West Kaua’i

Tonight: Partly cloudy with scattered showers in the evening, then mostly cloudy with showers likely after midnight. Lows 66 to 71 near the shore to around 58 above 3000 feet. East winds up to 10 mph. Chance of rain 70 percent.

Sunday: Mostly cloudy with showers likely. Highs 80 to 85 near the shore to around 69 above 3000 feet. Southeast winds up to 10 mph. Chance of rain 70 percent.

Sunday Night: Mostly cloudy with occasional showers and slight chance of thunderstorms. Lows 66 to 71 near the shore to around 58 above 3000 feet. Southwest winds up to 10 mph. Chance of rain 80 percent.

South Kaua’i

Tonight: Partly cloudy with scattered showers in the evening, then mostly cloudy with showers likely after midnight. Lows around 68. Northeast winds up to 10 mph in the evening becoming light. Chance of rain 60 percent.

Sunday: Mostly cloudy with showers likely. Highs 76 to 84. South winds up to 10 mph. Chance of rain 70 percent.

Sunday Night: Mostly cloudy with showers likely and slight chance of thunderstorms. Lows around 69. Light winds. Chance of rain 70 percent.

Kaua’i Mountains

Tonight: Partly cloudy with scattered showers in the evening, then mostly cloudy with showers likely after midnight. Lows 61 to 67 in the valleys to around 56 above 4000 feet. South winds up to 10 mph. Chance of rain 70 percent.

Sunday: Mostly cloudy with showers likely. Highs 71 to 81 in the valleys to around 63 above 4000 feet. South winds up to 10 mph. Chance of rain 70 percent.

Sunday Night: Mostly cloudy with showers likely and slight chance of thunderstorms. Lows 62 to 67 in the valleys to around 58 above 4000 feet. Southwest winds up to 10 mph. Chance of rain 70 percent.

North Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 62 to 69. Southeast winds up to 10 mph. Chance of rain 50 percent.

Sunday: Partly sunny. Showers likely in the morning, then scattered showers in the afternoon. Highs 75 to 82. Light winds. Chance of rain 70 percent.

Sunday Night: Mostly cloudy with showers likely and slight chance of thunderstorms. Lows 62 to 69. Light winds. Chance of rain 70 percent.

East Kaua’i

Tonight: Partly cloudy with isolated showers in the evening, then mostly cloudy with showers likely after midnight. Lows 61 to 71. South winds up to 10 mph in the evening becoming light. Chance of rain 60 percent.

Sunday: Partly sunny. Showers likely in the morning, then scattered showers in the afternoon. Highs 71 to 83. South winds up to 10 mph. Chance of rain 70 percent.

Sunday Night: Mostly cloudy with showers likely and slight chance of thunderstorms. Lows 62 to 72. Light winds. Chance of rain 70 percent.

Detailed Forecast

Synopsis

The plume of moisture that is producing some beneficial rainfall this afternoon over the windward areas of the Big Island will shift towards Kauai late tonight into Sunday. A front approaching the state Sunday night into Monday, will bring added instability which should enhance rainfall intensity with the main band of moisture over Kauai Sunday night and moving towards Oahu on Monday. The frontal boundary is expected stall and dissipate near Oahu Monday night. Locally heavy showers will be possible Sunday night into Monday with some flooding impacts possible. Weather conditions should improve Monday night into Tuesday with trade winds returning across portions of the state on Tuesday.

Discussion

This afternoon's satellite and radar imagery shows scattered showers over the east and southeast Big Island. Rainfall totals have been relatively light with anywhere from 0.1 to 0.5 inches of rain in the past 6 hours. The scattered showers this afternoon should begin to taper off this evening over the Big Island, while this plume of moisture heads towards Kauai tonight. Showers should be on the increase late tonight over Kauai and linger through the much of the day on Sunday. Rainfall amounts should stay on the lighter side due to stable conditions aloft. Some scattered showers will also be possible on Oahu on Sunday especially during the afternoon.
As a front approaches the state Sunday night into Monday, instability will increase but overall the instability looks marginal at best with 500mb temperatures warmer than -10C throughout the event. But nonetheless with precipitable water values approaching 2 inches, showery weather is expected with embedded isolated thunderstorms possible Sunday night into Monday for the western half of the state. By Monday afternoon, the main band of moisture should be centered around Oahu. Some flooding impacts will be possible, but due to the marginal instability, extreme rainfall rates are not expected with this event. The front is expected to weaken Monday evening, but details regarding where the frontal boundary will stall still remains uncertain. Our latest forecast reflects the boundary stalling slightly more westward than our previous forecast with the main moisture boundary stalling around Oahu Monday evening. This forecast remains in good agreement with the latest 12Z ensemble guidance.
High pressure will build north to northeast of the state Tuesday into Wednesday, which should bring the return of our trade winds at least to our eastern half of the state on Tuesday. Some weak troughing north of Kauai may cause the winds to be lighter and out of the east-southeast direction for the western half of the state. Some lingering moisture will also produce scattered showers across the state through Wednesday. A drier pattern is expected for the second half of next week as an upper level ridge of high pressure builds over the state.

Aviation

Light to moderate ESE trade winds will continue through tonight as high pressure moves north of the islands. An area of increased clouds and showers is currently moving across the eastern half of the state this afternoon, bringing periodic MVFR conditions to windward slopes and coasts. A cold front will approach the state from the northwest tomorrow. Ahead of the front, winds will veer to become southerly across the western half of the state and bring widespread clouds and showers to Kauai.
Outside of the low clouds and showers moving into windward areas this afternoon/evening and the low clouds and showers that will move across Kauai and perhaps Oahu from the south tomorrow, VFR conditions will generally prevail.
AIRMET Sierra is in effect for mountain obscuration across windward and SE portions of Oahu, Molokai, Maui and the Big Island, with conditions lingering through the evening. AIRMET Sierra will likely be needed for Kauai tomorrow as showers increase across the western end of the state.

Marine

Moderate trades weaken and veer to SE from Oahu and points east and S in the vicinity of Kauai tonight as low pressure tracks well N of the area. The attendant cold front enters the western waters Sunday night where there will be a slight chance for thunderstorms. Moderate winds retain a SE component through mid- week.
The current large, medium period NW swell is tracking close to model guidance and has declined sharply since its early morning peak at the Hanalei and Waimea buoys. Surf has fallen below the advisory threshold for all shorelines and High Surf Advisories (HSA) have been allowed to expire. Small surf expected Sunday as this swell steadily diminishes. Deep low pressure presently situated just southeast of Russia's Kamchatka Peninsula will deliver a large, long period swell WNW (310) swell early next week with long period forerunners arriving by Sunday night. The distant source region of this swell compared to the present swell will focus energy in the longer period bands increasing confidence that surf will rise to well within the HSA range during Monday and perhaps approach warning criteria for N and W facing shores as it peaks Monday night through Tuesday. The swell then gradually subsides during the second half of the week.
Moderate SE flow maintains small chop along E facing shores while background energy out of the southerly quadrant maintains small surf for S facing shores.

HFO Watches/Warnings/Advisories

None.

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