Daily Weather

Nov 28, 2014 4:00 AM HST

Synopsis
Surface high pressure far north of the islands will maintain strong and gusty trade winds across the state into this weekend. Lingering low-level moisture will continue to generate scattered showers primarily across windward sections of the Big Island and Maui. Elsewhere, expect brief showers mainly over windward and mauka sections for the next few days. Trade wind speeds are forecast to diminish slightly early next week, with trade showers remaining focused mainly across windward facing slopes and terrain.

Discussion
The tight pressure gradient south of a nearly stationary 1031 mb surface high near 38°N 161°W, or about 1100 miles north of Honolulu, is maintaining strong and gusty trade winds across the main Hawaiian island chain early this morning. Aloft, a mid-tropospheric ridge just east of the islands is also producing stable atmospheric conditions across the region. This is keeping the height of the trade wind inversion relatively low across the island chain. This low inversion is also allowing locally higher wind gusts to occur, especially in channeled areas in the vicinity of higher terrain.

Satellite imagery and radar reflectivity data continue to show low clouds and scattered showers over the windward sections of Maui and the Big Island. Elsewhere, the dry and stable atmospheric conditions are allowing mainly light trade showers to develop over some of the islands west of Maui and the Big Island.

The forecast models indicate the tight pressure gradient will remain across the state through Saturday as the surface high shifts slowly eastward. The coverage of trade showers will remain focused mainly along windward and mauka areas through the remainder of this weekend. The best chance for significant rainfall totals will remain across the windward Big Island, with rainfall amounts expected to be meager elsewhere.

There will likely be little change in the the weather pattern from late this weekend through early next week. As the surface high far north of the state weakens slightly as it shifts eastward, the pressure gradient will start to relax beginning Sunday. This will result in some gradual weakening of trade winds. By mid-week, a modest increase in low-level moisture may occur. The forecast models appear to indicate a mid-tropospheric trough may approach the state from the west next Wednesday. This feature may allow the trade wind inversion height to lift slightly. The increased moisture and the higher inversion combined with some dynamic lift as the mid-level trough arrives may allow the coverage of trade showers to increase across windward and mauka areas starting around mid-week.

Aviation
Fresh and gusty trade winds beneath a well defined inversion aloft are expected to persist today. Wind speeds in excess of 25 kt are expected along northeastern exposures and over mountain ridges, with some local channeled areas potentially experiencing stronger wind gusts. As a result, airmet tango remains in effect for moderate low-level turbulence in the air space leeward of all mountains today.

Trade wind showers are expected to continue affecting mainly windward facing slopes and terrain. Low clouds and numerous showers across the northeast Big Island have required the issuance of airmet sierra for mountain obscuration over the windward Big Island. These conditions are expected to persist beyond sunrise this morning. Brief MVFR cigs may also occur along some windward sections of the smaller islands, especially Maui. Otherwise, VFR conds are expected to prevail across most other areas through this afternoon.

Marine
The current large north northeast swell continues to slowly subside early this morning. The most recent readings from the Hilo, Pauwela, mokapu and Waimea buoys indicate the significant wave height is still around 9 feet with a wave period of about 13 seconds. As a result, the high surf advisory remains in effect for east facing shores of Kauai, Oahu, Molokai, Maui and the Big Island today. This swell is forecast to gradually shift to a more northeasterly direction, which combined with locally generated wind waves, will likely keep surf elevated and choppy along east facing shores through most of this weekend.

The current north northeast swell may also cause moderate harbor surges within north facing harbors, such as Kahului and Hilo harbors, through today. Large breaking waves are also possible near the entrances to these north facing harbors. A marine weather statement has been issued to cover this potential threat.

A small craft advisory /sca/ remains in effect for all Hawaiian coastal waters, except the leeward Maui county marine zone through Saturday afternoon due to a combination of strong trade winds and rough seas. Even though seas may eventually subside later this weekend, the typically windy waters adjacent to the islands of Maui and Hawaii counties will likely remain in the SCA from Saturday night into early next week due to the trades remaining locally strong.

Watches/Warnings/Advisories
High surf advisory until 6 pm HST this evening for east facing shores of Kauai, Oahu, Molokai, Maui and the Big Island.

Small craft advisory until 6 pm HST Saturday for all Hawaiian waters, except the Maui county leeward waters.





Advisory and Warning Criteria
Location Advisory Warning
North-Facing Shores 15 Feet (8') 25 Feet
West-Facing Shores - Big Island 8 Feet (4'+) 12 Feet
West-Facing Shores - Remaining Islands 12 Feet (7') 20 Feet
South-Facing Shores 8 Feet (4'+) 15 Feet
East-Facing Shores 8 Feet (4'+) 15 Feet

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