West Kaua’i

Today: Breezy. Partly sunny with scattered showers and isolated thunderstorms in the morning, then mostly sunny with isolated showers and thunderstorms in the afternoon. Highs 83 to 89 near the shore to around 75 above 3000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Tonight: Breezy. Partly cloudy with isolated showers and thunderstorms in the evening, then mostly cloudy with scattered showers and isolated thunderstorms after midnight. Lows around 70 near the shore to around 60 above 3000 feet. Northeast winds up to 20 mph. Chance of rain 50 percent.

Friday: Mostly sunny with isolated showers and thunderstorms. Highs 82 to 88 near the shore to around 74 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

South Kaua’i

Today: Breezy. Mostly cloudy with occasional showers and isolated thunderstorms in the morning, then mostly sunny with scattered showers and isolated thunderstorms in the afternoon. Highs 79 to 87. Northeast winds 15 to 20 mph. Chance of rain 90 percent.

Tonight: Breezy. Mostly cloudy with scattered showers and isolated thunderstorms. Lows around 71. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Friday: Breezy. Mostly cloudy with numerous showers and isolated thunderstorms in the morning, then mostly sunny with scattered showers and isolated thunderstorms in the afternoon. Highs 79 to 87. Northeast winds 10 to 20 mph. Chance of rain 70 percent.

Kaua’i Mountains

Today: Breezy. Occasional showers and isolated thunderstorms in the morning, then partly sunny with numerous showers and isolated thunderstorms in the afternoon. Highs 73 to 81 in the valleys to around 64 above 4000 feet. East winds 10 to 20 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy. Breezy. Numerous showers and isolated thunderstorms in the evening, then occasional showers and isolated thunderstorms after midnight. Lows 62 to 68 in the valleys to around 58 above 4000 feet. East winds up to 20 mph. Chance of rain 90 percent.

Friday: Breezy. Occasional showers and isolated thunderstorms in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 72 to 80 in the valleys to around 64 above 4000 feet. East winds up to 15 mph increasing to 10 to 20 mph in the afternoon. Chance of rain near 100 percent.

North Kaua’i

Today: Partly sunny. Numerous showers and isolated thunderstorms in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 75 to 85. East winds up to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with scattered showers and isolated thunderstorms. Lows 63 to 71. East winds 10 to 15 mph decreasing to up to 10 mph after midnight. Chance of rain 50 percent.

Friday: Partly sunny with numerous showers and isolated thunderstorms in the morning, then mostly sunny with scattered showers and isolated thunderstorms in the afternoon. Highs 75 to 85. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 70 percent.

East Kaua’i

Today: Occasional showers and isolated thunderstorms in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 72 to 84. Northeast winds 10 to 15 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy. Scattered showers and isolated thunderstorms in the evening, then numerous showers and isolated thunderstorms after midnight. Lows 63 to 74. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Friday: Partly sunny. Numerous showers and isolated thunderstorms in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 72 to 84. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

High pressure will meander far northwest of Hawaii through Saturday, keeping moderate to locally breezy trades blowing. A low aloft will enhance incoming trade wind showers today through Friday. A few downpours are possible, and a rumble or two of thunder can't be ruled out, mainly for Kauai and Oahu which will be closest to the upper low. The holiday weekend will trend much drier and more settled as the upper low moves away. The drier weather will continue through the middle of next week with the trade winds easing a bit.

Discussion

Currently at the surface, a 1034 mb high is centered around 1250 miles north-northwest of Honolulu, and is driving moderate to locally breezy trade winds across the island chain early this morning. Infrared satellite imagery shows partly to mostly cloudy conditions across the state, with cloud coverage the greatest over windward and mauka areas. Radar imagery shows scattered to numerous showers moving into windward slopes and coasts, with a few showers occasionally reaching leeward communities. Main short term focus continues to revolve around rain chances and thunder potential.
High pressure will remain anchored to the north-northwest of the state today and tonight, then reposition to the northwest of the islands Friday through the weekend. This will maintain moderate to locally breezy trade winds across the island chain. The high will weaken and the associated ridge extending eastward from the high will settle southward and closer to the state Memorial Day through the middle of next week, resulting in an easing of the trade wind flow back to moderate speeds.
As for the remaining weather details, an upper level low currently evident in water vapor imagery around 250 miles north-northwest of Kauai, will pivot southeastward and over the western islands later today. This feature will remain nearly stationary in the vicinity of Kauai and Oahu tonight, then begin to lift northeastward on Friday. As the upper level low pivots over the western end of the state, model solutions continue to depict surface based CAPE values climbing to around 1000-1500 J/KG and H7-H5 lapse rates increasing to 6.5-7.0 C/KM. This appears sufficient to bring some locally heavy downpours in trade showers to the western islands, with a few rumbles of thunder not out of the question as well. Some thunder may affect the Big Island slopes this afternoon, similar to what was seen yesterday. Otherwise, a wetter than normal trade wind pattern should prevail during the next couple days, favoring windward and mauka areas, along with more leeward spread to the shower activity.
The upper low will continue to lift northeastward and away from the islands Friday night and Saturday, although lingering moisture will likely keep a somewhat showery trade wind pattern in place. The airmass will further stabilize Sunday and Memorial Day, leading to a decrease in trade showers, with fairly typical or slightly drier than normal conditions then expected through the middle of next week.

Aviation

Moderate to locally breezy trade winds will persist during the next 24 hours, strongest and most gusty during the late morning and afternoon hours. A low aloft will move over the western end of the state, bringing the potential for a few downpours mainly to windward areas, with a few rumbles of thunder not out of the question. A thunderstorm or two may pop over the high elevation slopes of the Big Island this afternoon as well.
AIRMET Sierra remains in effect for mountain obscuration over windward portions of Kauai, Oahu, Molokai and Maui. Conditions will likely improve later this morning on Molokai and Maui, but may linger through much of the day on Kauai. Oahu may see some improvement, but confidence in this occurring is not as high as over the islands further east.

Marine

A broad ridge of high pressure will remain nearly stationary north of the state, maintaining moderate to locally strong trade winds through the weekend. An upper level low will build over the northern waters today enhancing trade showers and may produce isolated thunderstorms, mainly over Kauai and Oahu waters, through Friday. The Small Craft Advisory for the windier waters and channels around Maui County and the Big Island has been extended through Friday and now includes windier zones around Kauai and Maui County as winds are expected to strengthen slightly today.
Surf along south facing shores will remain small to moderate as a long period south-southwest swell is expected to fill in today. A second pulse from the same direction will fill in over the weekend and keep small to moderate surf through the weekend before declining early next week.
East shore surf will remain near seasonal average during the next couple of days as trade winds persist. Tiny surf will prevail along north facing shores this week and into the weekend.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters, Kauai Leeward Waters.
Small Craft Advisory until 6 AM HST Friday for Kaiwi Channel, Maui County Windward Waters.
Small Craft Advisory until 6 PM HST Friday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.