West Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows around 73 near the shore to around 62 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Thursday: Partly sunny. Breezy. Isolated showers in the morning, then scattered showers in the afternoon. Highs 85 to 90 near the shore to around 76 above 3000 feet. East winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Thursday Night: Mostly cloudy. Numerous showers and isolated thunderstorms in the evening, then scattered showers and isolated thunderstorms after midnight. Lows 71 to 76 near the shore to around 62 above 3000 feet. East winds around 10 mph. Chance of rain 70 percent.

South Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows around 73. East winds 10 to 15 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy with numerous showers. Highs 81 to 89. East winds 10 to 15 mph. Chance of rain 70 percent.

Thursday Night: Mostly cloudy. Frequent showers and isolated thunderstorms in the evening, then numerous showers and isolated thunderstorms after midnight. Lows around 74. East winds 10 to 15 mph. Chance of rain 80 percent.

Kaua’i Mountains

Tonight: Breezy. Cloudy with numerous showers. Lows 64 to 71 in the valleys to around 59 above 4000 feet. East winds 10 to 25 mph. Chance of rain 60 percent.

Thursday: Cloudy and breezy. Occasional showers in the morning, then numerous showers in the afternoon. Highs 75 to 83 in the valleys to around 66 above 4000 feet. East winds 10 to 25 mph. Chance of rain 80 percent.

Thursday Night: Breezy. Frequent showers and isolated thunderstorms. Lows 64 to 71 in the valleys to around 60 above 4000 feet. East winds 10 to 20 mph. Chance of rain near 100 percent.

North Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 65 to 73. East winds up to 15 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy with numerous showers. Highs 77 to 88. East winds up to 15 mph. Chance of rain 70 percent.

Thursday Night: Mostly cloudy with frequent showers and isolated thunderstorms. Lows 66 to 73. East winds 10 to 15 mph. Chance of rain 80 percent.

East Kaua’i

Tonight: Cloudy with scattered showers. Lows 65 to 75. East winds 10 to 15 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy with numerous showers. Highs 74 to 86. East winds 10 to 15 mph. Chance of rain 70 percent.

Thursday Night: Cloudy with frequent showers and isolated thunderstorms. Lows 65 to 77. East winds 10 to 15 mph. Chance of rain 90 percent.

Detailed Forecast

Synopsis

A wet trade wind pattern begins developing early Thursday morning and matures this weekend into next week. Thunderstorms will be possible at times, particularly at night, during this stretch.

Discussion

Closed low positioned about 300 miles NE of Honolulu is slowly advancing southwestward toward the western half of the island chain where it will reside through at least the first half of the weekend. Observed soundings indicate a trade wind inversion sloping from around 6.5kft at Hilo to 13kft at Lihue, indicating loss of stability over the westernmost islands. Beneath the closed low itself, over the open ocean, visible imagery indicates that showers are not particularly widespread but are topping out around 15kft to go along with a few thunderstorms where lapse rates are steepest, around 6.5C/km.
This environment will spread over the islands late tonight into early Thursday, particularly over Oahu and Kauai where the mid-level low is modeled to reside by tomorrow morning. The first wave of deeper moisture will reach the islands during this time bringing an increase in shower coverage and intensity. Low stability and breezy trades suggest a high likelihood of showers penetrating leeward over Oahu. Lapse rates will gradually steepen to 7C/km by Saturday night suggesting a tendency for shower intensity, but not necessarily shower coverage, to increase during the same time period. Isolated thunderstorms will also be possible, particularly where low-level convergence is maximized within island plumes over the waters west of the islands. Thunder cannot be ruled out elsewhere, but is comparably less likely over the islands themselves. The exception may be the Big Island this weekend, where potential for afternoon thunderstorms will be worth monitoring as mid-level lapse rates steepen. All told, expect a transition to a wetter trade wind pattern with embedded heavy showers and isolated thunderstorms beginning early Thursday morning. Breezy trade winds will favor progressive, fast-moving showers that will not be prone to causing flooding issues.
Renewed upper low development early next week sets the stage for a continued wet trade wind pattern through much of next week.

Aviation

High pressure far northeast of the islands will maintain moderate to locally breezy easterly trade winds through tomorrow night. An upper low to the northeast will drift southwest and position itself just to the west of state. This upper low will partially destabilize the atmosphere tonight through tomorrow, allowing some bands of passing showers to become more intense, and cloud tops to become higher.
Most of the showers are expected over windward and mountain areas, with the exception of leeward Big Island, where afternoon cloud and shower development is expected. Although there will be increased instability from the upper low, the presence of dry mid level air should inhibit widespread shower coverage, keeping most TAF sites with prevailing VFR and occasional MVFR conditions. Isolated thunderstorms are possible, primarily for the northwest half of the state tomorrow.
No AIRMETs are in effect, however mtn obsc is possible early tomorrow morning due to increasing clouds and showers. AIRMET Tango will likely be needed later this afternoon as the upper low increases the chances of mid to upper level turbulence.

Marine

Moderate to fresh trade winds have filled in across the Hawaiian coastal waters as a weak surface trough over the northern waters advances west of the area and dissipates. A Small Craft Advisory (SCA) for winds around Maui County and the Big Island has been extended through 6 PM HST Friday. An approaching upper low may bring isolated thunderstorms to the northern waters from late tonight into the weekend.
Moderate to long period NW swell energy will generally persist over the next couple of days as a series of NW swells filter across the area, maintaining near to above average surf along north-facing shores through Friday. A brief, NNW swell will move through late Friday, but should taper off quickly on Saturday. Surf along north-facing shores will trend down through the weekend.
A series of small, medium to long period S swells will continue to filter into the area through the rest of the week and into the weekend, keeping south-facing shores from going flat. East-facing shores will remain small through the near term, but will see increasing short period action as trades strengthen over the latter part of the week.

Fire weather

Wind speeds and afternoon minimum humidity levels will likely remain below critical fire weather thresholds this week. Unstable conditions will produce increasing wet weather across the state into next week. Locally heavy rain and thunderstorms are possible during this time. Inversion height over Maui and the Big Island is around 6.5kft this afternoon. During the next several days, it will likely tend to lift and weaken.

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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