Climate & Environment

Kauai Weather: Dry Tonight, Heavy Rain and Flood Threats Expected Tuesday

By Kauai Now

For the rest of tonight, most of Kauai will see clear skies. Lows will be around 71 °F near the shore and about 62 °F above 3,000 ft. Light winds are expected. On Tuesday, the weather will shift to partly sunny with scattered showers in the morning and numerous showers in the afternoon. Highs will range from 82 °F to 87 °F near the shore and about 73 °F above 3,000 ft, with a 60 % chance of rain. Tuesday night will be mostly cloudy with numerous showers in the evening and scattered showers after midnight. Lows will be 64 °F to 72 °F near the shore and about 59 °F above 3,000 ft, again with a 60 % chance of rain.

South Kauai will also stay mostly clear tonight, with lows of 68 °F to 75 °F and northeast winds around 10 mph. Tuesday will be mostly sunny with isolated showers in the morning and mostly cloudy with numerous showers in the afternoon, highs between 77 °F and 87 °F, east winds up to 10 mph, and a 60 % chance of rain. Tuesday night will be mostly cloudy with numerous showers in the evening and scattered showers after midnight, lows 66 °F to 73 °F, northeast winds 10 mph to 15 mph, and a 60 % chance of rain.

The mountains will see partly cloudy skies tonight with scattered showers early and isolated showers after midnight. Lows will be about 66 °F in the valleys and 59 °F above 4,000 ft, with a 40 % chance of rain. Tuesday will be partly sunny with scattered showers, highs 72 °F to 80 °F in the valleys and about 65 °F above 4,000 ft, light winds, and a 50 % chance of rain. Tuesday night will be mostly cloudy with numerous showers in the evening and scattered showers after midnight, lows 59 °F to 64 °F in the valleys and 55 °F above 4,000 ft, northeast winds up to 10 mph in the evening becoming light, and a 60 % chance of rain.

North Kauai will be partly cloudy tonight with isolated showers, lows 62 °F to 73 °F, light winds, and a 20 % chance of rain. Tuesday will be partly sunny with scattered showers, highs 68 °F to 83 °F, east winds up to 10 mph, and a 50 % chance of rain. Tuesday night will be mostly cloudy with scattered showers, lows 57 °F to 70 °F, east winds up to 10 mph, and a 50 % chance of rain.

East Kauai will be partly cloudy tonight with isolated showers, lows 63 °F to 74 °F, light winds, and a 20 % chance of rain. Tuesday will be mostly sunny with isolated showers in the morning and mostly cloudy with numerous showers in the afternoon, highs 75 °F to 83 °F, light winds becoming east around 10 mph in the afternoon, and a 60 % chance of rain. Tuesday night will be mostly cloudy with numerous showers in the evening and scattered showers after midnight, lows 61 °F to 73 °F, northeast winds up to 10 mph, and a 60 % chance of rain.

A broad, deep surface low about 950 mi northeast of the islands is keeping trade winds weak and the pressure gradient loose. A ridge aloft just south of the islands maintains stable conditions with a solid inversion. Little rain has fallen from Kauai to Maui, where moisture has been limited and an inversion was observed at 6,000 ft. Scattered showers have been seen on Big Island slopes where a shallow pocket of moisture remains stalled. Westerly winds on the highest elevations of the Big Island summits will support a wind advisory into the evening and then ease overnight.

A surface trough originating in the western Pacific will drift to within 300 mi southwest of Hawaii tonight, while an upper‑level trough 1,000 mi west of Kauai will advance eastward, tapping moisture from thunderstorms along the surface trough and raising high clouds. The upper‑level trough will erode the ridge aloft early Tuesday and provide large‑scale ascent later in the day, pulling the surface trough toward the islands and spreading a deep tropical airmass with precipitable water rising to about 2 inches by afternoon. The flood watch for all islands will remain in effect, with the greatest flood threat from Tuesday night through Wednesday night as the trough moves over the state and a deep tropical airmass with precipitable water exceeding 2.25 inches arrives.

Heavy rainfall is expected to be greatest Tuesday night into Wednesday, with the jet stream providing strong ascent and possibly generating an elongated surface low near or over the islands. The GFS and ECMWF models favor Oahu and Maui County for the greatest rainfall totals, but all islands face a heavy rain threat. Slow‑moving cells anchored on terrain could drop very heavy rainfall in a short period, leading to flash flooding.

The threat of heavy rainfall will diminish Thursday and Friday as the jet stream lifts northward and weak upper‑level anticyclonic flow develops. A weak surface trough and ample moisture may linger, providing some chance for spotty heavy showers that could require the flood watch to be extended. In the extended forecast, trade winds may return Friday night and Saturday as the lingering moisture is pushed south of the islands, and breezy northeast winds may develop Sunday as a shallow front moves near or over the state.

Aviation conditions will remain VFR through the night, with MVFR to IFR possible on Tuesday afternoon and into Wednesday as a band of deep tropical moisture arrives from the southwest. The heaviest rain bands will likely arrive Tuesday night into Wednesday, where LIFR conditions may occur under the heavier rain bands. No AIRMETs are in effect, but an AIRMET SIERRA for mountain obscuration and an AIRMET ZULU for moderate icing between FL180 and FL270 on Tuesday are possible.

Marine conditions will see a surface low nearly 800 nm northeast of the Hawaiian Islands and the remnants of Tropical Storm Olana about 1,000 nm southwest. A trough will approach offshore waters by late tonight and reach local coastal waters on Tuesday, bringing heavy showers with embedded thunderstorms over the next several days. The current medium‑to‑long‑period northwest swell will continue to build, bringing surf near advisory levels along north and exposed west‑facing shores Tuesday, then gradually decline. Additional swells will arrive mid‑week, and south‑swell may bump surf Thursday and Friday. East‑facing shores will remain small through the first half of the week but may see moderate surf if the north‑northeast swell materializes later.

Fire weather concerns will remain low through the next several days as gentle to moderate trades and dry conditions persist through tonight. Deep tropical moisture will move in Tuesday, bringing a threat for heavy rain and thunderstorms through at least Wednesday night and possibly into Thursday. Inversion heights will range from 6,500 ft to 8,500 ft tonight and will be eliminated by Tuesday afternoon.

Flood watch is in effect from 6 AM HST Tuesday through late Wednesday night for the entire Hawaiian Islands, including all major regions and summits. A wind advisory will remain until 6 AM HST Tuesday for Big Island summits. The weather information is provided by Blue Hawaiian Helicopters and NOAA.gov.