West Kaua’i

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 66 to 71 near the shore to around 59 above 3000 feet. East winds around 10 mph. Chance of rain 50 percent.

Tuesday: Partly sunny. Scattered showers and isolated thunderstorms in the morning, then showers likely and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs 79 to 86 near the shore to around 72 above 3000 feet. Southeast winds 10 to 15 mph. Chance of rain 70 percent.

Tuesday Night: Mostly cloudy with showers likely and isolated thunderstorms. Locally heavy rainfall possible. Lows 66 to 71 near the shore to around 59 above 3000 feet. South winds around 10 mph. Chance of rain 70 percent.

South Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows around 69. East winds around 10 mph. Chance of rain 50 percent.

Tuesday: Partly sunny. Scattered showers and isolated thunderstorms in the morning, then showers likely and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs 78 to 85. Southeast winds around 10 mph. Chance of rain 70 percent.

Tuesday Night: Mostly cloudy with showers likely and isolated thunderstorms. Locally heavy rainfall possible. Lows around 69. South winds around 10 mph. Chance of rain 70 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 60 to 68 in the valleys to around 57 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Tuesday: Mostly cloudy. Numerous showers and isolated thunderstorms in the morning, then showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs 74 to 80 in the valleys to around 63 above 4000 feet. South winds 10 to 15 mph. Chance of rain 80 percent.

Tuesday Night: Mostly cloudy. Showers and isolated thunderstorms in the evening, then showers likely and isolated thunderstorms after midnight. Locally heavy rainfall possible. Lows 60 to 68 in the valleys to around 57 above 4000 feet. South winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 90 percent.

North Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 62 to 70. East winds up to 10 mph. Chance of rain 50 percent.

Tuesday: Mostly cloudy. Numerous showers and isolated thunderstorms in the morning, then showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs 75 to 83. South winds up to 10 mph. Chance of rain 80 percent.

Tuesday Night: Mostly cloudy with showers likely and isolated thunderstorms. Locally heavy rainfall possible. Lows 63 to 70. South winds up to 10 mph. Chance of rain 70 percent.

East Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 63 to 72. East winds up to 10 mph. Chance of rain 70 percent.

Tuesday: Mostly cloudy. Scattered showers and isolated thunderstorms in the morning, then showers likely and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs 72 to 84. Southeast winds up to 10 mph. Chance of rain 70 percent.

Tuesday Night: Mostly cloudy with showers likely and isolated thunderstorms. Locally heavy rainfall possible. Lows 63 to 72. South winds around 10 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

Trades will dissipate tonight as a deepening low to the far northwest of the state drifts southward and lingers west of the state the next several days. Southerly winds will help to bring increased showers across the state. Unsettled weather conditions are expected Tuesday through Wednesday with scattered showers across the state especially during the afternoons. Heavier showers and isolated thunderstorms will be possible around Kauai late Tuesday into Wednesday.

Discussion

This afternoon's satellite and radar imagery shows scattered showers across leeward areas. These showers should decrease this evening as we lose day time heating. Trades will begin to dissipate tonight as a low pressure system approaches the state from the northwest.
A surface low pressure system, coupled with its associated upper level low, is northwest of the islands, and this will be the focus for our next weather event. This system is expected to strengthen as it sinks southward over the next few days, bringing with it a round of somewhat unstable and wet weather to the islands. There is good agreement with the GFS and ECMWF that both the surface and upper level lows will remain to the west of the islands. This means that Kauai will have the greatest chance for heavy showers and isolated thunderstorms that will be associated with the system. With above normal precipitable water (PW) values of around 1.5 to 1.7 inches and some marginal instability (500 mb temps of around -10C) the atmosphere will be more than capable of producing periods of heavy rain. Although the latest ensemble guidance is in fairly good agreement with the main moisture axis staying just west of Kauai, we decided to go with a Flood Watch for Kauai due to some of the National Water models showing a few streams approaching critical levels. With the flow beginning to veer toward the south tonight, we should begin to see an increase of showers tonight. But the best chance for the heaviest showers and isolated thunderstorms will likely be Tuesday afternoon through Wednesday morning.
For rest of the state, moist southerly flow will produce humid conditions and scattered showers. With the added moisture, some brief downpours are possible especially during the afternoon and evening hours. Widespread rainfall is not expected for rest of the state, but some showery conditions will be possible at times.
Thursday into Friday, the upper level low should lift further north and away from the state which should bring the return of stable conditions, although somewhat unsettle weather may continue near Kauai. Winds will continue to remain light to moderate and out of the southeast direction, which will produce warm and humid conditions through the rest of the week. Trade winds should return sometime during the first half of next week.

Aviation

Light to moderate trade winds will continue through the evening, ushering in a few brief showers to windward and mountain areas. Winds have been light enough to support sea breeze activity this afternoon, bringing clouds a few showers to leeward and interior areas (particularly across the Big Island). Winds will remain light enough for land breezes to develop overnight and help to clear out leeward and interior areas.
Tomorrow, winds will veer out of the south in advance of a developing low pressure system to the northwest of Kauai. This flow will focus showers over south-facing coasts and slopes, along with some sea breeze development bringing clouds and a few showers to north and east-facing coasts and slopes as well. The proximity of the low will enhance showers and bring the potential for isolated thunderstorm activity to the western end of the state beginning tomorrow afternoon.
MVFR conditions will be possible in any showers, otherwise VFR should prevail.
No AIRMETs are currently in effect.

Marine

High pressure far northeast of Hawaii will gradually weaken as a surface through roughly 600 nm northwest of Kauai deepens into a complex low. In response, moderate to locally fresh easterly trade winds will ease this evening then veer to the southeast by tomorrow afternoon. The primary low center is expected to become anchored 400 nm northwest of Kauai Tuesday afternoon, causing winds over the western end of the state to further veer to the south. Near the Big Island and Maui, southeast winds will increase to moderate to fresh levels, likely necessitating a Small Craft Advisory (SCA) Wednesday through Friday. The low will also bring a chance of heavy showers and thunderstorms, mainly over the western end of the island chain.
Small surf is expected along north shores through this evening. A tiny, medium period northwest swell will fill in later tonight and provide a small boost in surf into Wednesday. Minimal surf is expected later Wednesday through Friday, followed by another small pulse of northwest swell next weekend.
South shore surf will be remain small through this evening. A small, long period south-southwest swell is expected to arrive late tonight, pushing surf along south facing shores above summer average. The swell will hold through Wednesday, then decline Thursday and Friday.
Surf along east facing shores will remain near or below average due to the lack of strong trade winds over or upstream of the islands.

HFO Watches/Warnings/Advisories

Flood Watch from Tuesday afternoon through Wednesday afternoon for Niihau, Kauai,

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