Photo Credit: Tyler Rooke

West Side

Today: Mostly sunny. Breezy. Scattered showers early in the morning, then isolated showers in the late morning and afternoon. Highs 80 to 89. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Partly cloudy with isolated showers. Lows 69 to 75. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Wednesday: Mostly sunny with isolated showers. Highs 81 to 89. Northeast winds up to 15 mph shifting to the northwest in the afternoon. Chance of rain 20 percent.

South Side

Today: Sunny and breezy. Highs around 89. Northeast winds 15 to 25 mph.

Tonight: Mostly clear. Breezy. Lows around 71. North winds 15 to 25 mph.

Wednesday: Sunny and breezy. Highs around 89. North winds 10 to 20 mph.

North Shore

Today: Mostly cloudy. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 80 to 86 near the shore to around 66 near 5000 feet. East winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with scattered showers. Lows 68 to 73 near the shore to around 54 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs 80 to 87 near the shore to around 67 near 5000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Central Maui

Today: Mostly sunny. Breezy. Scattered showers early in the morning. Highs around 88. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Partly cloudy. Breezy. Lows 68 to 74. Northeast winds 10 to 20 mph.

Wednesday: Mostly sunny. Breezy. Highs around 89. Northeast winds up to 20 mph.

Upcountry

Today: Mostly sunny with scattered showers early in the morning, then mostly sunny with isolated showers in the late morning and afternoon. Highs around 60 at the visitor center to around 55 at the summit. East winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Partly cloudy with isolated showers. Lows around 47 at the visitor center to around 43 at the summit. East winds around 15 mph. Chance of rain 20 percent.

Wednesday: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 62 at the visitor center to around 58 at the summit. East winds up to 15 mph. Chance of rain 50 percent.

East Maui

Today: Mostly cloudy. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 80 to 86 near the shore to around 66 near 5000 feet. East winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with scattered showers. Lows 68 to 73 near the shore to around 54 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs 80 to 87 near the shore to around 67 near 5000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Lanai City

Today: Mostly sunny. Breezy. Isolated showers early in the morning. Isolated showers in the afternoon. Highs 74 to 83. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows around 68. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Wednesday: Sunny with isolated showers. Highs 75 to 82. Northeast winds up to 10 mph. Chance of rain 20 percent.

Kaunakakai

Today: Mostly sunny. Breezy. Isolated showers early in the morning. Isolated showers in the afternoon. Highs 75 to 89. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Tonight: Partly cloudy. Isolated showers early in the evening. Lows 61 to 74. East winds around 15 mph. Chance of rain 20 percent.

Wednesday: Mostly sunny. Highs 76 to 89. East winds up to 15 mph.

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