West Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then scattered showers and slight chance of thunderstorms after midnight. Locally heavy rainfall possible after midnight. Lows around 73 near the shore to around 61 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Friday: Mostly cloudy. Breezy. Scattered showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible in the morning. Highs 85 to 90 near the shore to around 76 above 3000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Friday Night: Mostly cloudy. Scattered showers in the evening, then scattered showers and slight chance of thunderstorms after midnight. Locally heavy rainfall possible after midnight. Lows around 73 near the shore to around 63 above 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

South Kaua’i

Tonight: Mostly cloudy. Showers likely in the evening, then showers likely and slight chance of thunderstorms after midnight. Locally heavy rainfall possible after midnight. Lows around 73. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Friday: Mostly cloudy. Breezy. Showers likely and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible in the morning. Highs 81 to 89. Northeast winds 10 to 20 mph. Chance of rain 70 percent.

Friday Night: Mostly cloudy. Scattered showers in the evening, then showers likely and slight chance of thunderstorms after midnight. Locally heavy rainfall possible after midnight. Lows around 74. Northeast winds around 15 mph. Chance of rain 70 percent.

Kaua’i Mountains

Tonight: Cloudy with occasional showers and slight chance of thunderstorms. Locally heavy rainfall possible after midnight. Lows 64 to 71 in the valleys to around 60 above 4000 feet. East winds up to 15 mph. Chance of rain 90 percent.

Friday: Mostly cloudy. Showers likely and slight chance of thunderstorms in the morning, then showers likely in the afternoon. Locally heavy rainfall possible in the morning. Highs 74 to 84 in the valleys to around 66 above 4000 feet. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 70 percent.

Friday Night: Mostly cloudy. Breezy. Showers likely in the evening, then occasional showers and slight chance of thunderstorms after midnight. Locally heavy rainfall possible after midnight. Lows 65 to 71 in the valleys to around 60 above 4000 feet. East winds 10 to 20 mph. Chance of rain 90 percent.

North Kaua’i

Tonight: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 65 to 73. Southeast winds around 10 mph. Chance of rain 70 percent.

Friday: Mostly cloudy. Showers likely and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible in the morning. Highs 77 to 88. East winds up to 15 mph. Chance of rain 70 percent.

Friday Night: Mostly cloudy. Scattered showers in the evening, then showers likely and slight chance of thunderstorms after midnight. Locally heavy rainfall possible after midnight. Lows 66 to 74. East winds 10 to 15 mph. Chance of rain 70 percent.

East Kaua’i

Tonight: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 65 to 77. Northeast winds up to 10 mph. Chance of rain 70 percent.

Friday: Mostly cloudy. Showers likely and slight chance of thunderstorms in the morning, then showers likely in the afternoon. Locally heavy rainfall possible in the morning. Highs 74 to 86. East winds 10 to 15 mph. Chance of rain 70 percent.

Friday Night: Mostly cloudy. Showers likely in the evening, then showers likely and slight chance of thunderstorms after midnight. Locally heavy rainfall possible after midnight. Lows 66 to 77. East winds 10 to 15 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

A relatively unstable moderate to locally breezy trade wind weather pattern will continue into Saturday. An upper low centered just NW of Kauai, along with low level convergent cloud bands riding in on easterly trade winds, bring periods of showers to all islands. Some of these showers will be locally heavy at times, with a slight chance of thunderstorms mainly over the western half of the state through Saturday. Enhanced showers will linger through next week as another upper low moves in from the northeast and lingers over the area.

Discussion

Radar and satellite show isolated to scattered showers over the islands this afternoon. The heaviest showers have been over Kauai. Thunderstorms have remained outside the coastal waters so far, but there is a slight chance we could see some lightning close to, or even on, the islands tonight. And this slight chance will continue through Saturday.
As the upper low drifts slowly southwest and weakens over the next few days, several rounds of increased low-level moisture will move in from the east as trade winds continue. The latest model guidance shows the most significant moist pushes coming in Friday night and again Monday night. But even without these enhanced periods, windward areas will see numerous showers over the next few days. The best chance for thunderstorms near or on land will be over the western islands, as they will be closer to the coldest air aloft. Moderate to heavy rain is expected with the stronger showers and any thunderstorms that manage to form, but conditions are not especially favorable for flooding, and although we continue to watch for potential changes, a flood watch is not warranted at this time.
The first upper low will be replaced by a second one late Monday. This next one will stall to the northeast of the state, then weaken and move off to the east. An upper trough connecting the remnants of the first low to our SW and the second one to the NE will remain over the area. However, the coldest air aloft will have moved off well to the NE, and what gets left near us will not be cold enough to continue the threat of thunderstorms past Saturday.
Trade winds will continue at 15 to 20 kts through at least the middle of next week, keeping showers focused on typical windward and mauka areas.

Aviation

Moderate to locally breezy trades will persist through Friday and deliver periods of low level clouds and showers to windward and mountain areas. The presence of an upper level low centered near Kauai will allow some of these showers to become enhanced and bring periods of heavy rain and possibly isolated thunderstorms. Isolated showers can be seen moving into windward and mountain areas this afternoon, with clouds and showers also building up over leeward portions of the Big Island. Upstream, a band of moisture can be seen just to the east of the island chain on visible satellite imagery and is expected to move through this evening into the overnight hours. As it moves through, trade wind showers will increase in coverage and intensity and isolated thunderstorms will be possible. Additional bands of moisture will filter in on the trades on Friday, prolonging the unsettled pattern. Heavier shower and storm activity will be capable of producing MVFR and isolated IFR ceilings and visibility.
AIRMET SIERRA for tempo mountain obscuration will likely be needed later this evening as clouds and showers increase in coverage.
AIRMET TANGO is in effect for across the island chain for tempo moderate turbulence between FL300 and FL380. This is expected to continue through the day.

Marine

Moderate to locally strong trades will remain in place over the Hawaiian coastal waters through the forecast period. An upper low north of Kauai is drifting southwestward today and will keep isolated thunderstorms in the forecast over the waters into the weekend.
Moderate, long period NW swell maintains near to above average surf along N and W facing shores through the end of the week. A brief moderate, long period NNW swell advances through the area on Friday in maintenance of elevated surf and then tapers off Saturday. Surf along N and W shores then trends down through the weekend. A hurricane force low is forecast to develop east of Japan later today and could bring another round of moderate, long period northwest swell early next week.
A series of small, medium to long period south to southwest swells filter into the area into the weekend, keeping S shores from going flat. Short period surf along E shores picks up today into the weekend as trades strengthen.

Fire weather

Wind speeds and afternoon minimum humidity levels will remain below critical fire weather thresholds this week. Unstable conditions will produce increasing wet weather trends across the state into Saturday. Locally heavy rain and thunderstorms are possible during this time period. Temperature inversion heights near Maui and the Big Island will range from the 7,500 to 9,000 feet MSL into the weekend.

HFO Watches/Warnings/Advisories

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