West Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 64 to 69 near the shore to around 58 above 3000 feet. Northeast winds up to 10 mph. Chance of rain 50 percent.

Monday: Mostly cloudy. Scattered showers in the morning. Isolated thunderstorms in the late morning and afternoon. Showers in the afternoon. Locally heavy rainfall possible in the late morning and afternoon. Highs 78 to 86 near the shore to around 71 above 3000 feet. East winds up to 10 mph shifting to the southwest in the afternoon. Chance of rain 90 percent.

Monday Night: 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.

South Kaua’i

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

Monday: Mostly cloudy. Scattered showers in the morning. Isolated thunderstorms in the late morning and afternoon. Numerous showers in the afternoon. Locally heavy rainfall possible in the late morning and afternoon. Highs 77 to 85. Northeast winds around 10 mph. Chance of rain 70 percent.

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

Kaua’i Mountains

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers and slight chance of thunderstorms after midnight. Lows 57 to 67 in the valleys to around 56 above 4000 feet. East winds up to 10 mph. Chance of rain 70 percent.

Monday: Numerous showers and slight chance of thunderstorms early in the morning, then showers and isolated thunderstorms in the late morning and afternoon. Locally heavy rainfall possible in the late morning and afternoon. 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.

Monday Night: Mostly cloudy with scattered showers. Lows 58 to 68 in the valleys to around 57 above 4000 feet. East winds up to 10 mph. Chance of rain 50 percent.

North Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then scattered showers and slight chance of thunderstorms after midnight. Lows 60 to 67. Southeast winds up to 10 mph. Chance of rain 50 percent.

Monday: Mostly cloudy. Scattered showers and slight chance of thunderstorms early in the morning, then numerous showers and isolated thunderstorms in the late morning and afternoon. Locally heavy rainfall possible in the late morning and afternoon. Highs 74 to 82. Southeast winds up to 15 mph. Chance of rain 70 percent.

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

East Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then scattered showers and slight chance of thunderstorms after midnight. Lows 61 to 71. East winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Monday: Scattered showers and slight chance of thunderstorms early in the morning, then showers and isolated thunderstorms in the late morning and afternoon. Locally heavy rainfall possible in the late morning and afternoon. Highs 71 to 82. East winds up to 10 mph. Chance of rain near 100 percent.

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

Detailed Forecast

Synopsis

The potential for locally heavy showers and a few thunderstorms will persist into Monday 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 spotty showers over interior and leeward areas. A return to moderate to breezy easterly trade winds is expected late Tuesday and Wednesday, with showers primarily favoring windward areas through the rest of the week.

Discussion

A moist and unstable east-southeast flow remains over the region. A broad upper-level trough stretching across the Central Pacific is maintaining instability across the island chain, and a surface low over 700 miles west-northwest of Kauai is keeping the moderate low-level wind flow over the islands out of the east- southeast. This veered flow has produced a broad zone of convergence just north of the islands that has been producing scattered heavy showers, with one small cluster of thunderstorms persisting all day about 30 miles northeast of Oahu. Even with the deepest moisture remaining just north, most windward gages have picked up between a quarter to a half inch of rain during the past 12 hours, with a handful measuring over an inch. Due to sea breezes over leeward areas, isolated heavy showers have flared over some leeward areas, with the bulk of the activity occurring across the north shore of Oahu.
The threat of additional heavy rainfall will continue through at least tonight. The primary threat will be across windward areas of all islands, where ensemble guidance shows potential for another 2 or so inches. While the threat for heavy rainfall covers all islands, the GFS and ECWMF are hinting at a subtle shortwave aloft moving over the western end of the island chain this evening, which could serve as a trigger to produce another round of heavy showers and thunderstorms around Oahu and Kauai. Even on these islands, chances for flooding are not high enough to warrant a Flood Watch, but we will be closely monitoring tonight. While showers should favor windward slopes, we cannot rule out an isolated briefly heavy shower developing over leeward areas.
The moist and unstable east-southeast flow will persist on Monday, 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 spotty afternoon showers leeward, but with less potential for heavy rainfall.
Tuesday and Wednesday will transition back into moderate to perhaps locally breezy trade winds. Trades will return as the low west of the state fills. The upper-level trough will be weakening as it drifts over the island chain, but it will remain vigorous enough to maintain some instability into Wednesday. As a result, an active shower pattern will focus rainfall over windward slopes with some showers briefly spreading leeward. Trade winds will prevail through at least Friday, followed by a potential decline next weekend.

Aviation

Light to moderate east southeast winds with localized land/sea breezes should continue tonight through tomorrow as a surface low and trough linger to the west of the Hawaiian Islands. There is lingering marginal instability due to broad upper level troughing. Thus, expect scattered showers to affect mainly windward areas, with localized sea breezes leading to isolated to scattered showers over leeward locations as well, which should decrease overnight. Isolated thunderstorms are possible tonight, mostly north of the islands. MVFR ceilings and visibility will be possible in the showers with isolated IFR conditions in the heavier showers and/or thunderstorms.
SIGMET Victor is in place for thunderstorms northeast of Oahu. These thunderstorms have been periodically dying out and then flaring up again.
AIRMET Sierra is in effect for mountain obscuration for all islands except Kauai.

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 Monday, mainly for the western windward waters, due to the proximity of the low. As the low weakens and drifts westward during the first half of this week, 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 to moderate, medium-period north-northwest swell (340-350 degree) is expected to arrive tonight, peak Monday then slowly fade into Wednesday, providing a boost in surf for north and some exposed west facing shores. In the long range, a series of small, 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.
Expect choppy surf along east facing shores through early next 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 may start as early as Monday night as series of small, long-period south swells move through the state through the upcoming week.

HFO Watches/Warnings/Advisories

None.

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