West Kaua’i

Tonight: Mostly clear. Slight chance of thunderstorms and isolated showers early in the evening. Locally heavy rainfall possible early in the evening. Lows around 66 near the shore to around 56 above 3000 feet. North winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Saturday: Sunny with isolated showers. Highs around 81 near the shore to around 71 above 3000 feet. Light winds. Chance of rain 20 percent.

Saturday Night: Partly cloudy until early morning then becoming mostly cloudy. Scattered showers until early morning, then numerous showers early in the morning. Lows around 67 near the shore to around 58 above 3000 feet. Light winds. Chance of rain 40 percent.

South Kaua’i

Tonight: Mostly clear. Slight chance of thunderstorms and isolated showers early in the evening. Locally heavy rainfall possible early in the evening. Lows around 66. Light winds. Chance of rain 20 percent.

Saturday: Sunny with isolated showers. Highs 78 to 83. Light winds. Chance of rain 20 percent.

Saturday Night: Partly cloudy early in the evening then becoming mostly cloudy. Scattered showers. Lows around 67. Light winds. Chance of rain 40 percent.

Kaua’i Mountains

Tonight: Partly cloudy. Scattered showers and slight chance of thunderstorms early in the evening. Locally heavy rainfall possible early in the evening. Lows 58 to 65 in the valleys to around 55 above 4000 feet. Light winds. Chance of rain 30 percent.

Saturday: Mostly sunny. Isolated showers until late afternoon, then scattered showers late in the afternoon. Highs 71 to 78 in the valleys to around 64 above 4000 feet. Light winds. Chance of rain 30 percent.

Saturday Night: Partly cloudy with scattered showers until early morning, then mostly cloudy with numerous showers early in the morning. Lows 60 to 66 in the valleys to around 57 above 4000 feet. Light winds. Chance of rain 50 percent.

North Kaua’i

Tonight: Mostly clear. Slight chance of thunderstorms and isolated showers early in the evening. Locally heavy rainfall possible early in the evening. Lows 60 to 68. West winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Saturday: Sunny with isolated showers. Highs 73 to 80. Light winds. Chance of rain 20 percent.

Saturday Night: Partly cloudy until early morning then becoming mostly cloudy. Isolated showers until early morning, then scattered showers early in the morning. Lows 61 to 69. Light winds. Chance of rain 40 percent.

East Kaua’i

Tonight: Mostly clear. Scattered showers and slight chance of thunderstorms early in the evening. Locally heavy rainfall possible early in the evening. Lows 60 to 70. Northwest winds up to 10 mph in the evening becoming light. Chance of rain 30 percent.

Saturday: Sunny with isolated showers. Highs 72 to 82. Light winds. Chance of rain 20 percent.

Saturday Night: Partly cloudy with isolated showers early in the evening, then mostly cloudy with scattered showers in the late evening and overnight. Lows 62 to 71. Light winds. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A low northwest of the state is being absorbed by a stronger, nearly stationary, low far northwest of the state. A broad band of showers along a weak trough will continue eastward across the state through tonight. Relatively drier air and lighter winds are expected to fill in across the state on Saturday as the trough moves east and weakens. Chance of showers will increase again across the state Saturday night through Sunday night. Land/seabreezes will dominate much of the weather pattern next week as the low remains north of the state.

Discussion

This afternoon, moderate to heavy showers remain focused over Oahu and have started move into Maui County. Rain rates have been consistently in the 1 to 2 inch per hour range across the north shore of Oahu for the last several hours, causing significant flooding and flooding-related impacts, such as land slides.
Tonight into Saturday. The low to the far northwest of the state continues to draw up deep tropical moisture across the state within the southwest flow. The axis of moisture and weak surface convergence along a surface trough is expected to gradually shift eastward tonight into Saturday morning, with some relatively drier air filling in behind it. Upper level support is trending down as the shortwave continues to lift further northeast away from the state and upper level heights gradually rise through Saturday. As such, heavy rain and thunderstorm potential will also gradually decline. The Flood Watch was cancelled for Kauai as the deepest moisture and weak area of convergence shifts east, and has been extended for the rest of the island chain through 6 AM HST Saturday morning. With that said, the island of Oahu might be in the clear sooner than that as the radar and satellite trends show drier air trying to fill in from the west. On Saturday, showers will remain focused over Maui County and the Big Island, though they are expected to decline in intensity. Winds have also come down across Oahu, so the Wind Advisory has been cancelled. At the Big Island summits, advisory level winds are expected to continue into tonight before tapering off by Saturday morning, so the Wind Advisory has been extended through tonight. The Winter Weather Advisory also has been extended through tonight.
Saturday Night through Sunday Night. A deep low will persist to the far northwest with a persist southwest flow bringing up more deep moisture from the tropics. Aloft, another shortwave will rotate round the the base of the longwave trough, lowering upper level heights over the western end of the state and possibly helping to enhance shower development within the southwest flow once again. However, surface level forcing does not look impressive.
Next Week. Once that shortwave moves out, upper level heights will gradually rise again. Deep moisture will remain draped across the state through much of next week and surface winds are expected to start off rather light and variable with land and sea breezes dominating, then gradually veer out of the east by mid to late week as high pressure to the northeast takes over at the surface.

Aviation

Showers, some heavy, were moving ENE through the area this afternoon. The heaviest showers were over the island of Oahu and Maui County, and these will bring widespread MVFR with isolated IFR. Showers will continue into the evening before weakening after midnight. Saturday is expected to be mostly VFR with areas of MVFR in showers and low clouds.
AIRMET Sierra is in effect for mountain obscuration above 2000 ft for all islands.
AIRMET Tango remains in effect for moderate turbulence between 130 and FL360 across the entire area, and for moderate turbulence below 9000 ft downwind of terrain for Oahu and Kauai.
AIRMET Zulu remains in effect for moderate icing between 140-FL200 for all areas.

Marine

A gale force low northwest of Hawaii extends a front across the central waters. The front will continue to bring heavy rain and isolated thunderstorms to the region tonight as southerly winds trend down. The front weakens to a trough on Saturday and lingers in the vicinity of the islands through next week, keeping winds lighter than seasonal average starting Saturday. East of the trough, moderate to fresh easterly trade winds veer southeasterly near the trough axis, while areas to the west of the trough will see light and variable winds which may become southerly next week, but remain generally light.
Decreasing southerly winds will lead to declining surf along southern shores tonight and a return to smoother, less choppy conditions. However, surf remains moderate on Saturday with a sustained medium- period south swell. Continued pulses of southerly swell will keep surf along south facing shores at least small through mid-next week.
Along north facing shores, a moderate, short-period northwest swell generated by the gale force low to the northwest will bring a quick jump of small to moderate surf to north and west facing shores which peaks on Saturday before slowly diminishing into early next week.
Surf along east facing shores remains below seasonal average with lighter than average trade winds near and upstream of the islands forecast through next week.

HFO Watches/Warnings/Advisories

Flood Watch through late tonight 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.
Wind Advisory until 6 AM HST Saturday for Big Island Summits.
Winter Weather Advisory until 6 AM HST Saturday for Big Island Summits.

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Data Courtesy of NOAA.gov