West Kaua’i

Today: Mostly cloudy with showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 78 to 86 near the shore to around 71 above 3000 feet. Southeast winds up to 10 mph shifting to the south in the afternoon. Chance of rain 90 percent.

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 65 to 70 near the shore to around 58 above 3000 feet. Light winds. Chance of rain 40 percent.

Tuesday: Partly sunny with isolated showers. Highs 78 to 85 near the shore to around 71 above 3000 feet. Northwest winds up to 10 mph. Chance of rain 20 percent.

South Kaua’i

Today: Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 77 to 85. East winds around 10 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 68. Northeast winds around 10 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 77 to 84. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Kaua’i Mountains

Today: Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 72 to 78 in the valleys to around 63 above 4000 feet. East winds up to 10 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy with scattered showers. Lows 58 to 68 in the valleys to around 57 above 4000 feet. Light winds. Chance of rain 50 percent.

Tuesday: Mostly cloudy. Scattered showers in the morning, then numerous showers in the afternoon. Highs 71 to 77 in the valleys to around 62 above 4000 feet. East winds up to 15 mph. Chance of rain 70 percent.

North Kaua’i

Today: Mostly cloudy. Numerous showers and isolated thunderstorms early in the morning, then showers and isolated thunderstorms in the late morning and afternoon. Locally heavy rainfall possible. Highs 74 to 82. Southeast winds up to 15 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy with scattered showers. Lows 61 to 68. Southeast winds up to 10 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 73 to 82. East winds up to 15 mph. Chance of rain 50 percent.

East Kaua’i

Today: Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 71 to 82. East winds up to 10 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy with numerous showers. Lows 62 to 71. East winds up to 10 mph. Chance of rain 70 percent.

Tuesday: Mostly cloudy with scattered showers. Highs 70 to 82. Northeast winds up to 15 mph shifting to the east in the afternoon. Chance of rain 50 percent.

Detailed Forecast

Synopsis

The potential for locally heavy showers and a few thunderstorms will persist today, especially over the western end of the state, as an upper-level disturbance produces unstable conditions. Light to moderate east-southeast winds will focus rainfall across windward and mauka locations, while daytime sea breezes trigger showers over interior and leeward areas. A return to moderate to breezy easterly trade winds is expected later tonight through Tuesday, with showers primarily favoring windward areas through the rest of the week. Trade winds may become disrupted once again next weekend as a front approaches the region.

Discussion

Short-term model guidance remains in good agreement, indicating that the unstable pattern will persist today as a deep area of low pressure lingers to the west of the state. Pulses of energy rotating around this upper low will lead to periods of heavy showers and thunderstorms, especially over the western end of the state. Convergent southeast flow interacting with the island terrain will maintain a high potential for localized heavy showers and storms, which could develop and anchor periodically. This scenario quickly evolved over the windward side of Oahu overnight, where 3 to 5 inches of rain were observed within a six- hour period, driving water levels in the streams to flood stage. Given the sensitivity and saturated grounds, along with the potential for additional heavy showers, a Flood Watch has been issued for Niihau, Kauai and Oahu through this afternoon.
The moist and unstable east-southeast flow could persist into tonight, but chances for widespread heavy rainfall will slowly diminish. The upper-level trough stretching over the region will begin to slowly weaken, allowing the mid-levels to warm slightly, and the zone of low-level convergence will lift farther north. Expect continued windward rainfall with trades gradually filling in from east to west.
By midweek, a return to moderate to breezy trade winds is expected as high pressure builds to the north and the surface low west of the state moves farther away. Meanwhile, the upper low will weaken into an open trough as it drifts eastward over the region from Tuesday through Thursday. This pattern will shift showers back to the typical windward and mauka locations statewide, especially as pockets of low-level moisture move through. More stable conditions are anticipated later in the week as upper heights rise and the weakening upper trough exits east of the state.
The long-term guidance is now in slightly better agreement for next weekend compared to 24 hrs ago, and shows the trades easing once again with a cold front approaching the region.

Aviation

Light to moderate ESE winds as a surface low and trough move closer to the islands. There is some instability, so expect SHRA and a few TSRA. IFR/MVFR conds can be expected SHRA/TSRA. Elsewhere, VFR conds should prevail.
AIRMET Sierra is in effect for mtn obsc for all of Oahu and windward portions of the rest of the islands.

Marine

An area of low pressure just northwest of the state is generating light to moderate east-southeast winds over the western half of the state and moderate to locally fresh east southeast winds over the eastern half. Isolated thunderstorms are possible through today, mainly for the western waters, due to the proximity of the low. As the low weakens and drifts westward today and Tuesday, a couple of strong high pressure systems will pass north of the state. Winds will gradually back to a more typical easterly direction and increase to moderate to locally fresh speeds statewide by Tuesday night, before becoming locally strong during the second half of the week when Small Craft Advisories are likely for the winds and rough seas.
A small, medium-period north-northwest swell (340-350 degree) will peak today before fading into Tuesday, providing a boost in surf for north and some exposed west facing shores. In the long range, a series of small to moderate, long-period northwest (310-330 degree) swells look to keep surf from going flat for north and west facing shores Tuesday through much of next week. The next significant northwest swell may arrive next weekend.
Expect choppy surf along east facing shores through the first half of the week. East shore surf may pick up a notch or two by midweek as wind speeds increase locally and upstream. A small boost in south shore surf is possible Tuesday through the rest of the week as series of small, long-period south swells move through.

HFO Watches/Warnings/Advisories

Flood Watch in effect for Niihau, Kauai and Oahu through 6 PM HST today.