West Kaua’i

Tonight: Mostly cloudy with scattered showers and slight chance of thunderstorms in the evening, then partly cloudy with isolated showers after midnight. Locally heavy rainfall possible in the evening. Lows around 68 near the shore to around 58 above 3000 feet. Light winds. Chance of rain 50 percent.

Saturday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 80 to 86 near the shore to around 72 above 3000 feet. Light winds becoming northeast up to 10 mph in the afternoon. Chance of rain 50 percent.

Saturday Night: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows around 68 near the shore to around 59 above 3000 feet. Northeast winds up to 10 mph. Chance of rain 50 percent.

South Kaua’i

Tonight: Mostly cloudy. Scattered showers and slight chance of thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows around 69. Northeast winds around 10 mph. Chance of rain 50 percent.

Saturday: Partly sunny with scattered showers. Highs 78 to 85. Northeast winds around 10 mph. Chance of rain 50 percent.

Saturday Night: Mostly cloudy with scattered showers. Lows around 69. Northeast winds around 10 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Numerous showers and slight chance of thunderstorms in the evening, then occasional showers after midnight. Locally heavy rainfall possible in the evening. Lows 59 to 68 in the valleys to around 57 above 4000 feet. Light winds. Chance of rain 90 percent.

Saturday: Mostly cloudy. Occasional showers in the morning, then numerous showers in the afternoon. Highs 72 to 78 in the valleys to around 63 above 4000 feet. Light winds becoming east up to 10 mph in the afternoon. Chance of rain 90 percent.

Saturday Night: Mostly cloudy. Numerous showers in the evening, then occasional showers after midnight. Lows 60 to 68 in the valleys to around 57 above 4000 feet. East winds around 10 mph. Chance of rain 90 percent.

North Kaua’i

Tonight: Mostly cloudy. Scattered showers and slight chance of thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 62 to 69. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Saturday: Partly sunny with scattered showers. Highs 74 to 83. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

Saturday Night: Mostly cloudy with scattered showers. Lows 62 to 70. East winds around 10 mph. Chance of rain 50 percent.

East Kaua’i

Tonight: Mostly cloudy. Scattered showers and slight chance of thunderstorms in the evening, then numerous showers after midnight. Locally heavy rainfall possible in the evening. Lows 62 to 72. Northeast winds up to 10 mph in the evening becoming light. Chance of rain 70 percent.

Saturday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 70 to 83. Northeast winds up to 10 mph. Chance of rain 70 percent.

Saturday Night: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 63 to 73. Northeast winds around 10 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

An active convective pattern with mostly light winds will continue through this evening, with showers and thunderstorms favoring island interiors, then transitioning to mostly windward areas overnight. Moderate trades are expected to return tomorrow and persist through the weekend, with lingering moisture and instability maintaining a rather showery pattern across the island chain. A new storm system will disrupt the trades once again and bring another chance for heavy rainfall and thunderstorms to the islands Tuesday through Thursday.

Discussion

A cold core upper level low remains centered near Maui County this afternoon and will gradually begin to push eastward tonight. The instability this upper level feature is providing to the state combined with ample lingering moisture (PWats over 1.5 inches as measured on this afternoon's upper air soundings) has lead to heavy showers and thunderstorms impacting the majority of the state throughout the day today, which caused flash flooding across portions of Kauai, Oahu and Maui County. High resolution models indicate that heavy rain and thunderstorms will persist through this evening. In addition, snow is expected across the high elevation summits of the Big Island and the Winter Weather Advisory has been extended through 6 AM Saturday for elevations above 11,000 feet.
At the surface, a strong high pressure system to far north and weakening surface trough retreating to west will bring light to moderate southeast winds through this evening. This lighter flow will allow land breezes to help to push some of the shower and thunderstorm activity offshore overnight. Showers caught up in the southeast flow will also periodically move into windward areas, particularly during the overnight hours, and occasionally pass over into leeward areas.
Over the weekend, moderate easterly trades will return as the surface trough pushes further away and dissipates, allowing the high pressure system to the north to take more control of the local winds. Aloft, we'll lose some of the instability as the upper low opens into a trough, weakens and pushes further eastward, but showers are expected to stick around as moisture lingers. As a result, expect a showery tradewind pattern through the weekend, with showers occasionally spilling over to leeward areas.
A new, potent storm system will disrupt the tradewind flow much of next week bringing light to moderate southeasterly winds back to the area late Monday into Tuesday and another round of potentially significant rainfall Tuesday through Thursday.

Aviation

Interior showers and thunderstorms are bringing widespread MVFR to Oahu through Maui and portions of the Big Island with pockets of IFR within more intense activity. SHRA/TSRA will gradually dissipate this evening with the loss of diurnal heating. The development of land breezes tonight will accelerate clearing, though convective debris will likely contribute to a mid-level/VFR cloud deck that will persist well into tonight. Moderate trades return early Saturday, but will not be strong enough to eliminate the inland shower threat. Instead, the resulting hybrid sea breeze pattern will focus moderate to locally heavy showers over western portions of the smaller islands, particularly Oahu and Maui.
AIRMET Sierra remains in effect for the islands of Oahu, Molokai, and Maui.

Marine

High pressure far to the northeast will gradually strengthen through the weekend. Light east to southeast winds across Hawaii will become light to moderate east to northeast winds by tomorrow night. Trade wind flow will predominate Sunday and Monday, but a deep low developing to the northwest of the state will disrupt that flow on Tuesday. Winds will yet again weaken and shift to be out of the south or southeast and remain so through most of the week.
Meanwhile, a passing upper level disturbance has triggered isolated to scattered thunderstorms across much of the state. These storms are capable of producing strong gusty winds, locally heavy downpours, and small hail. The potential for strong thunderstorms will persist through tonight and possibly beyond.
A lingering north-northwest swell that peaked last night will continue its gradual decline through Saturday. On Sunday, surf along north and west facing shores will dip below average, then remain so well into next week. Guidance does depict a gale forming on Monday near the Kurils, then tracking northeast to the western Aleutians by Wednesday. If this materializes, Hawaii could experience an uptick in the northwest swell next weekend.
Surf along east facing shores will remain below the April average through most of next week due to the lack of consistent trade wind flow.
A small long period background south swell will move through this weekend, but will have little impact on surf heights. A larger pulse of long-period south- southwest swell is expected for the coming week.

HFO Watches/Warnings/Advisories

Winter Weather Advisory until 6 AM HST Saturday for Big Island Summits.

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