Hilo

Today: Mostly cloudy with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs around 84 near the shore to 66 to 73 at 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 65 to 71 near the shore to around 56 at 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Mostly cloudy with scattered showers. Highs around 84 near the shore to 68 to 74 at 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Kona

Today: Sunny in the morning, then mostly cloudy with scattered showers in the afternoon. Highs 82 to 88 near the shore to around 69 near 5000 feet. West winds around 10 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers early in the evening, then mostly cloudy with isolated showers late in the evening. Mostly clear after midnight. Lows 70 to 75 near the shore to around 54 near 5000 feet. South winds around 10 mph shifting to the north after midnight. Chance of rain 50 percent.

Wednesday: Sunny in the morning, then mostly cloudy with scattered showers in the afternoon. Highs 82 to 88 near the shore to around 70 near 5000 feet. West winds up to 10 mph in the morning becoming light. Chance of rain 40 percent.

Waimea

Today: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs around 81 near the shore to 71 to 80 near 3000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 65 to 70 near the shore to 56 to 63 near 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs around 81 near the shore to 72 to 80 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Kohala

Today: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs around 81 near the shore to 71 to 80 near 3000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 65 to 70 near the shore to 56 to 63 near 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs around 81 near the shore to 72 to 80 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

South Big Island

Today: Breezy. Sunny in the morning, then mostly cloudy with scattered showers in the afternoon. Highs around 85 near the shore to around 71 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers early in the evening, then partly cloudy in the late evening and overnight. Lows around 72 near the shore to around 53 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Breezy. Sunny in the morning, then partly sunny with isolated showers in the afternoon. Highs around 85 near the shore to around 72 near 5000 feet. East winds up to 20 mph. Chance of rain 20 percent.

Puna

Today: Mostly cloudy with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs around 84 near the shore to 66 to 73 at 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 65 to 71 near the shore to around 56 at 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Mostly cloudy with scattered showers. Highs around 84 near the shore to 68 to 74 at 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Waikoloa

Today: Mostly sunny. Isolated showers in the afternoon. Highs around 86 near the shore to 68 to 74 above 4000 feet. North winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy. Isolated showers early in the evening. Lows around 73 near the shore to 52 to 58 above 4000 feet. Northeast winds around 10 mph. Chance of rain 20 percent.

Wednesday: Mostly sunny. Highs around 86 near the shore to 69 to 75 above 4000 feet. North winds up to 15 mph shifting to the northwest in the afternoon.

Detailed Forecast

Synopsis

Moderate trade winds will remain in place into Wednesday, then gradually diminish through the end of the week. The trade winds will deliver clouds and passing showers to windward areas, with an area of moisture lingering over most islands this morning, then gradually diminishing. As winds weaken, afternoon sea breezes will bring clouds and showers to leeward areas as well. Shower coverage and intensity may increase from Wednesday into Friday as moisture increases and the atmosphere becomes unstable. Light to moderate east to southeast winds are expected next weekend.

Discussion

A mid-level trough NW of Kauai and a mid-level ridge extending over the Big Island from the E will move little through tonight, keeping modest instability mainly W of the island chain. The trough will move SE over the islands as the ridge weakens from Wednesday through Friday, potentially leading to increased shower coverage and intensity as trade winds weaken.
The forecast into Wednesday expects moderate trade winds delivering passing clouds and showers to windward areas, with showers most active during nights and mornings. In the short term, an area of moisture that has been providing showers to windward portions of Maui and the Big Island overnight will likely spread showers to Oahu this morning before gradually moving W of all islands this afternoon and tonight. After receiving just 0.43″ of rain during the entire month of August, our rain gage in Honokaa received 0.70″ in the last 12 hours. The bulk of this moisture is expected to pass S of Kauai, but a few windward showers can be expected there.
From Wednesday into Friday, the mid-level trough NW of Kauai will move slowly SE over the islands, while low-level moisture gradually pools in the area as a low-level trough develops nearby. Subsequent surface pressure falls over and N of the islands, and surface ridging to the E will cause trade winds to veer to E to SE and weaken. This will in turn give rise to either a hybrid trade wind / sea breeze, or a more pure land-sea breeze type of pattern from Thursday into the weekend. The combination of moisture and instability will increase chances for some locally heavy downpours, and potentially thunderstorms. Afternoon leeward sea breezes could be the strongest forcing for heavy shower development, but some windward downpours appear possible as well.
Over the weekend, modest instability lingers, with the surface ridge just N of Kauai supporting moderate E winds over the Big Island, with light SE winds over Kauai. A few windward showers may move in, but there will also be a chance of leeward afternoon clouds and showers. Long range guidance depicts a mid-level ridge strengthening over the islands early next week, with trade winds likely remaining light to moderate.

Aviation

Moderate trade wind flow will steer bands of showers off the Pacific towards the Islands. As of 2 am Tuesday, late night satellite and radar imagery revealed a broad band of scattered showers draped across Windward Big Island and Maui County. The band is moving slowly westward and should move into eastern Oahu by morning. Showery conditions will likely persist across much of the state through early evening.
The air mass over the state remains moderately unstable. This will enhance shower intensity, especially when combined with daytime heating. Brief IFR conditions in moderate to heavy showers are possible today. More stable, less showery conditions are expected tonight.
AIRMET Sierra for mountain obscuration is in effect for Molokai, Maui, and windward Big Island north of Cape Kumukahi. Conditions may spread to additional locations this morning before improving in the afternoon.

Marine

The slack pressure gradient down upon the islands from weak surface high pressure centered about 900 nm northeast of the state will maintain generally moderate trade winds through the week. Surface high pressure north of the state will expand eastward through the middle of the week. This will result in slightly strengthened trades, especially through eastern island channels and south of Big Island today. A Small Craft Advisory is in effect through 6 AM Wednesday for these notoriously windier eastern island channels and bays surrounding Maui County and Big Island.
The near three foot small, medium to long period south swell passing around the islands this morning will slowly fade through tonight. A similar medium period southeast swell is showing up and this swell should last through late week. This swell will maintain elevated surf along many southeastern and eastern-facing shores, in particular windward Big Island, the next few days. The next mentionable swells may arrive from both the northwest early next week and the south in a little over a week.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Wednesday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

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Data Courtesy of NOAA.gov