Hilo

Tonight: Cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 59 to 67 near the shore to 48 to 53 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 70 percent.

Wednesday: Mostly cloudy with scattered showers. Highs around 77 near the shore to around 63 at 4000 feet. Northeast winds up to 10 mph. Chance of rain 50 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 59 to 66 near the shore to 47 to 54 at 4000 feet. Northeast winds up to 10 mph. Chance of rain 50 percent.

Kona

Tonight: Cloudy. Scattered showers in the evening, then isolated showers after midnight. Haze through the night. Lows 68 to 73 near the shore to 45 to 52 near 5000 feet. Light winds. Chance of rain 50 percent.

Wednesday: Mostly cloudy. Haze through the day. Scattered showers in the afternoon. Highs around 82 near the shore to around 63 near 5000 feet. Light winds. Chance of rain 40 percent.

Wednesday Night: Mostly cloudy with isolated showers in the evening, then partly cloudy after midnight. Haze through the night. Lows around 70 near the shore to 45 to 52 near 5000 feet. Light winds. Chance of rain 20 percent.

Waimea

Tonight: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 60 to 68 near the shore to 52 to 59 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Wednesday: Mostly cloudy with isolated showers. Highs 72 to 78 near the shore to 66 to 75 near 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Wednesday Night: Mostly cloudy in the evening then becoming partly cloudy. Isolated showers. Lows 59 to 68 near the shore to 51 to 59 near 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Kohala

Tonight: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 60 to 68 near the shore to 52 to 59 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Wednesday: Mostly cloudy with isolated showers. Highs 72 to 78 near the shore to 66 to 75 near 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Wednesday Night: Mostly cloudy in the evening then becoming partly cloudy. Isolated showers. Lows 59 to 68 near the shore to 51 to 59 near 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

South Big Island

Tonight: Cloudy…breezy and haze. Lows around 71 near the shore to around 51 near 5000 feet. Northeast winds up to 20 mph.

Wednesday: Mostly cloudy. Breezy. Haze through the day. Isolated showers in the afternoon. Highs around 83 near the shore to around 62 near 5000 feet. East winds up to 20 mph. Chance of rain 20 percent.

Wednesday Night: Mostly cloudy in the evening then becoming partly cloudy. Haze. Lows around 71 near the shore to around 51 near 5000 feet. East winds 10 to 15 mph.

Puna

Tonight: Cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 59 to 67 near the shore to 48 to 53 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 70 percent.

Wednesday: Mostly cloudy with scattered showers. Highs around 77 near the shore to around 63 at 4000 feet. Northeast winds up to 10 mph. Chance of rain 50 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 59 to 66 near the shore to 47 to 54 at 4000 feet. Northeast winds up to 10 mph. Chance of rain 50 percent.

Waikoloa

Tonight: Mostly cloudy. Isolated showers in the evening. Haze through the night. Lows 68 to 73 near the shore to 48 to 54 above 4000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Wednesday: Mostly cloudy. Haze. Highs around 82 near the shore to 62 to 69 above 4000 feet. Northeast winds up to 15 mph.

Wednesday Night: Mostly cloudy in the evening then becoming partly cloudy. Haze. Lows around 70 near the shore to 48 to 53 above 4000 feet. East winds up to 10 mph.

Detailed Forecast

Synopsis

Moderate to locally breezy trade winds will focus showers over windward slopes as high clouds diminish on Wednesday. A dissipating front will move across the islands Thursday night and Friday, leading to an increase in rainfall and stronger trade winds. Drier trade wind weather is favored on Saturday, followed by a potential increase in showers Sunday or Monday.

