West Kaua’i

Today: Cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 83 to 88 near the shore to around 73 above 3000 feet. Light winds. Chance of rain 70 percent.

Tonight: Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 73 near the shore to around 61 above 3000 feet. Northeast winds up to 10 mph shifting to the southeast after midnight. Chance of rain 70 percent.

Friday: Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 84 to 89 near the shore to around 75 above 3000 feet. South winds around 10 mph in the morning becoming light. Chance of rain 70 percent.

South Kaua’i

Today: Cloudy. Numerous showers and isolated thunderstorms in the morning, then occasional showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs 80 to 88. Light winds. Chance of rain 90 percent.

Tonight: Cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 73. Light winds becoming south up to 15 mph after midnight. Chance of rain 70 percent.

Friday: Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 80 to 88. Southwest winds 10 to 15 mph. Chance of rain 70 percent.

Kaua’i Mountains

Today: Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 72 to 80 in the valleys to around 64 above 4000 feet. Light winds. Chance of rain 90 percent.

Tonight: Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 64 to 70 in the valleys to around 59 above 4000 feet. South winds up to 10 mph. Chance of rain 70 percent.

Friday: Cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 74 to 83 in the valleys to around 65 above 4000 feet. Southwest winds up to 10 mph. Chance of rain 70 percent.

North Kaua’i

Today: Cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 74 to 86. Northeast winds up to 10 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Scattered showers and isolated thunderstorms in the evening, then numerous showers and isolated thunderstorms after midnight. Locally heavy rainfall possible in the evening. Lows 65 to 73. Light winds becoming south up to 10 mph after midnight. Chance of rain 70 percent.

Friday: Cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 76 to 86. Southwest winds up to 10 mph shifting to the north in the afternoon. Chance of rain 70 percent.

East Kaua’i

Today: Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 72 to 85. Light winds. Chance of rain 90 percent.

Tonight: Cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 64 to 76. Light winds becoming southwest around 10 mph after midnight. Chance of rain 70 percent.

Friday: Cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 75 to 87. South winds up to 10 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

A low pressure system north of the Hawaiian islands moves slowly southward, bringing an increase in shower activity especially to the Niihau, Kauai, and portions of Oahu through Friday. The heaviest showers will be fixated on Kauai, where isolated thunderstorms will also be possible. Additional showers and thunderstorms may trigger on the Big Island Friday and Saturday afternoons. Trades return to the forecast under a more stable weather pattern by the end of the weekend.

Discussion

Latest satellite imagery continues to depict a low pressure system and associated cold frontal passage northeast of the Hawaiian Islands, meandering southwestward slowly. As the low tracks toward the state, it will begin to bring colder air aloft, destabilizing the atmosphere today and tomorrow, increasing cloud coverage and shower activity. Heaviest showers will be isolated to the northern portion of the state, namely across Niihau, Kauai and portions of Oahu throughout the day today and into early Friday.
The aforementioned front is progged to weaken to a trough later today. Winds veer southerly during this time, but remain light in speed, allowing localized sea breezes to develop during the afternoon hours, resulting in cloud coverage and shower activity across interiors of all islands. Latest model guidance suggests Kauai will be engulfed in widespread showers throughout the latter half of today, with northern aspects of Oahu seeing showers from the southern edge of the shower band, though lingering showers may make way across Oahu at times.
Latest model guidance suggests overall shower activity becomes dispersed by late Friday, however, maintained lighter winds will continue to allow localized sea breezes during the afternoons, still capable of producing showers on interiors of all islands. Meanwhile, deeper unstable tropical moisture will get pulled into the sea breeze pattern, triggering a brief period of enhanced shower activity and thunderstorms across the Big Island Friday and Saturday.
Toward the latter end of the weekend, the low pressure system will begin to propagate westward and allow ridging to rebuild back over the islands. As a result, winds will back into a more easterly direction, reintroducing stable trades and associated showers to the forecast. Expect moderate to locally breezy trades along with brief passing showers through much of early next week, favoring windward and mauka areas in the overnight to early morning hours.

Aviation

Light and variable winds persist as a weak frontal passage continues to approach the islands from the north. Expect periods of MVFR conditions due to shower activity and cloud coverage predominately for PHLI, PHNL, and PHJR throughout the day. Isolated thunderstorms possible. Otherwise, VFR conditions prevail elsewhere.
No AIRMETs are currently in effect.

Marine

A low pressure system about 600 nm northeast of the coastal waters has an associated cold front that is moving into the coastal waters near Kauai early this morning. The front and low are expected to move south, with the front weakening to a trough, which is expected to stall near Kauai or Oahu this afternoon. This system has disrupted the trade wind flow, and brought lighter winds to the region. Trade winds are expected to build back in by Sunday in response to a high pressure ridge moving in north of the islands.
The ECMWF and GFS are in good agreement with an upper level low, associated with the above mentioned surface feature, moving south today, and being north of Kauai tonight and tomorrow. This will provide some instability that could lead to a few thunderstorms over the coastal and offshore waters.
This low pressure system is sending a moderate medium period N-NNE swell to N facing shores that will peak this afternoon. There is also a small NW swell that will be filling in through the weekend, helping to keep surf heights slightly elevated. A small medium period NW swell is then expected to fill in Monday and decline slowly through next week. Tiny to small background N and NW pules will keep north shores from going completely flat for the remainder of next week.
Surf for south facing shores will remain small today, before a small long period south swell arrives on Friday and peaks over the weekend at, or slightly above, the September average (5 ft).
Surf for east facing shores will likely see an increase from the N-NNE swell today and Friday. The islands to see the greatest increase would be Kauai and Oahu, as these islands are most directly within the fetch area of the low to the north. This swell will fade out by the weekend. Trade wind chop could increase by Sunday as trade winds return.

Fire weather

Light and variable winds with increasing humidity values across the state through the weekend will help maintain below critical fire weather thresholds over the next several days. Moderate trades and drier conditions expected by next week. Temperature inversion heights near Maui and the Big Island today will range from 7,000 to 8,000 feet elevation.

HFO Watches/Warnings/Advisories

None.