE flow exists in the low levels over the islands, with island-scale land/sea breeze circulations overriding this flow, especially at the surface. A low aloft is centered about 500 miles ENE of Oahu and moving slowly E, with an associated weak trough extending W over the state. High clouds S of the trough axis are streaming over the Big Island and adjacent waters.

Overnight, expect skies to become generally clear over leeward areas, while the light e/se winds at cloud level push a few showers over windward areas. Radar shows that some of these small showers have intense cores, most recently noted over windward Kauai, Kauai leeward waters, and SW of Maui and Lanai.

A somewhat less active weather pattern is expected to develop over the islands by Thursday, as a slightly drier airmass spreads in from the E, and mid level temps warm as the low aloft departs. Models indicate that a weak trade wind flow will develop Thursday and persist through Friday, as the ridge drifts N slightly. Still expect afternoon seabreezes Thursday and Friday afternoon to drive interior and leeward clouds and showers, but the showers are not expected to as widespread or intense as we have observed the past few days. A hybrid weather pattern seems likely, with limited showers and clouds across windward areas during the nights and early mornings, and clouds with scattered showers across leeward and interior in the afternoons. Warm and muggy conditions will continue, with above normal sea surface temperatures surrounding the islands contributing to rather high dewpoints. High clouds near the Big Island should clear out by Thursday as the trough aloft weakens.

During the weekend and into the middle next week, a cold front several hundred miles NW of the state will move slowly SE, pushing the surface ridge over the state, supporting a pure land/sea breeze pattern over the islands. Scattered showers will develop mainly across interior and leeward areas each afternoon, with clearing each night as land breezes take over. Continued above normal dewpoints and the lack of trade winds will maintain warm and muggy conditions. Developing light SE background flow may allow areas of volcanic haze to spread over the smaller islands during this time. The front is forecast to retreat northward by the middle of next week, allowing the ridge to shift N, and a light trade wind flow is expected to return.

Aviation
Light background wind flow will allow land breezes over the islands tonight. Expect decreasing low cloud cover as a result, with VFR conditions prevailing. Sea breezes will produce low clouds over the leeward and island interior sections by Thursday afternoon. Mvfr cigs, isol or tempo mtn obsc will be possible as the cloud cover builds through the day. On Thursday, the atmosphere will be a little bit more stable than the past few days, so expecting shallow or moderate cloud tops and lighter associated showers than the past few days.

Marine
Surf observations and buoy data indicate that the current S swell remains sufficiently high to maintain advisory level surf along S facing shores. The advisory currently extends through the day Thursday, with surf expected to decrease below advisory levels Thursday evening. No other significant S swells are expected through the weekend. A moderate NW swell appears to have peaked earlier today, and will gradually subside through Friday, just as a new moderate NW swell begins to build. This swell is expected to peak late Saturday and diminish into Monday, just as another similarly- sized NW swell begins to arrive. Resultant surf is expected to remain below advisory levels along N and W facing shores. After a bit of a hiatus, the Oahu surf discussion /srdhfo/ was updated today, and contains details that can be loosely applied statewide.

Winds and seas will remain below small craft advisory levels through the forecast period.

Watches/Warnings/Advisories
High surf advisory until 6 pm HST Thursday for south facing shores of all islands.


DISCUSSION/MARINE...BIRCHARD

AVIATION...FOSTER