Area Forecast Discussion for Hawaii

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Issued: Dec 14, 2017 3:42 PM HST

Synopsis
Cool and windy conditions will remain into Friday before winds decrease and the chance of rain increases. An upper level trough will bring unsettled weather to portions of the state Friday and through the weekend. Partly cloudy skies with scattered showers are expected over the weekend with pockets of heavy rain and isolated thunderstorms.

Discussion
High pressure north of the state is providing gusty north to northeast winds with quick hitting showers affecting windward and north facing slopes with more enhanced showers over windward Maui County and the Hamakua Coast. Dew points remain below normal for Hawaii making sensible temperatures feel cooler. A wind advisory continues for most areas through tonight. Changes are afoot for Friday as winds slowly decrease and a wetter weather pattern moves in. A surface trough will gradually move over the state from the east and push the gusty trade winds westward Friday night.

Inclement weather will be possible starting Friday as the upper low drops from the higher latitudes and reaches a position northeast of the islands Friday. The upper low will bring instability and cold temperatures to the middle and upper atmosphere allowing for increasing, and heavier showers. The instability aloft, the trough at the surface, and increasing low level moisture will lead to the possibility of heavy rain and isolated thunderstorms, and the chance of snow for the Big Island summits. The current forecast favors the eastern half of the state for the heavier rain with the chance of afternoon thunderstorms Friday and Saturday with daytime heating and sea breezes expected to tap into the instability aloft.

The upper low is expected to maintain a position just NE of the islands on Saturday, and then slowly pull away to the NE on Sunday. The low will maintain at least slight instability through the weekend, with the threat of heavy rain remaining. Saturday looks to be a convective day with light winds and clear skies in the morning, switching to sea breezes with clouds, showers, and isolated thunderstorms in the afternoon. By Sunday, the surface trough is forecast to be located west of the state, bringing southerly winds across the area with scattered showers with some embedded downpours. Widespread showers are becoming a distinct possibility Monday and Tuesday with another upper level trough dropping over the islands from the northwest, and deep tropical moisture pushing up from the southeast.

Aviation
A strong and gusty north-northeast flow will persist tonight then gradually decrease on Friday as a developing cutoff low drifts eastward toward the state. The low level flow remains rather stable over the western end of the island chain, with some enhanced low level moisture affecting the eastern half of the state. Widespread, moderate low level turbulence will occur over and downwind of island terrain tonight. AIRMET Tango for moderate low level turbulence remains in effect for all islands and will likely be extended into Friday.

Enhanced low level moisture across the eastern half of the island chain will produce MVFR ceilings over northeast slopes of the Big Island and Maui tonight, and we will monitor for potential mountain obscuration. Expect VFR conditions to prevail over the rest of the state.

Marine
The current northwest swell is on the decline, with the Waimea buoy (51201) down around 9 feet and 15 seconds. This is still enough to keep surf at advisory level, so the High Surf Warning has been downgraded to an Advisory through tonight, with the north Big Island High Surf Advisory remaining in effect through tonight as well. Surf will subside below advisory level along north and west facing shores tomorrow through Saturday. Strong northeast winds will cause rough surf on east facing shores tonight into the weekend. This surf is expected to reach HSA level for east facing shores during the day Friday.

A Small Craft Advisory (SCA) (and Gale Warning for Alenuihaha Channel) remains in effect through tonight for strong north to northeast winds, and large seas. Northeast winds will begin to subside tomorrow into Saturday as the trough approaches from the east.

The approaching trough will bring increasing moisture and instability, so thunderstorms are possible as early as this evening over the windward waters of Maui and the Big Island, with the chance lingering through Saturday.

See the Collaborative Surf Discussion for details on surf and swell.

Watches/Warnings/Advisories
Wind Advisory until 6 AM HST Friday for Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe, Maui Windward West, Maui Leeward West, Maui Central Valley, Windward Haleakala, Leeward Haleakala, Big Island.

High Surf Advisory until 6 AM HST Friday for Kauai Windward, Kauai Leeward, Waianae Coast, Oahu North Shore, Oahu Koolau, Molokai, Maui Windward West, Maui Central Valley, Windward Haleakala, Big Island North and East.

Small Craft Advisory until 6 AM HST Friday 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, Maalaea Bay, Pailolo Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Gale Warning until 6 AM HST Friday for Alenuihaha Channel.


Morrison...Discussion

TS...Marine

Wroe...Aviation