National Weather Service United States Department of Commerce

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Issued: Dec 10, 2018 8:26 PM HST

Synopsis
A series of high pressure systems passing north of the islands will maintain breezy, and strong trade winds through most of the week. The high will begin weakening on Friday and into the weekend, leading to calmer trades. A band of clouds and showers is expected to fall apart in the next couple of days with less trade wind showers on tap for most of the second half of the week. A change in the weather is expected early next week as a front approaches the islands from the northwest.

Discussion
Satellite and radar imagery this evening shows a wet band of unstable cumulus clouds moving across Kauai, Oahu and Molokai. Short range models continue to predict a wet weather pattern for much of the Hawaiian Islands tonight and tomorrow. The highest chances for measurable rainfall will trend towards the windward and mountain slopes with the stronger shower bands producing scattered showers over leeward areas.

In the large scale weather pattern a strong west to east oriented jet stream will drive a several high pressure systems across 30 degrees north latitude this week just to the north of Hawaii. These high centers will reinforce the strong pressure gradient over the islands through Thursday before weakening a bit with more moderate trade winds on Friday. A wind advisory remains in effect for the windier areas of Maui and Hawaii Counties until Tuesday evening.

A combination of good low level forcing due to strong winds mixing through the boundary layer with convergence along mountain ranges and upper level divergence, will help elevate the trade wind subsidence inversion into the 7000 to 9000 foot range through much of the work week. This higher inversion layer will allow deeper clouds to form producing a wet weather pattern across the Hawaiian Islands through Friday. These showers will tend to favor windward and mountain slopes of all islands with some of the stronger showers passing over leeward sections especially in the overnight hours. Smaller scale changes in coverage and intensity of the shower bands will vary from day to day, however the overall wet trend will remain in the forecast.

The large scale weather pattern changes a bit by this weekend. The high pressure ridge to the north of Hawaii will weaken as a cold front dives towards the islands from the northwest. Trade winds will diminish as the ridge breaks down and pressure gradients weaken. By Sunday the winds at the 850 MB level will turn southeasterly creating a blocking effect on precipitation. This blocking effect occurs when the smaller islands fall in the lee, or downwind, of the Big Island. The Big Island creates a bow wave effect (imagine a large rock in a stream with water flowing around it) pushing clouds and showers away from the islands while enhancing stable downward motions aloft. Expect the drier period to last through next Monday as lighter winds also allow a land and sea breeze wind regime to develop across all islands. Clouds will trend towards interior sections of each island due to sea breeze convergence with clearing at night due to land breeze divergence.

On Tuesday the pattern changes back towards trade winds, as a strong high pressure center tracks across the Central Pacific north of the islands with a shallow cold front dropping down out of the northwest. Looks like more wet weather for the middle part of next week in the extended weather outlook. Stay tuned.

Aviation
Moderate to strong winds will continue through twentyfour hours and possibly beyond. High topped stratocumulus and scattered showers will continue to favor windward and mauka areas. Isolated MVFR conditions may become more widespread towards morning. An AIRMET for mountain obscuration could become necessary for winward facing slopes, especially on Kauai, Oahu, and Molokai.

AIRMET Tango for low-level turbulence below 8,000ft will likely remain necessary through at least Tuesday afternoon. Easing of the moderate to strong winds is not expected until later in the workweek. AIRMET Tango also addresses strong surface winds greater than 30kts.

Marine
A series of highs building to the north of the state will keep strong trade winds blowing across the coastal waters through the week. The first in the series of highs will shift steadily eastward well to the north of the islands over the next couple of days as a cold front approaches from the west. This cold front will then pass by north of the state on Wednesday, bringing a brief and subtle easing to the trade winds across the marine area. The next high will then build north of the islands Wednesday night through Friday, resulting in strengthening trade winds once again. This high will then slowly settle southward and closer to the state over the weekend into early next week, and this will result in a gradual weakening of the trade winds across the marine area.

A Gale Warning remains in effect for the Alenuihaha Channel through Tuesday and a Small Craft Advisory (SCA) remains in effect for the rest of the coastal waters through Tuesday night. The Gale Warning may need to be extended through Tuesday night for the Alenuihaha Channel. Meanwhile, the SCA will likely need to be extended for the entire marine area through the end of the work week due to winds, seas, or a combination of the two. Additionally, another round of Gales could affect the Alenuihaha Channel Thursday and Thursday night as the trade winds associated with a new strong high get cranking once again.

The current north-northwest will continue to decline through Tuesday night. The first forerunners of a new long-period northwest swell are expected to begin arriving late tonight or early Tuesday, with this swell slowly building through the remainder of the day and Tuesday night, before peaking on Wednesday. Surf is expected to near High Surf Advisory (HSA) levels on Wednesday along exposed north and west facing shores. A smaller northwest swell is then expected on Friday, followed by a new and larger northwest swell late in the weekend into early next week, which could bring another round of advisory level surf to north and west facing shores.

Meanwhile along east facing shores, strong trades are producing rough choppy surf. The Mokapu buoy is showing an increase of short-period energy over the last 24 hours and we should continue to see an increase in surf along east facing shores on Tuesday. The HSA remains in effect for east facing shores of Kauai, Oahu, Molokai, Maui, and the Big Island through Wednesday night but will likely need to be extended due to the strong trade winds persisting through the end of the week.

Watches/Warnings/Advisories
High Surf Advisory until 6 AM HST Thursday for Kauai Windward, Oahu Koolau, Olomana, Molokai Windward, Maui Windward West, Windward Haleakala, South Big Island, Big Island North and East.

Wind Advisory until 6 PM HST Tuesday for Lanai, Kahoolawe, Maui Central Valley, South Big Island, Big Island North and East, Kohala, Big Island Interior.

Small Craft Advisory until 6 AM HST Wednesday 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 PM HST Tuesday for Alenuihaha Channel.


DISCUSSION...Bohlin

AVIATION...Bedal

MARINE...Jelsema