West Kaua’i

Tonight: Mostly cloudy. Scattered showers and isolated thunderstorms in the evening, then isolated showers after midnight. Locally heavy rainfall possible in the evening. Lows 65 to 70 near the shore to around 59 above 3000 feet. Light winds. Chance of rain 40 percent.

Tuesday: Partly sunny with isolated showers. Highs 79 to 87 near the shore to around 72 above 3000 feet. Northwest winds up to 10 mph. Chance of rain 20 percent.

Tuesday Night: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows 66 to 71 near the shore to around 59 above 3000 feet. East winds up to 10 mph increasing to 10 to 15 mph after midnight. Chance of rain 20 percent.

South Kaua’i

Tonight: Mostly cloudy. Scattered showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows around 69. Northeast winds around 10 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 79 to 86. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows around 69. Northeast winds 10 to 15 mph. Chance of rain 40 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Scattered showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 59 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 73 to 79 in the valleys to around 64 above 4000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Tuesday Night: Mostly cloudy with numerous showers. Lows 59 to 69 in the valleys to around 57 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

North Kaua’i

Tonight: Mostly cloudy. Scattered showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 62 to 69. Southeast winds up to 10 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 74 to 82. East winds up to 15 mph. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 62 to 69. East winds 10 to 15 mph. Chance of rain 50 percent.

East Kaua’i

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms in the evening, then numerous showers after midnight. Locally heavy rainfall possible in the evening. Lows 62 to 72. East winds up to 10 mph. Chance of rain 70 percent.

Tuesday: Mostly cloudy with scattered showers. Highs 72 to 84. Northeast winds up to 15 mph shifting to the east in the afternoon. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 63 to 73. East winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

The potential for locally heavy showers and a few thunderstorms will continue this evening and tonight over the western end of the state. Therefore, the Flood Watch has been extended through tonight. A return to moderate to breezy easterly trade winds is expected by Tuesday, with showers primarily favoring windward areas through the rest of the week. Drier conditions are expected near the end of the week as trades weaken and a front approaches the island chain from the northwest.

Discussion

This afternoon, an area of low pressure northwest of the islands continues to create unstable conditions, supporting occasionally heavy showers and isolated thunderstorms over the western end of the state. While the majority of the islands remained dry from late morning into early afternoon, latest radar imagery shows that shower activity has significantly increased along interior and leeward portions of Oahu as of mid-afternoon. Less rainfall is observed over the remaining islands, with only a few clouds and showers embedded within light to moderate east- southeast flow.
Based on the latest radar trends and hi-res model guidance, the Flood Watch for Niihau, Kauai, and Oahu that was previously in effect through late this afternoon has been extend through tonight. Although chances for heavy rain will generally be decreasing throughout this evening and tonight, there could still be some showers and isolated thunderstorms that develop and become anchored over terrain, which would have the potential to produce downpours and cause some flooding issues. Therefore, the forecast has been updated accordingly and the Flood Watch was extended in time.
While the moist and unstable east-southeast flow is expected to persist into tonight, chances for widespread heavy rainfall will gradually diminish by Tuesday. The surface low will become less defined Tuesday as the associated upper-level trough stretching over the region slowly weakens, allowing the mid-levels to warm and the zone of low-level convergence to lift farther north and away from the island chain. Expect continued windward rainfall chances, however, as trades gradually fill 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 and weakens. Meanwhile, the upper low will weaken into an open trough as it drifts eastward over the region 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.
By the weekend and into early next week, guidance suggests that the trades will ease once again as a cold front approaches the region.

Aviation

Light to moderate east-southeast winds will focus clouds and showers over windward coasts and slopes, particularly during the overnight to early morning time periods. These lighter winds will also allow sea breezes to form during the afternoon hours, which will bring clouds and showers to island interiors and leeward areas. Island plumes (convergent bands downstream of island terrain) may also help to bring clouds and showers to leeward areas at times. In addition, there may be instability enough to support heavy downpours and isolated thunderstorm activity over the western end of the state through this evening.
MVFR/IFR conditions can be expected in showers. Otherwise, VFR conditions should prevail.
AIRMET Sierra is in effect for mountain obscuration for all of Oahu and may be needed for portions of the state through tomorrow morning.

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 the rest of this afternoon, mainly for the western half of the island chain, due to the proximity of the low. As the low weakens and drifts westward through 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. Winds will become locally strong during the second half of the week, so Small Craft Advisories are likely for the winds and rough seas.
A small, medium-period north-northwest swell (340-350 degree) will peak today, providing a boost in surf for north and some exposed west facing shores, then fade tonight into Tuesday. 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 moves through.

HFO Watches/Warnings/Advisories

Flood Watch through late tonight for Niihau, Kauai, and Oahu.

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