Area Forecast Discussion for Hawaii

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Issued: Nov 20, 2017 8:28 PM HST

Synopsis
A surface low far northeast of the islands will continue to move away from the region, while an upper level trough will remain over the area through this weekend. Low clouds and showers will remain focused along north facing slopes and coasts tonight. A strong new surface high will build far northwest of the area on Wednesday, then move southeast into early next week. Gusty trade winds are expected Thanksgiving and Friday, before easing slightly this weekend. Expect windward trade showers, occasionally spreading leeward.

Discussion
The pressure gradient between a surface high far northwest of the islands and a surface low far northeast of the region is producing light to locally moderate northerly winds. This low- level flow continues to advect cool and relatively dry air into the islands. Aloft, loops of water-vapor satellite imagery indicate middle- and upper-tropospheric troughing extends northeast from the islands. This upper-level troughing is having little significant affect on atmospheric stability across the state. A significant low-level temperature inversion of around 5 thousand feet was observed on Kauai, and increased to near 7 thousand feet over the eastern Big Island in the early afternoon balloon soundings.

The northerly winds continue to transport isolated showers over north and northeast facing slopes and coasts early this evening according to loops of satellite imagery and radar reflectivity data. However, rainfall will continue to be relatively light over most areas. Note that there is a chance the north winds, and some slightly higher inversion heights could boost rainfall along portions of the north and northeast slopes of the Big Island later tonight and early Tuesday morning.

The trough aloft continues to produce strong west winds over the summits of Mauna Kea and Mauna Loa on the Big Island of Hawaii. As a result, a High Wind Warning has been issued for the summits through early Tuesday morning. The winds are expected to gradually subside as the trough weakens later tonight and Tuesday. We expect to issue a Wind Advisory for the summits on Tuesday once the High Wind Warning is lowered.

Through Wednesday, we anticipate the surface low far northeast of the area will move northeastward and weaken, while the surface high moves southeastward and weakens. Winds over the islands will gradually shift out of the northeast Tuesday. The forecast guidance shows a low-cloud band moving down from the northwest and reaching Kauai County by Tuesday afternoon. This feature is expected to then move over Oahu and Maui County Tuesday night, before pushing down over the Big Island by Wednesday evening. This cloud band will bring an increase in low clouds and showers, especially over windward areas.

Starting Wednesday night, a new strong surface high far northwest of the islands will start moving southeast. The northeast trade winds over the islands will strengthen and become breezy. Wind speeds will likely decrease again starting Saturday, but trade winds will likely persist through early next week.

Aviation
Cool northerly winds will continue to push bands of clouds and showers over the islands tonight. The highest coverage for shower activity will occur over mountain areas and the northern slopes of all islands. Relatively drier air will lead to higher than normal cloud bases. VFR conditions prevail with only isolated MVFR in moderate showers.

No AIRMETs are in effect and none are expected.

Marine
Surf along the north-facing shores of the state remains elevated. Evening buoy observations show little change during the past several hours, and heights remain above Wave Watch III guidance. The effect of the low bias in the guidance is that surf height will remain above High Surf Advisory (HSA) criteria through Tuesday morning. The north swell is expected to diminish Tuesday afternoon.

A Small Craft Advisory (SCA) remains in effect for all Hawaiian waters except Maalaea Bay and waters south of the Big Island due to the large north swell causing the combined seas to be above the 10-foot threshold. With wave heights running above guidance, the SCA has been extended through Tuesday morning.

Winds over the coastal and offshore waters remain generally from the north due to a low pressure system northeast of the state. The low is in the process of weakening and moving northeastward away from the island chain. As this occurs, a high pressure ridge will slowly build north of the forecast area through the week. Winds will slowly veer toward a trade wind direction and increase, reaching SCA levels Wednesday with near gale-force winds possible in some of the channels by Thursday.

In addition to the increasing trades, a new north-northeasterly swell is forecast to arrive later this week. The swell is currently projected to be a few feet larger than the current swell with a slightly longer period. This will translate to possible warning level surf for north facing shores, building late Wednesday and peaking on Thanksgiving Day. The swell direction will result in some wrap into the east facing shores so warning level surf is a possibility for these areas as well.

Watches/Warnings/Advisories
High Surf Advisory until noon HST Tuesday for north facing shores of Niihau, Kauai, Oahu, Molokai, Maui, and the Big Island.

High Wind Warning until 6 AM HST Tuesday for the Big Island Summits.

Small Craft Advisory until noon HST Tuesday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters.