West Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows around 69 near the shore to around 58 above 3000 feet. East winds up to 10 mph. Chance of rain 50 percent.

Monday: Partly sunny with scattered showers. Haze. Highs around 83 near the shore to around 69 above 3000 feet. East winds up to 15 mph shifting to the southeast in the afternoon. Chance of rain 50 percent.

Monday Night: Mostly cloudy. Scattered showers in the evening, then showers after midnight. Haze through the night. Lows around 69 near the shore to around 59 above 3000 feet. Southeast winds 10 to 15 mph. Chance of rain 80 percent.

South Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows around 67. East winds around 10 mph. Chance of rain 50 percent.

Monday: Mostly cloudy with scattered showers. Haze. Highs 76 to 82. East winds around 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy. Showers likely in the evening, then showers after midnight. Haze through the night. Lows around 69. Southeast winds 10 to 15 mph. Chance of rain 70 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 62 to 67 in the valleys to around 56 above 4000 feet. East winds up to 10 mph. Chance of rain 70 percent.

Monday: Mostly cloudy with scattered showers. Haze. Highs 70 to 80 in the valleys to around 62 above 4000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy. Showers likely in the evening, then showers after midnight. Haze through the night. Lows around 66 in the valleys to around 58 above 4000 feet. Southeast winds up to 15 mph shifting to the south after midnight. Chance of rain 80 percent.

North Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 62 to 70. Southeast winds up to 15 mph. Chance of rain 50 percent.

Monday: Partly sunny with scattered showers. Haze. Highs 74 to 83. Southeast winds up to 15 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Haze through the night. Lows 63 to 71. Southeast winds up to 10 mph increasing to 10 to 15 mph after midnight. Chance of rain 70 percent.

East Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 63 to 72. East winds up to 15 mph. Chance of rain 50 percent.

Monday: Mostly cloudy with scattered showers. Haze. Highs 70 to 81. Southeast winds up to 15 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy. Showers likely in the evening, then showers after midnight. Haze through the night. Lows 64 to 73. Southeast winds up to 10 mph shifting to the south after midnight. Chance of rain 80 percent.

Detailed Forecast

Synopsis

A powerful Kona storm is expected to impact the state this week into next weekend, bringing numerous hazards across the islands. A combination of flash flooding, damaging winds, and strong thunderstorms, some potentially severe, will threaten Hawaii from Tuesday through at least Saturday. The risk for flash flooding could begin as early as Tuesday over Kauai and Oahu as deep tropical moisture spreads into the western end of the state. This threat will expand eastward across the state by the latter half of the week, with the likelihood for severe thunderstorms increasing late this week and into the weekend.

Discussion

A significant transition in the large-scale weather pattern is expected to unfold across the Hawaiian Islands over next couple of days and continuing through the upcoming weekend. The current dry and relatively stable regime, characterized by moderate east- southeast flow, will persist through Monday. Conditions will begin to deteriorate Monday night into Tuesday as a deep upper- level trough amplifies northwest of the islands.
Deterministic models and their respective ensembles remain in strong agreement regarding the evolution of this system. A powerful jet streak with core winds of 100 to 140 kt at 250 mb is forecast to dig southward along the western flank of the trough. As this occurs, the trough will gradually acquire a negative tilt while expanding toward the island chain through the week. The islands will increasingly fall beneath the jet streaks left-exit region, supporting enhanced divergence aloft and large-scale ascent across the region.
At the surface, model guidance depicts a broad area of low pressure consolidating northwest of the islands in response to strong upper-level height falls. Central pressures could fall to near 990 mb while remaining well northwest of the state. The resulting pressure pattern will shift winds across the islands out of a southerly direction, drawing a plume of deep tropical moisture northward across the region.
Precipitable water values are expected to increase into the 1.5 to 2.0 inch range beginning Tuesday across the western islands, then spread eastward across the remainder of the state through midweek. This pattern will then persist through next weekend, with PWAT values potentially rising into the 2.0 to 2.3 inch range statewide later this week.
The combination of strong upper-level forcing for ascent, deep tropical moisture, and persistent southerly flow will create a favorable environment for periods of heavy rainfall with embedded thunderstorms. Repeated rounds of convection may occur as smaller disturbances rotate through the broader upper trough, increasing the potential for training rainfall bands along terrain-favored areas.
Flooding concerns are expected to increase statewide as the week progresses, particularly where the heavier rainfall persists over the same areas for multiple days. By late week and next weekend, soils will already be saturated, increasing the likelihood that additional rainfall will produce rapid runoff and dangerous flooding conditions.
In addition to the rainfall threat, strengthening south to southwest winds may develop during the latter portion of the week and upcoming weekend as the pressure gradient tightens between the developing low to the northwest and high pressure to the east. If this materializes, strong kona winds and localized downslope wind events will be likely.
While thunderstorms are certainly possible earlier in the event, the environment will become increasingly favorable for severe storms later in the week and into the weekend as as the thermodynamics and kinematics (MU CAPE of 1500 to 2500 J/kg, 50 kt 850 winds with up to 40 kt or more of 0-6 km bulk shear) become more supportive.
While uncertainty remains regarding the exact timing and location of the heaviest rainfall, confidence continues to increase that the islands will experience a prolonged period of unsettled and potentially impactful weather this week. Residents and interests across the state should continue to monitor forecasts as details regarding the timing and location of impacts become clearer.

Aviation

Mostly VFR with areas of MVFR in showers and low clouds tonight and tomorrow. Areas of MVFR will be mainly on eastern and southern sections of each island. ESE surface winds will weaken slightly Monday.
AIRMET Sierra for mtn obsc is in effect above 020 for windward locations of Kauai, Oahu, and Maui.

Marine

Strong high pressure far northeast of the state will maintain fresh to locally strong trade winds tonight, mainly over the eastern half of the state. The Small Craft Advisory (SCA) has been scaled back to include just the central and eastern waters through tonight. As winds slowly decrease, the SCA will likely be trimmed back to just the windier waters and channels around Maui County and the Big Island through Monday afternoon.
A front approaching from the west will begin to ease the trades further and shift them southeasterly on Monday, with the winds becoming southerly and increasing to fresh to locally strong speeds Tuesday through late this week. Heavy rain, thunderstorms, and rough seas are expected to accompany this storm system for the rest of this week and into the weekend.
Surf along east facing shores will gradually trend downward on Monday, then fall below normal levels Tuesday through late this week. A series of small long period west-northwest swells will keep some small surf in place along north and west facing shores during the forecast period. A small medium period north swell is possible late this week.
Surf along south-facing shores will remain small, with the exception of areas exposed to trade wind swell wrap. Rough and choppy conditions will develop Tuesday and continue through late week as southerly winds increase in advance of a front. A series of small long-period south swells will also move through Monday through late this week.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Monday for Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

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Data Courtesy of NOAA.gov