Hilo

Tonight: Mostly cloudy with occasional showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 63 to 71 near the shore to 54 to 59 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 90 percent.

Saturday: Cloudy. Occasional showers in the morning, then numerous showers in the afternoon. Highs 79 to 84 near the shore to around 70 at 4000 feet. Southeast winds up to 15 mph. Chance of rain 90 percent.

Saturday Night: Mostly cloudy with numerous showers. Lows 65 to 71 near the shore to around 59 at 4000 feet. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 70 percent.

Kona

Tonight: Mostly cloudy with scattered showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 65 to 74 near the shore to around 55 near 5000 feet. Light winds. Chance of rain 40 percent.

Saturday: Partly sunny with scattered showers. Highs 84 to 90 near the shore to around 71 near 5000 feet. West winds up to 10 mph. Chance of rain 40 percent.

Saturday Night: Mostly cloudy with scattered showers in the evening, then partly cloudy after midnight. Lows 67 to 75 near the shore to around 57 near 5000 feet. Light winds. Chance of rain 40 percent.

Waimea

Tonight: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 64 to 72 near the shore to 59 to 65 near 3000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Highs around 83 near the shore to 74 to 80 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Saturday Night: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 65 to 73 near the shore to 60 to 67 near 3000 feet. East winds up to 10 mph. Chance of rain 50 percent.

Kohala

Tonight: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 64 to 72 near the shore to 59 to 65 near 3000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Highs around 83 near the shore to 74 to 80 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Saturday Night: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 65 to 73 near the shore to 60 to 67 near 3000 feet. East winds up to 10 mph. Chance of rain 50 percent.

South Big Island

Tonight: Breezy. Mostly cloudy with scattered showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 75 near the shore to around 58 near 5000 feet. Northeast winds up to 20 mph. Chance of rain 50 percent.

Saturday: Breezy. Mostly cloudy with scattered showers. Highs around 87 near the shore to around 71 near 5000 feet. East winds up to 25 mph. Chance of rain 50 percent.

Saturday Night: Breezy. Mostly cloudy with scattered showers in the evening, then partly cloudy after midnight. Lows around 76 near the shore to around 59 near 5000 feet. East winds 10 to 20 mph. Chance of rain 40 percent.

Puna

Tonight: Mostly cloudy with occasional showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 63 to 71 near the shore to 54 to 59 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 90 percent.

Saturday: Cloudy. Occasional showers in the morning, then numerous showers in the afternoon. Highs 79 to 84 near the shore to around 70 at 4000 feet. Southeast winds up to 15 mph. Chance of rain 90 percent.

Saturday Night: Mostly cloudy with numerous showers. Lows 65 to 71 near the shore to around 59 at 4000 feet. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 70 percent.

Waikoloa

Tonight: Partly cloudy with slight chance of thunderstorms and isolated showers in the evening, then mostly cloudy with scattered showers and slight chance of thunderstorms after midnight. Locally heavy rainfall possible. Lows 68 to 74 near the shore to around 56 above 4000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

Saturday: Partly sunny in the morning then becoming mostly sunny. Isolated showers. Highs 85 to 90 near the shore to 67 to 74 above 4000 feet. North winds up to 10 mph shifting to the northwest in the afternoon. Chance of rain 20 percent.

Saturday Night: Mostly cloudy with isolated showers in the evening, then partly cloudy after midnight. Lows 70 to 75 near the shore to 55 to 60 above 4000 feet. Light winds. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Heavy showers and thunderstorms will remain possible across most of the state through at least tonight, while easterly trade winds that remain in place across Big Island and Maui spread to Oahu and Kauai. The upper level disturbance creating the active weather will slowly move eastward and weaken on Saturday, leading to decreased chances for heavy rainfall across most islands. Somewhat wet easterly trade winds will focus most rainfall along windward slopes Sunday into early next week. A more stable, moderate to breezy trade wind flow is expected by Wednesday.

Discussion

Heavy showers and thunderstorms diminished sharply late this afternoon and evening over and around Oahu. The upper level low that dropped over the state earlier today is now weakening, and the surface trough that it induced over Oahu has filled and lifted to the northwest. This resulted in the dramatic decrease in convection on Oahu as easterly trade winds filled back in, but not before many stations on Oahu measured between 4 and 7 inches of rain, leading to flash flooding. That said, the weakening upper level low will linger overhead and maintain some threat of additional flooding tonight from Oahu to Big Island.
Chances of heavy showers and thunderstorms will diminish from west to east on Saturday as the upper low drifts eastward and weakens. Moderate easterly trade winds will focus rainfall over windward areas, and the GFS and ECMWF are showing increased moisture moving over the Big Island and possibly Maui, suggesting more rainfall across those windward and southeast slopes. Given the lingering instability from the upper level low, heavy rainfall may remain a threat on the Big Island and perhaps Maui, and we will determine if the Flood Watch needs to remain in effect for those islands.
Moderate easterly winds and a somewhat wet pattern of mainly windward and mauka showers is expected by Sunday. The upper low will be replaced by a broad and flat upper trough, which will not allow the atmosphere to become very stable. As a result, we cannot rule out a briefly heavy shower, mainly over the heated leeward slopes of the Big Island and Maui during the afternoon hours.
Increased stability and stronger trade winds are due late Tuesday or Wednesday. Expect a typical pattern of mainly windward rainfall and mostly dry conditions leeward.

Aviation

An upper level low moving over Maui County this evening will keep unsettled easterly wet trade winds in the forecast with periods of MVFR conditions across the eastern half of the state lasting into Saturday. Shower activity will decrease from west to east this weekend as the upper low weakens and continues to march eastward.
AIRMET Sierra remains in effect for mountain obscuration over Kauai, Oahu, Molokai, Lanai and Maui.

Marine

Winds will remain slightly east southeasterly this weekend at moderate to fresh levels. By Saturday, the upper disturbance that gave us some severe weather today will be moving northeast away from the state, with stability slowly returning to the region. As a result, expect decreasing heavy showers and thunderstorms across the waters. A more typical easterly trade wind pattern will return next week.
A small, medium period north northwest (320-330 degree) swell that peaked last night will continue to slowly decline tonight. Surf along north and west facing shores will follow suit, easing during the weekend.
A mix of small, medium to long period south swells will maintain small surf along south-facing shores this weekend. A series of gale lows passing south of New Zealand should send small south swell energy into the southern waters through next week. Surf should lift to summer averages around Sunday and hold through Wednesday. Moderate to fresh trades will maintain rough and choppy surf along east-facing shores.
Tides will peak around 2.5 to 3.0 ft MLLW this weekend. Minor overwash of low lying coastal areas will be possible during times of high tide.

HFO Watches/Warnings/Advisories

Flood Watch until 6 AM HST Saturday for Waianae Coast, Oahu North Shore, Olomana, Central Oahu, Waianae Mountains, Lanai Mauka, Kahoolawe, Maui Windward West, Maui Leeward West, Haleakala Summit, Kona, Kohala, Big Island Interior, Big Island Summits, East Honolulu, Honolulu Metro, Ewa Plain, Koolau Windward, Koolau Leeward, Molokai, Lanai Windward, Lanai Leeward, Lanai South, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala, Big Island South, Big Island Southeast, Big Island East, Big Island North.

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Data Courtesy of NOAA.gov