West Kaua’i

Tonight: Mostly clear. Breezy. Isolated showers after midnight. Lows 66 to 71 near the shore to around 57 above 3000 feet. East winds 10 to 20 mph. Chance of rain 20 percent.

Friday: Mostly sunny with isolated showers. Highs around 83 near the shore to around 73 above 3000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Friday Night: Partly cloudy with isolated showers. Lows around 68 near the shore to around 57 above 3000 feet. Northeast winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 20 percent.

South Kaua’i

Tonight: Breezy. Partly cloudy with isolated showers. Lows 65 to 70. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Friday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 82. East winds 10 to 15 mph. Chance of rain 40 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 64 to 69. East winds 10 to 15 mph. Chance of rain 40 percent.

Kaua’i Mountains

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows 56 to 64 in the valleys to around 54 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Friday: Partly sunny with scattered showers. Highs 72 to 77 in the valleys to around 65 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 56 to 64 in the valleys to around 53 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

North Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 61 to 69. Southeast winds 10 to 15 mph. Chance of rain 40 percent.

Friday: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs 74 to 81. East winds up to 15 mph. Chance of rain 40 percent.

Friday Night: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 61 to 68. Southeast winds up to 15 mph becoming around 10 mph after midnight. Chance of rain 40 percent.

East Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows 59 to 72. East winds 10 to 15 mph. Chance of rain 50 percent.

Friday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 73 to 83. East winds 10 to 15 mph. Chance of rain 50 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 59 to 71. East winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Breezy and gusty trade winds will gradually decline on Friday as surface high pressure to the northeast slowly weakens. Rainfall will remain focused on windward areas, with some afternoon showers developing across leeward Big island slopes. East to southeast trades will be light enough for afternoon sea breezes and spotty showers to develop over all leeward areas Saturday into early next week, and a disturbance aloft will bring some instability, resulting in some briefly heavy showers.

Discussion

Breezy and gusty trade winds will persist into Friday. Trades winds are down slightly compared to the past couple of days but still remain breezy and gusty, thanks to 1028 mb surface high located roughly 650 miles northeast of the islands. The high will meander with only slight weakening through Friday morning, keeping the gusty trades in place. Rainfall will be modest and largely confined to windward slopes over most islands, though a small pocket of moisture will produce more shower activity on windward Big Island this evening. A sharp upper-level trough just south of the state will drift over the Big Island tonight and produce some instability. As a result, showers on the Big Island could be heavier, with a couple of freezing rain showers possible on the high summits.
Trade winds will decline late Friday afternoon through Saturday as more instability develops. The trades will ease and shift out of the east to southeast as the weakening surface high moves away from the region and surface low pressure develops less than 700 miles northwest of Kauai. While the upper-level trough generating the surface low will initially be well to the west, 500 mb heights over the islands will fall as a weak subtropical jet creeps toward the islands. As a result, developing instability will wipe out the inversion and may induce increased shower activity locally. Rainfall will favor east and windward slopes, but the weaker trades will increase chances for afternoon clouds and spotty showers over sheltered leeward terrain on all islands.
A brief increase in trade winds is possible Sunday, followed unstable light to moderate east to east-southeast winds early next week. Persistent upper-level troughing over the region will keep the atmosphere slightly unstable, while surface troughing to the northwest will favor weakened and veered winds Sunday night into Tuesday. The GFS and ECMWF are showing the highest chances for rainfall just north of the island chain during this time, but there will be a chance of heavy showers over windward slopes, with spotty afternoon showers leeward.
A return to a more stable trade wind flow is expected by Wednesday.

Aviation

Locally strong trade winds will continue overnight before declining slightly Friday. VFR conditions are generally expected, especially over Kauai and Oahu, with brief periods of MVFR VIS/CIG in SHRA more likely over over windward portions of Maui and the Big Island. AIRMET Tango for moderate low level turbulence over and downwind of island terrain will remain in place into Friday. No other AIRMETs are expected.

Marine

Surface high pressure about 600 nm northeast of Oahu is slowly drifting east. This will weaken the pressure gradient over the state enough to allow ongoing fresh trades to gradually fall back to more moderate speeds by tomorrow night. Locally strong trades will persist across the notoriously windier bays and channels surrounding Maui County and Big Island, as well as south of Big Island, through Friday. An all-water Small Craft Advisory (SCA) is in effect through the day to account for these persistent strong winds and subsequent elevated seas. The SCA for primarily winds has been pared back from the west from tonight through Friday to only include those windward waters east of the Kaiwi Channel. High pressure will be moving further off to the east this weekend as weak troughing develops west of the state. This will subtly veer gentle to moderate trades more east southeast into early next week.
The current small to moderate size, medium period north northwest (330 degree) swell that peaked late yesterday has been gradually falling through the day. A similar sized west northwest (300-320 degree) swell will move into the northwest waters tonight and peak across the islands Friday afternoon. North and west-facing shore surf will remain under advisory thresholds.
Surf along east-facing shores will remain rough and choppy but stay just under advisory thresholds. As winds fall off this weekend, so will this east chop.

HFO Watches/Warnings/Advisories

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

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