Discussion

Showers have diminished over most islands within a moderate to locally breezy trade wind flow this afternoon, though conditions continue to be somewhat gloomy due to persistent high clouds. The tail end of a nearly stationary front is sitting about 200 miles north of Kauai, which is easing the local pressure gradient driven by surface high pressure centered far to the north. As a result, trade winds have been holding at moderate strength today, with a few exposed locales measuring breezy winds. The trade wind flow carried in a diffuse band of moisture earlier today, leading to a quarter to nearly a half inch of rainfall over windward areas from Big Island to Oahu. These modest showers are shifting to Kauai, where an uptick in rainfall is expected into the evening. A mid level ridge will maintain stable conditions, while a weakening subtropical jet stream continues to send high clouds overhead.
Aside from diminishing high clouds, little change is expected Wednesday and New Year's Day. Trades will remain at moderate strength as the shallow front to the north is pushed toward the state by building high pressure farther north. Little organized moisture is noted south of the front in the guidance, suggesting a typical pattern of mainly windward and mauka showers. High clouds will thin considerably as the subtropical jet stream is displaced south of the islands.
The dissipating front will move down the island chain Thursday night and Friday, leading to increased showers and strengthening trade winds. The shallow band of moisture along the feature will reach Kauai around sundown on New Year's Day, sag southward to Oahu and Maui County overnight, then reach Big Island on Friday. The remnant front will quite shallow, and with a weak mid level ridge holding, there will be no upper level support for heavier showers. As a result, expect wet conditions windward, and as the surface high the surface high to the north strengthens, breezy trades will likely carrying brief periods of showers over leeward areas from Kauai to Maui. Breezy trade winds should push much of the moisture clear of the islands on Saturday, with the highest chances for showers lingering on windward Big Island.
Uncertainty enters the picture by Sunday. The GFS and ECMWF models show a continued highly amplified pattern over the Pacific. This pattern features a splitting West Pacific jet stream carving out a deep trough south of 30N near the Dateline, a ridge aloft north of Hawaii, and an upper level trough between the islands and the continental U.S. The GFS and ECMWF are both intensifying the upper trough northeast of Hawaii late Sunday or Monday as surface low pressure lingers west of Kauai, but the models have been inconsistent with the details and placement of key features. For now, the official forecast leans more toward the ECMWF solution, which keeps a stronger and less stable trade wind flow that favors windward rainfall.

Aviation

Trade winds will continue to increase and become more easterly A band of enhanced, but increasingly diffuse, showers continues to move east to west, impacting the western islands this afternoon. Expect some MVFR conditions for Kauai and Oahu for the rest of the afternoon with these low clouds and showers, while the eastern half of the state improves back to VFR. Showers will resume their typical windward/mauka focus later tonight through Wednesday.
AIRMET Sierra for mountain obscuration is in effect for Oahu and Kauai. Conditions are expected to improve later this evening through tonight as the shower band dissipates.

Marine

Moderate to locally strong easterly trade winds will continue through the week as high pressure builds north of the islands. A Small Craft Advisory remains in effect for the typically windy waters around Maui County and the Big Island through Wednesday evening.
Overlapping moderate to large, medium period northwest (320 degree) and north-northwest (340-350 degree) swells will maintain advisory-level surf, with little change through Wednesday evening. Recent buoy obs continue to show the swells running around 1 to 2 feet above guidance so the High Surf Advisory for north and west facing shores of Niihau, Kauai, Oahu, and Molokai and north facing shores of Maui remains in effect through the day on Wednesday. These swells will then gradually decline starting late Wednesday, with surf falling back below advisory criteria at that time. A moderate, medium period north (360-020 degree) swell will then move across the waters later this week and may produce moderate surges in north facing harbors.
Choppy east shore surf will continue though mid-week as locally breezy trade winds strengthen. Surf along south facing shores will remain tiny to small through the forecast period.
Peak astronomical monthly tides may result in minor coastal flooding from Thursday through early next week, and may be exacerbated along north facing shores later this week due to the north swell.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST Wednesday for Niihau, Kauai Leeward, Waianae Coast, Oahu North Shore, Maui Windward West, Kauai North, Molokai Windward, Molokai North, Molokai West, Maui Central Valley North, Windward Haleakala.
Small Craft Advisory until 6 PM HST Wednesday for Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

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Data Courtesy of NOAA.gov