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 overnight as a deepening low to the far northwest of the state drifts southward Tuesday, lingering west of the state over 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

No changes to the forecast package this evening, but anticipate making some tweaks with the morning package to factor in the latest guidance. Don't expect that to change the flavor of the forecast.
Monitoring radar closely tonight as winds are beginning to veer to the southeast, with precipitable water (PW) values on the increase. This can lead to showers forming over the Koolau mountain range on Oahu that hang up on the mountains. Otherwise, passing showers are expected over the islands tonight.
The complex surface low pressure system is about 900 miles northwest of Kauai this evening, with its developing frontal boundary roughly 500 miles to the northwest of Kauai. Thunderstorms have begun to form along this developing surface feature, with an isolated thunderstorm or two reaching the northern offshore waters this evening. The GFS and ECMWF continue to be in good agreement with the surface system and its associated strengthening upper level low sinking south over the next few days. While both models bring PW values well above normal (1.5 to 1.7) over the western islands, the ECMWF is a little wetter than the GFS. Both models bring 500 mb temperatures of -10 to -12C which is colder than normal. This amount of moisture and instability increases confidence of heavier showers and thunderstorms over the western islands. Models continue to keep the system just west of Kauai, which places Kauai and Niihau in the main focus, and thus the ones under the Flood Watch starting at noon on Tuesday.
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 trades will continue through the evening, ushering in a few SHRA to windward and mauka locations. Winds will remain light enough for land breezes to develop overnight and help to clear out leeward and interior areas.
Tomorrow (Tuesday), winds will veer southerly in advance of a developing low pressure system to the northwest of Kauai. This should focus SHRA over south-facing coasts and slopes, along with some sea breeze development bringing low cigs and SHRA to north and east- facing coasts and slopes. The proximity of the low could bring +RA and isol TSRA to the western end of the state beginning tomorrow afternoon.
MVFR/IFR conds will be possible in any SHRA/TSRA, otherwise VFR prevails.
No AIRMETs are currently in effect.

Marine

High pressure far northeast of Hawaii will gradually weaken as a gale low develops and becomes anchored roughly 500 nm northwest of Kauai Tuesday. In response, moderate to locally fresh easterly trade winds will ease this evening then veer south around the western end of the state and southeast for the eastern end of the state by Tuesday afternoon. Near the Big Island and Maui, southeast winds may increase to moderate to locally strong levels that could necessitate 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. The low is expected to move north Friday into next weekend allowing winds to gradually ease and shift more easterly.
A tiny, medium period northwest swell is currently moving through the state as band energy in the 12-14 second range has been steadily increasing since this afternoon based on the Hanalei near shore buoy. This swell will provide a small boost in surf along north and west facing shores as it peaks Tuesday before slowly subsiding into Wednesday. Minimal surf is expected later Wednesday through Friday. A slightly larger small, long period northwest swell is expected next weekend.
South shore surf will remain small through this evening. Long period forerunners of a small, long period south-southwest swell is expected to arrive late tonight into early Tuesday morning, pushing surf along south facing shores above summer average. The swell will is expected to peak Tuesday night then hold through Wednesday, before declining 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. East shore surf may see some side shore chop from the southeasterly wind swell that will develop next week.

HFO Watches/Warnings/Advisories

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