Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

835 pm hst Wed Jan 17 2018

Synopsis

Strong and gusty trade winds are in store through Thursday as robust surface high pressure passes north of the state. Clouds and showers will favor windward and mauka areas, though a few showers will periodically spread leeward on the gusty trades. Winds will gradually decrease from Friday into the weekend as the surface high is pushed far northeast of the islands.

Discussion

Strong and gusty trade winds continue to develop this evening. A 1036 mb surface high is located about 1,100 miles north of Kauai, while a decaying front is stalling about 50 miles north of Kauai. A rather strong ridge aloft over the region is maintaining a solid inversion based at 3,500 to 5,500 ft, according to the afternoon soundings. Winds have been steadily ramping up into this evening, with numerous stations on Maui County and the Big Island reporting advisory level winds.

A further increase in winds is expected into Thursday as the surface high passes north of the state and builds to 1037 mb. The inversion will remain unchanged in most areas, causing the trade winds to be accelerated over and around island terrain. As a result, the Wind Advisory remains in effect for all islands. With models showing winds near the inversion level increasing another 5 kt or so, we will have to closely monitor locations such as the leeward Kohala districts for the potential for High Wind Warning conditions. The Wind Advisory currently runs through Thursday afternoon, though it will likely need to be extended through Thursday night for the state and possibly into Friday for portions of the Big Island and Maui.

Showers will focus across windward slopes during the next couple of days. The atmosphere will remain stable as the inversion holds firm. The above mentioned decaying frontal band will sag southward and likely reach Kauai tonight, making windward portions of the Garden Isle wetter than most areas. This band of moisture is expected to slowly lift northward late Thursday and Friday. A separate area of poorly-defined low level moisture is expected to fill in across windward Big Island tonight, then spread up the island chain on Friday.

Trade winds will begin to decrease on Friday and continue decreasing into the weekend. During this time, the surface high will weaken and move to the northeast of the state, leaving an associated surface ridge about 300 miles north of Kauai by Friday night. A North Pacific trough will erode the surface ridge during the weekend. During this time, trade winds will veer out of the east to east- southeast and may become light enough for daytime sea breezes to dominate across the western end of the island chain.

Beyond the weekend, models are not in good agreement on the details of a developing low near Midway Atoll. However, models do show moderate easterly trades hanging on over the Big Island with southeasterly flow across the smaller islands. The mid level ridge currently over the state will likely erode significantly, leading to some instability and the potential for a few heavier showers.

Aviation

Robust high pressure passing N of the area will support strong trade winds through Thursday while a ridge aloft will maintain a stable island atmosphere. This will lead to strong low-level winds, with AIRMET TANGO in effect for moderate to severe turbulence near and downwind of the terrain, and for surface wind speeds.

Two bands of low clouds embedded within the trade flow will make for locally cloudy conditions, with a stalled frontal band moving over Kauai from the E, and a more diffuse cloud band near and upwind of the Big Island, with fewer clouds over Maui county and Oahu. Radar is detecting isolated to scattered light showers within these cloud bands, and thus far AIRMET SIERRA for mountain obscuration is not anticipated, and observations will be monitored for its possible issuance.

Marine

For the evening update, the high surf warning for north and west facing shores has been lowered to an advisory this evening based on the latest buoy observations and trends. Surf will dip below advisory levels early Thursday for north and west facing shores. Along east facing shores, a high surf advisory is now in effect from 6 AM HST Thursday through 6 PM HST Friday due to strong trades locally and upstream.

The ASCAT pass earlier today showed strong- to gale-force winds over the local waters (gales were shown across the Alenuihaha Channel), which is in line with the current forecast package. Seas at the exposed nearshore PacIOOS buoys reflect this and show 8 to 12 ft due to a combination of rising wind waves and a moderate northwest swell that is trending down. Strong- to gale-force winds are forecast to hold through Friday across the waters as strong high pressure shifts eastward to the north. As a result, the small craft advisory for all waters remains in effect through Thursday night and will likely be extended for most waters through Friday in later packages.

For the upcoming weekend, forecast confidence begins to lower due to significant differences shown between the ECMWF and GFS solutions. The GFS advertises light and variable winds, while the ECMWF depicts moderate to fresh trades holding. The current forecast is more in line with the GFS solution, but is subject to changes over the coming days. Despite these differences, winds will likely trend down over the weekend, which will allow surf along east facing shores to begin lowering, as well as seas across all waters.

A small northwest swell is expected Sunday, that will be reinforced with a moderate northwest swell by Monday, potentially reaching advisory levels. This reinforcement will peak by late Monday, then gradually lower through midweek.

Watches, Warnings, Advisories

Wind Advisory until 6 PM HST Thursday for Niihau-Kauai-Oahu- Molokai-Lanai-Kahoolawe-Maui Windward West-Maui Leeward West- Maui Central Valley-Windward Haleakala-Leeward Haleakala-Kona- South Big Island-Big Island North and East-Kohala-Big Island Interior.

High Surf Advisory until 6 AM HST Thursday for Niihau-Kauai Leeward-Waianae Coast-Oahu North Shore-Molokai Leeward-Maui Central Valley.

High Surf Advisory until 6 PM HST Friday for Kauai Windward-Oahu Koolau-Molokai Windward-Maui Windward West-Windward Haleakala.

High Surf Advisory from 6 AM Thursday to 6 PM HST Friday for Olomana-South Big Island-Big Island North and East.

Gale Warning until 6 AM HST Friday for Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.

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-Big Island Windward Waters.

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