West Kaua’i

Today: Partly sunny. Scattered showers and isolated thunderstorms in the morning, then numerous showers 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.

Tonight: Mostly cloudy. Occasional showers and isolated thunderstorms in the evening, then numerous showers and isolated thunderstorms after midnight. 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 80 percent.

Wednesday: Partly sunny. Numerous showers and isolated thunderstorms in the morning, then scattered showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs 80 to 85 near the shore to around 72 above 3000 feet. South winds 10 to 15 mph. Chance of rain 70 percent.

South Kaua’i

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

Tonight: Mostly cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 69. South winds around 10 mph. Chance of rain 90 percent.

Wednesday: Partly sunny. Numerous showers and isolated thunderstorms in the morning, then scattered showers in the afternoon. Highs 78 to 85. South winds 10 to 15 mph. Chance of rain 70 percent.

Kaua’i Mountains

Today: Mostly cloudy. Numerous showers and isolated thunderstorms in the morning, then occasional 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.

Tonight: Mostly cloudy. Occasional showers and isolated thunderstorms in the evening, then numerous showers 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.

Wednesday: Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible in the afternoon. Highs 74 to 80 in the valleys to around 63 above 4000 feet. South winds 10 to 15 mph. Chance of rain 70 percent.

North Kaua’i

Today: Mostly cloudy. Numerous showers and isolated thunderstorms in the morning, then occasional 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.

Tonight: Mostly cloudy. Occasional showers and isolated thunderstorms in the evening, then numerous showers and isolated thunderstorms after midnight. Locally heavy rainfall possible. Lows 63 to 70. South winds up to 10 mph. Chance of rain 90 percent.

Wednesday: Partly sunny. Numerous showers and isolated thunderstorms in the morning, then scattered showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 75 to 82. South winds around 10 mph. Chance of rain 70 percent.

East Kaua’i

Today: Mostly cloudy. Scattered showers and isolated thunderstorms in the morning, then numerous showers 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.

Tonight: Mostly cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 63 to 72. South winds around 10 mph. Chance of rain 90 percent.

Wednesday: Partly sunny. Numerous showers and isolated thunderstorms in the morning, then scattered showers in the afternoon. Highs 71 to 84. South winds around 10 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

A deepening low to the far northwest of the state drifts southward and lingers 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 today through Wednesday, with scattered showers across the state especially during the afternoon hours. Heavier showers and isolated thunderstorms will be possible around Kauai later today and into Wednesday.

Discussion

As expected, there are some modifications to the forecast this morning, but no significant changes.
The surface low is about 800 miles to the northwest, and the associated front is about 500 miles northwest of Kauai early this morning. Thunderstorms have been observed ahead of the front overnight, including some making it into the offshore waters to the northwest of the coastal waters. 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 precipitable water (PW) values well above normal (1.5 to 1.7) over the western islands, the ECMWF is the wetter of the two models. The 2 am sounding from Lihue saw PW increase from 1.18 inches yesterday afternoon to 1.59 inches. Meanwhile, the Hilo sounding was 1.37 inches at 2 am. PW values are expected to continue to increase over the western end of the state. Both models bring 500 mb temperatures of -10 to -12C which is colder than normal. The 2 am sounding had -10C at 500 MB for Lihue and -08C for Hilo.
This amount of moisture and instability continues to support the heavier showers and thunderstorms in the forecast over the western islands. Models continue to keep the system just west of Kauai, which will keep Kauai and Niihau as the main focus for the heavier showers, and thus the ones under the Flood Watch starting at noon today.
Southerly flow over the remainder of the state will bring increased moisture which will result in produce humid conditions and scattered showers. With PW values being so far above normal, 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

Winds will veer southerly in advance of a developing low pressure system to the northwest of Kauai today. 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 this afternoon.
MVFR/IFR conds will be possible in any SHRA/TSRA, otherwise VFR prevails.
No AIRMETs are currently in effect.

Marine

A low roughly 700 NM northwest of Kauai will dig south today before stalling roughly 500 NM northwest of Kauai Wednesday. In response, gentle to moderate east southeast winds will continue to veer southerly around the western end of the state and moderate to locally fresh southeast will develop over the eastern end of the state. The western end of the state will have the highest chance of heavy showers and thunderstorms due to the proximity of the low. The low is expected to move north Friday into the weekend allowing winds to gradually ease and shift more easterly.
A 2-3 foot, 12-14 second northwest swell will peak today, providing a little boost in surf along north and west facing shores. This swell will subside into Wednesday. Expect minimal surf late Wednesday through Friday. A slightly larger small, long period northwest swell is expected to fill in over the weekend.
A small, long period south-southwest swell is filling in this morning with wave energy increasing in the 14 to 18 second band range as seen in the Barbers Point, Kaumalapau and Pear Harbor near shore CDIP buoys. This new swell is expected push surf along south facing shores above summer average as it peaks tonight and holds into Wednesday. The swell will slowly decline the rest of the week.
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 wrap in from the southeasterly wind swell over the next several days.

HFO Watches/Warnings/Advisories

Flood Watch from noon HST today through Wednesday afternoon for Niihau, Kauai,