West Kaua’i

Tonight: Mostly cloudy. Numerous showers and slight chance of thunderstorms in the evening, then scattered showers and slight chance of thunderstorms after midnight. Locally heavy rainfall possible. Lows 62 to 67 near the shore to around 53 above 3000 feet. East winds up to 10 mph in the evening becoming light. Chance of rain 70 percent.

Friday: Mostly cloudy. Scattered showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible. Highs around 83 near the shore to around 73 above 3000 feet. Light winds. Chance of rain 50 percent.

Friday Night: Mostly cloudy with scattered showers early in the evening, then partly cloudy with isolated showers in the late evening and overnight. Locally heavy rainfall possible early in the evening. Lows 65 to 70 near the shore to around 56 above 3000 feet. Northeast winds up to 10 mph. Chance of rain 50 percent.

South Kaua’i

Tonight: Mostly cloudy. Numerous showers and slight chance of thunderstorms until early morning, then scattered showers and slight chance of thunderstorms late in the night. Locally heavy rainfall possible. Lows 60 to 66. Northeast winds around 10 mph. Chance of rain 70 percent.

Friday: Mostly cloudy. Scattered showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible. Highs around 81. East winds around 10 mph. Chance of rain 50 percent.

Friday Night: Mostly cloudy with scattered showers. Locally heavy rainfall possible early in the evening. Lows 63 to 69. Northeast winds around 10 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Frequent showers and slight chance of thunderstorms until early morning, then numerous showers and slight chance of thunderstorms late in the night. Locally heavy rainfall possible. Lows around 56 in the valleys to around 48 above 4000 feet. Light winds. Chance of rain near 100 percent.

Friday: Mostly cloudy. Numerous showers and slight chance of thunderstorms in the morning, then numerous showers in the afternoon. Locally heavy rainfall possible. Highs around 76 in the valleys to around 65 above 4000 feet. Light winds becoming east up to 10 mph in the afternoon. Chance of rain 70 percent.

Friday Night: Mostly cloudy. Numerous showers early in the evening, then occasional showers in the late evening and overnight. Locally heavy rainfall possible early in the evening. Lows around 59 in the valleys to around 51 above 4000 feet. East winds around 10 mph. Chance of rain 90 percent.

North Kaua’i

Tonight: Frequent showers and slight chance of thunderstorms in the evening, then numerous showers and slight chance of thunderstorms after midnight. Locally heavy rainfall possible. Lows 56 to 65. Southeast winds up to 10 mph in the evening becoming light. Chance of rain near 100 percent.

Friday: Mostly cloudy. Numerous showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible. Highs 76 to 82. East winds up to 10 mph. Chance of rain 70 percent.

Friday Night: Mostly cloudy with scattered showers. Locally heavy rainfall possible early in the evening. Lows 59 to 68. East winds around 10 mph. Chance of rain 50 percent.

East Kaua’i

Tonight: Frequent showers and slight chance of thunderstorms in the evening, then numerous showers and slight chance of thunderstorms after midnight. Locally heavy rainfall possible. Lows 54 to 67. Light winds. Chance of rain near 100 percent.

Friday: Mostly cloudy. Numerous showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible. Highs 74 to 80. East winds up to 10 mph. Chance of rain 70 percent.

Friday Night: Mostly cloudy. Scattered showers early in the evening, then numerous showers in the late evening and overnight. Locally heavy rainfall possible early in the evening. Lows 57 to 70. East winds around 10 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

A convective pattern with mostly light winds will hold in place during the next couple days, with showers favoring areas near the coast at night and the island interiors during the afternoon and evening hours. Locally heavy rain and a slight chance of thunderstorms is forecast through Friday as an upper low moves over the state. Lingering moisture will likely keep the pattern rather showery, even in leeward areas at times through the weekend. A new storm system could bring another chance for heavy rainfall and thunderstorms to the islands Tuesday and Wednesday.

