West Kaua’i

Tonight: Mostly cloudy with scattered showers and isolated thunderstorms. Lows around 70 near the shore to around 61 above 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Thursday: Breezy. Partly sunny with scattered showers and isolated thunderstorms in the morning, then mostly sunny with isolated showers in the afternoon. Highs 83 to 88 near the shore to around 75 above 3000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Thursday Night: 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 61 above 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

South Kaua’i

Tonight: Mostly cloudy. Breezy. Numerous showers in the evening, then occasional showers and isolated thunderstorms after midnight. Lows around 71. Northeast winds 15 to 20 mph. Chance of rain 90 percent.

Thursday: Breezy. Mostly cloudy with occasional showers and isolated thunderstorms in the morning, then mostly sunny with scattered showers in the afternoon. Highs 79 to 87. East winds 10 to 20 mph. Chance of rain 90 percent.

Thursday Night: Mostly cloudy with scattered showers and isolated thunderstorms. Lows around 71. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Breezy. Occasional showers and isolated thunderstorms. Lows 62 to 69 in the valleys to around 58 above 4000 feet. East winds 10 to 20 mph. Chance of rain near 100 percent.

Thursday: Breezy. Occasional showers and isolated thunderstorms in the morning, then partly sunny with numerous showers in the afternoon. Highs 74 to 81 in the valleys to around 65 above 4000 feet. East winds 10 to 25 mph. Chance of rain near 100 percent.

Thursday Night: Mostly cloudy. Breezy. Numerous showers and isolated thunderstorms in the evening, then occasional showers and isolated thunderstorms after midnight. Lows 62 to 69 in the valleys to around 58 above 4000 feet. East winds 10 to 25 mph. Chance of rain 90 percent.

North Kaua’i

Tonight: Mostly cloudy with numerous showers and isolated thunderstorms. Lows 64 to 72. East winds up to 15 mph. Chance of rain 70 percent.

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

Thursday Night: Mostly cloudy with scattered showers and isolated thunderstorms. Lows 64 to 71. East winds 10 to 15 mph. Chance of rain 50 percent.

East Kaua’i

Tonight: Occasional showers and isolated thunderstorms. Lows 64 to 75. Northeast winds 10 to 15 mph. Chance of rain near 100 percent.

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

Thursday Night: Mostly cloudy. Scattered showers and isolated thunderstorms in the evening, then numerous showers and isolated thunderstorms after midnight. Lows 64 to 74. East winds 10 to 15 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

High pressure will meander around far northwest of Hawaii through Saturday, keeping breezy trades blowing. A low aloft approaching the islands from the north will enhance incoming trade wind showers starting tonight, and continuing 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 see us trend to much drier and more settled tradewind weather as the upper low moves away.

Discussion

Breezy trade winds over the islands are being driven by a relatively strong and large high, shown on scatterometer data roughly 1200 mi NNW of Kauai. Models show this high will meander around the same general area through Sat. Meanwhile, a mid- to upper- level low located about 350 miles NNW of Kauai is pushing SSW. Global models are in good agreement that this low will settle in near or over the main Hawaiian Islands on Thu.
The low will be accompanied by fairly impressive cold air aloft, with 500 mb temperatures over Kauai expected to drop to about 2-sigma below normal Thu morning. Guidance thus shows about 1000-1500 J/kg of 500 m MLCAPE, along with a small pocket of upper level divergence over the northern main islands. However, at the same time, models insist that trades will continue in the boundary layer with bands of low-level moisture riding through from time to time, with one of those noteworthy bands arriving Fri.
This setup will help to enhance incoming bands of clouds and showers riding in on the trades, and might even lead to a couple of brief thundershowers as well (a pattern sometimes colloquially known as “thundertrades”). The latest NBM thunder guidance maxes out below 10 percent near Niihau Thu night, then fades quickly. However, our locally developed hekili guidance shows probabilities from 10-20 percent over and around Kauai and Oahu from both GFS and ECMWF, lingering into Friday before dropping as the upper low starts to move away to the NE. Usually in patterns like this, the most favorable areas for thunder are in the ribbons of low-level convergence downwind of the islands, but it's not impossible to get hefty downpours and a rumble or two windward and mauka too as trade wind shower areas move in from the ENE. Based on a blend of this data and pattern recognition, will keep a mention of t-showers in the forecast for portions of Kauai and Oahu tonight through Fri. Interior Big Island will also continue to have a small chance for a couple of upslope thundershowers Thu afternoon, just like we saw today over the south flank of Mauna Loa where NLDN detected a single lightning strike there.
The southern end of the chain should not see as much shower enhancement from the rather compact upper low, but trades will still be somewhat wet from time to time due to the proximity of the upper low, as any more organized pockets of moisture ride in.
The airmass stabilizes by the holiday weekend as the upper low moves quickly away. Wet trades will become much drier, with fewer clouds and showers, and more sunshine. Trade wind speeds will eventually ease some as the high to the N weakens to a much weaker subtropical ridge by the middle of next week.

Aviation

Moderate to locally breezy trade winds will strengthen slightly heading into tomorrow. Low clouds and showers will favor windward and mauka areas, particularly overnight and during the morning hours, with showers reaching leeward communities at times. MVFR conditions will be possible in showers. Otherwise, VFR conditions will prevail.
No AIRMETs are in effect this afternoon. AIRMET Sierra might be needed for mountain obscuration across windward areas overnight into tomorrow morning as shower coverage is expected to increase.

Marine

High pressure to the distant north- northwest will maintain moderate to locally strong trade winds through the weekend. An upper level low to the north of the waters may bring periods of instability and isolated thunderstorms over Kauai and portions of Oahu waters tonight through Thursday. A Small Craft Advisory is in effect for the windier waters and channels around Maui County and the Big Island through Thursday and may need to be extended through the weekend.
Surf along south facing shores will remain small to moderate as a long period south-southwest swell is expected to fill in late tonight into Thursday. 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 Thursday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

Kauai Now Weather is brought to you by Blue Hawaiian Helicopters.