West Kaua’i

Tonight: Partly cloudy. Breezy. Scattered showers after midnight. Lows around 75 near the shore to around 65 above 3000 feet. Northeast winds 10 to 20 mph decreasing to up to 15 mph after midnight. Chance of rain 50 percent.

Monday: Mostly sunny. Breezy. Isolated showers in the morning, then scattered showers in the afternoon. Highs 87 to 93 near the shore to around 79 above 3000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Monday Night: Mostly clear. Isolated showers after midnight. Lows around 74 near the shore to around 64 above 3000 feet. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

South Kaua’i

Tonight: Breezy. Partly cloudy with scattered showers in the evening, then mostly cloudy with showers likely after midnight. Lows 72 to 78. Northeast winds 15 to 20 mph. Chance of rain 70 percent.

Monday: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs 80 to 90. Northeast winds 15 to 25 mph. Chance of rain 50 percent.

Monday Night: Partly cloudy. Breezy. Isolated showers in the evening, then scattered showers after midnight. Lows 71 to 77. Northeast winds 10 to 20 mph. Chance of rain 40 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then showers likely after midnight. Lows around 70 in the valleys to around 62 above 4000 feet. East winds up to 20 mph. Chance of rain 70 percent.

Monday: Partly sunny. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 75 to 83 in the valleys to around 69 above 4000 feet. East winds up to 20 mph shifting to the northeast in the afternoon. Chance of rain 70 percent.

Monday Night: Partly cloudy in the evening then becoming mostly cloudy. Breezy. Scattered showers. Lows around 69 in the valleys to around 61 above 4000 feet. Northeast winds 10 to 25 mph. Chance of rain 50 percent.

North Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 66 to 76. East winds 10 to 15 mph. Chance of rain 50 percent.

Monday: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs 72 to 89. East winds up to 20 mph. Chance of rain 50 percent.

Monday Night: Breezy. Partly cloudy with scattered showers. Lows 66 to 75. East winds 10 to 20 mph. Chance of rain 40 percent.

East Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 68 to 77. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Monday: Breezy. Partly sunny with scattered showers. Highs 79 to 87. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy with scattered showers. Lows 67 to 77. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Moderate to breezy trades will continue through at least the middle of next week. Showers will generally favor windward and mauka areas, particularly during the overnight through early morning hours each day. Tropical Storm Lowell remains far ESE of the main Hawaiian Islands, and uncertainty regarding its track and intensity is high at this time.

Discussion

Benign trade wind pattern through at least Wednesday as strong longwave ridging approaching 594dm remains anchored near 170W longitude before migrating east during the latter portion of the week. This in turn favors expansive surface high pressure in maintenance of breezy trades through this time. An uptick in showers for windward and mauka is expected next week as moisture originating off the California coast entrains in the trades, reaching the islands around Wednesday.
The tropics remain active with attention turning toward the Friday Sept 4th to Wednesday Sept 9th period. During this time, TC Lowell and TC Karina will make their approaches toward the islands within just a couple days of each other. During this time, uncertainty runs higher than normal owing to weak steering flow followed by the potential emergence of a closed upper low NW of the islands that will alter steering flow and introduce a component of shearing as well. TC Lowell and TC Karina both carry potential to come precariously close to the islands during this time window. For Lowell, the GFS remains on the northern side of the solution envelope while the new ECMWF, machine learning guidance, and official NHC forecast remain considerably further south. The forecast remains sensitive to upper air features that have yet to actually form and evolve so confidence remains low even as model consensus on a more southerly track has emerged. As TC Lowell tracks NW, Karina will be fast approaching from the east with a NW turn expected as well. Timing of this turn will be key and represents the primary forecast uncertainty for Karina which remains about a week out. Consult NHC for the official track and intensity forecast.
An approach by Lowell that is sufficiently far south to prevent direct impacts to the islands will still bring elevated humidity and a high liklihood of gusty winds for Labor Day Weekend.

Aviation

Conditions will be mostly VFR with moderate strength trades, leading to occasional turbulence downstream of mountains. VCSH for the windward parts of the islands through the period, best chances this evening into early Monday morning. Low ceilings with MVFR conditions may occur briefly in heavier showers.
AIRMET Tango is in effect for mod low-level turbulence lee of terrain and may be needed for a few days.

Marine

A strong surface high will remain nearly stationary far north of the Hawaiian Islands this week. Moderate to locally strong trade winds will prevail through the week. Tropical Cyclone Lowell may move into the Hawaiian Offshore Waters from Wednesday into Thursday. Wind and weather impacts and timing for the coastal waters, including wind waves and combined seas will be driven by the speed and track of Lowell as it moves towards the region.
The current south swell will slowly decrease over the next several days. Overlapping small, long- and medium-period south swell energy will keep small surf along south facing shores through the week. The next boost in south swell energy will arrive and fill in from Tuesday to Wednesday at around 2 to 3 feet with an 18 second period. This swell will peak on Wednesday, just below south shore advisory thresholds, and then slowly decline through the end of the week. Trade wind swell energy will continue to keep elevated surf heights along all east facing shores through the coming week. Surf along east and southeast facing shores could become especially rough and choppy Thursday into Friday as Lowell passes by to the south.

Fire weather

Breezy trades continue through midweek and then strengthen Thursday through Saturday as TC Lowell passes south of the islands. Dewpoints increase into the 70s during this time in maintenance of elevated min RH values. The exact track of TC Lowell remains uncertain and thus potential for meaningful rainfall remains uncertain at this time. The inversion will hover between 6k-7kft the next couple of days.

HFO Watches/Warnings/Advisories

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

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Data Courtesy of NOAA.gov