West Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 70 to 75 near the shore to around 62 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Wednesday: Mostly sunny with scattered showers. Highs 85 to 90 near the shore to around 76 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Wednesday Night: Mostly cloudy in the evening then becoming partly cloudy. Isolated showers. Lows around 73 near the shore to around 63 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

South Kaua’i

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then numerous showers after midnight. Lows around 74. Northeast winds 15 to 20 mph. Chance of rain 70 percent.

Wednesday: Mostly cloudy. Breezy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 81 to 89. Northeast winds 15 to 20 mph. Chance of rain 70 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows around 74. Northeast winds around 15 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Breezy. Frequent showers. Lows 64 to 70 in the valleys to around 60 above 4000 feet. Northeast winds 10 to 25 mph decreasing to up to 20 mph after midnight. Chance of rain near 100 percent.

Wednesday: Breezy. Frequent showers in the morning, then numerous showers in the afternoon. Highs 75 to 83 in the valleys to around 66 above 4000 feet. Northeast winds up to 20 mph shifting to the east 10 to 20 mph in the afternoon. Chance of rain near 100 percent.

Wednesday Night: Breezy. Mostly cloudy with scattered showers. Lows 65 to 71 in the valleys to around 60 above 4000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

North Kaua’i

Tonight: Mostly cloudy with numerous showers. Lows 66 to 73. East winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain 70 percent.

Wednesday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 77 to 88. East winds 10 to 15 mph. Chance of rain 70 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 66 to 75. East winds 10 to 15 mph. Chance of rain 50 percent.

East Kaua’i

Tonight: Mostly cloudy with numerous showers. Lows 66 to 77. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Wednesday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 74 to 86. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 66 to 78. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Surface high pressure to the north will dominate through the week resulting in breezy trade winds. Trade winds are expected to increase Thursday and be locally strong through Saturday. Frequent passing showers will generally favor night time and early morning hours as well as windward and mountain areas. An upper trough just N of the islands will help these showers become a little more intense and could provide a slight chance of thunder for the Big Island interiors during the afternoons.

Discussion

Pockets of moisture with a little bit of instability have allowed for measurable rain over windward and mountain areas across the state over the last 12 hours. An upper level trough just N of the islands is expected to move south and provide a small boost to trade wind showers over the next few days, helping windward and mountain areas receive numerous showers in the overnight and morning time frames. In addition to shower enhancement, the higher elevation areas of the Big Island could see an isolated thunderstorm or two during the afternoon hours as the upper trough axis moves through and cold air aloft influences Big Island stability. Showers that have been moving through the area with the breezy trade wind flow have being supported by a 1027 mb high to the north. This high pressure system is expected to gradually drift east over the week and strengthen the trade winds from Thursday onward.
By Thursday, trade winds are expected to strengthen to the locally windy category (20 to 30 mph) in response to a tightening pressure gradient as the high pressure to the north expands and moves east. Winds should also increase over Big Island summits by Thursday as a result of the gradient between the upper trough near the Big Island and upper high far northwest of the State. Winds could reach advisory thresholds for the summit areas. Drier air is also forecast to fill in from Thursday through Saturday and could decrease showers and humidity levels. The first batch of dry air looks to move through by Thursday morning, followed by a sliver of moisture early Friday morning, then another drier slot by mid morning. Predicting these exact moist and dry areas will be challenging and so confidence on humidities and moisture for that time frame are not high currently.
Confidence then decreases further by Sunday as some model guidance indicates the upper trough developing into an upper low and retrograding back towards the islands. This scenario would potentially bring more moisture and instability back over the islands by later Sunday into early next week. However not all models show this, so we will be keeping an eye it over the next few days.

Aviation

Moderate trades will continue for the next few days. Low cigs and SHRA expected along windward and mauka locations with some limited spillover to leeward areas. MVFR conds possible in heavier SHRA otherwise VFR should prevail.
AIRMET Tango is in effect for mod turb blw 080 downwind of island terrain as well as btn 100 and FL200 for mod turb.

Marine

High pressure established north of the islands maintains locally strong trades that will gradually strengthen during the latter portion of the week. The Small Craft Advisory remains in effect for the typical windy waters around the Big Island and Maui County and will likely continue through the forecast period. An expansion in coverage is possible late this week as winds near 25 kt and seas approach 10 ft over area waters.
Long period forerunners of a pair of NNW (330) swells have registered on the Waimea Bay PacIOOS Buoy this afternoon. At present, the bulk of wave action along the N shore remains tied to NE trades, but the swell will build until it peaks late Wednesday night into early Thursday. This swell will briefly peak near the High Surf Advisory (HSA) threshold. Swell then briefly subsides Thursday into Thursday night only to be followed by renewed long period energy from a slightly larger and slightly more NNW (340) swell that will peak Friday into Saturday. This swell holds a higher probability of exceeding HSA thresholds for N and W facing shores Fri into Sat.
Background medium to long period S to SSW swells will filter into the area through the week keeping surf along S facing shores from going flat. Rough, choppy surf along E shores will continue through the forecast period and will see a slight increase during the second half of the week into the weekend.

Fire weather

The current moisture and slight instability should keep conditions below critical fire weather thresholds through Wednesday. By Thursday morning, increasing trade winds and decreasing moisture will start increasing risks of fire weather. By Friday, model guidance shows an elevated risk for reaching critical fire thresholds due to the low moisture levels and strong trade winds. We will continue to monitor the fire weather risk as conditions evolve.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

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