Discussion

An upper level low continues to build over the state this evening generating significant amount of instability. A surface trough located just west of Kauai is generating light to moderate east to southeast flow over the state with precipitable water values around 1.5 inches. Infrared satellite imagery and radar show mostly cloudy conditions in place with numerous showers and isolated thunderstorms mostly along windward Maui and Oahu with a few showers developing over leeward Big Island and Kauai. Main short term focus revolves around the potential for heavy rain and thunderstorms during the next couple days.
The surface troughing will hold in place into Friday, continuing light to moderate east southeast winds allowing for nighttime land and daytime sea breezes. As the upper low slowly moves eastward tonight into Friday, 500 mb temperatures of -12 to -13 C and precipitable water values around 1.5 to 1.6 inches will continue the threat of heavy showers and isolated thunderstorms across any of the islands. Land breezes may help suppress the heavy rain over the islands by late tonight into Friday morning, but the focus will shift back over the islands by midday Friday. Snow is expected across the high elevation summits of the Big Island and Winter Weather Advisory remains in effect through 6 PM Friday for elevations above 11,000 feet.
The upper low will open up to a trough and weaken through the weekend. Thunderstorms or heavy showers could be possible after Friday given the remnant moisture around the islands, but the atmosphere over Hawaii will begin to stabilize and will lower the chances for this. Precipitable water values continue to hold close to 1.5 inches through the weekend and the latest model trends have been for weak trade winds. As a result, the pattern looks a bit showery through the weekend, with both windward showers, and chances for showers each afternoon and evening in leeward areas as well. A new, potent storm system will bring southerly winds beginning Monday and another round of potentially significant rainfall Tuesday through Thursday.

Aviation

Unstable east-southeast winds will maintain widespread MVFR conditions across most east and southeast slopes with pockets of heavy showers and thunderstorms producing IFR conditions tonight. A surface trough west of Kauai will creep away from the islands on Friday, though will gentle to moderate winds persist out of the east to southeast. An upper-level low, currently centered near Oahu, will drift eastward, maintaining instability statewide tonight, and the greatest chances for heavy showers and thunderstorms shifting to the eastern half of the island chain Friday.
Airmet Sierra is in effect for mountain obscuration over east and south facing areas of Kauai, Oahu, and Maui due to reduced visibility and low ceilings in heavy showers and thunderstorms.
Moderate upper-level turbulence will be possible near and west of the upper low center. AIRMET Tango remains in effect for Kauai and Oahu through midnight and will shift eastward and diminish on Friday.

Marine

Light to moderate east-southeast winds will persist into Friday due to a broad surface trough located west of Kauai. Over the western end of the islands, the light winds will allow a land and sea breeze pattern to continue near the coasts. For the eastern end, light to moderate east to southeast breezes, with localized fresh pockets, will likely persist, particularly around the Big Island. Expect spotty heavy showers and thunderstorms to continue into Friday due to an upper disturbance in the area.
A return to moderate easterly trades is possible by the weekend as high pressure builds to the north. Heavy showers and thunderstorms will remain possible into Friday due to an upper disturbance in the area. Guidance continues to show the trades becoming disrupted and shifting out of the south early next week as deep low pressure sets up nearby to the west
Surf along exposed north and west facing shores is quickly rising this evening as a fresh north-northwest builds down the island chain. Observations at the offshore buoys to the northwest peaked above the predicted levels this afternoon, which will correspond to the surf peaking overnight and into Friday morning. Heights could briefly near warning levels during this peak (around midnight). A downward trend is then expected over the weekend, with the swell shifting out of north.
Surf along east facing shores will remain below the April average through at least Saturday, then steadily rise to around average by early next week.
Surf along south facing shores will remain small, with mainly overlapping background south swells moving through. A larger pulse of south-southwest swell may arrive Tuesday.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 AM HST Friday for Waianae Coast, Oahu North Shore, Maui Windward West, Kauai North, Molokai Windward, Molokai North, Molokai West, Maui Central Valley North, Windward Haleakala.
Winter Weather Advisory until 6 PM HST Friday for Big Island Summits.

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