West Side

Tonight: Breezy. Partly cloudy with isolated showers. Lows 66 to 73. East winds up to 20 mph. Chance of rain 20 percent.

Thursday: Breezy. Mostly sunny with isolated showers. Highs 78 to 84. East winds up to 20 mph shifting to the northeast in the afternoon. Chance of rain 20 percent.

Thursday Night: Partly cloudy with isolated showers. Lows 67 to 73. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

South Side

Tonight: Mostly clear. Breezy. Lows 66 to 75. Northeast winds up to 20 mph.

Thursday: Sunny and breezy. Highs around 87. Northeast winds up to 20 mph.

Thursday Night: Mostly clear. Breezy. Lows 66 to 75. North winds 10 to 20 mph.

North Shore

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then numerous showers after midnight. Lows around 70 near the shore to around 51 near 5000 feet. East winds up to 20 mph. Chance of rain 70 percent.

Thursday: Breezy. Partly sunny with scattered showers. Highs 77 to 82 near the shore to around 65 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Thursday Night: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows around 70 near the shore to around 51 near 5000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Central Maui

Tonight: Mostly clear. Breezy. Lows around 68. Northeast winds 10 to 20 mph.

Thursday: Sunny and breezy. Highs 80 to 88. Northeast winds 10 to 25 mph.

Thursday Night: Breezy. Partly cloudy with isolated showers. Lows around 68. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Upcountry

Tonight: Breezy. Partly cloudy with scattered showers. Lows around 46 at the visitor center to around 40 at the summit. East winds up to 20 mph increasing to 10 to 20 mph after midnight. Chance of rain 50 percent.

Thursday: Partly sunny with scattered showers. Highs around 59 at the visitor center to around 63 at the summit. East winds up to 15 mph. Chance of rain 50 percent.

Thursday Night: Partly cloudy with scattered showers. Lows around 45 at the visitor center to around 40 at the summit. East winds 10 to 15 mph. Chance of rain 50 percent.

East Maui

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then numerous showers after midnight. Lows around 70 near the shore to around 51 near 5000 feet. East winds up to 20 mph. Chance of rain 70 percent.

Thursday: Breezy. Partly sunny with scattered showers. Highs 77 to 82 near the shore to around 65 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Thursday Night: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows around 70 near the shore to around 51 near 5000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Lanai City

Tonight: Mostly clear. Breezy. Lows around 64. Northeast winds up to 25 mph decreasing to up to 10 mph after midnight.

Thursday: Mostly sunny. Breezy. Isolated showers in the afternoon. Highs 72 to 81. Northeast winds 10 to 15 mph increasing to 10 to 25 mph in the afternoon. Chance of rain 20 percent.

Thursday Night: Mostly clear. Breezy. Lows 62 to 67. Northeast winds 10 to 20 mph.

Kaunakakai

Tonight: Breezy. Mostly clear with isolated showers. Lows 56 to 72. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Thursday: Breezy. Mostly sunny with isolated showers. Highs 69 to 87. East winds 10 to 25 mph. Chance of rain 20 percent.

Thursday Night: Partly cloudy with isolated showers. Lows 56 to 72. East winds 10 to 15 mph. Chance of rain 20 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.

Data Courtesy of NOAA.gov