West Kaua’i

Today: Mostly cloudy. Scattered showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible. Highs 78 to 85 near the shore to around 70 above 3000 feet. Light winds. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Locally heavy rainfall possible in the evening. Lows around 68 near the shore to around 58 above 3000 feet. Light winds. Chance of rain 50 percent.

Saturday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 80 to 86 near the shore to around 72 above 3000 feet. Light winds becoming northeast up to 10 mph in the afternoon. Chance of rain 50 percent.

South Kaua’i

Today: Mostly cloudy. Scattered showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible. Highs 77 to 85. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Locally heavy rainfall possible in the evening. Lows around 69. Northeast winds around 10 mph. Chance of rain 50 percent.

Saturday: Partly sunny with scattered showers. Highs 78 to 85. Northeast winds around 10 mph. Chance of rain 50 percent.

Kaua’i Mountains

Today: Mostly cloudy. Numerous showers and slight chance of thunderstorms in the morning, then numerous showers in the afternoon. Locally heavy rainfall possible. Highs 72 to 78 in the valleys to around 62 above 4000 feet. Light winds. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then occasional showers after midnight. Locally heavy rainfall possible in the evening. Lows 59 to 68 in the valleys to around 57 above 4000 feet. Light winds. Chance of rain 90 percent.

Saturday: Mostly cloudy. Occasional showers in the morning, then numerous showers in the afternoon. Highs 72 to 78 in the valleys to around 63 above 4000 feet. Light winds becoming east up to 10 mph in the afternoon. Chance of rain 90 percent.

North Kaua’i

Today: Mostly cloudy. Numerous showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible. Highs 73 to 82. Southeast winds up to 10 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with scattered showers. Locally heavy rainfall possible in the evening. Lows 62 to 69. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Saturday: Partly sunny with scattered showers. Highs 74 to 83. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

East Kaua’i

Today: Cloudy with numerous showers and slight chance of thunderstorms in the morning, then partly sunny with scattered showers in the afternoon. Locally heavy rainfall possible. Highs 71 to 83. East winds up to 10 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Locally heavy rainfall possible in the evening. Lows 62 to 72. Northeast winds up to 10 mph in the evening becoming light. Chance of rain 70 percent.

Saturday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 70 to 83. Northeast winds up to 10 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

An active convective pattern with mostly light winds will hold in place today, with showers favoring the island interiors during the afternoon and evening hours and along windward areas overnight. Locally heavy rain and a slight chance of thunderstorms is forecast through today as an upper low moves over the state. Moderate trades are expected to return Saturday through the weekend. Lingering moisture and instability will likely maintain a rather showery pattern, even for leeward areas through the weekend. A new storm system could bring another chance for heavy rainfall and thunderstorms to the islands Tuesday and Wednesday.

Discussion

A cold core upper level low remains centered near Maui County this morning and is expected to gradually push eastward through the day today. The instability this upper level feature is providing to the state combined with ample lingering moisture (PWats over 1.6 inches as measured on this morning's upper air soundings) will continue to keep heavy rain in the forecast for today.
Heavy showers have been impacting Kauai for a couple of hours already this morning, with rain gages reporting up to 2 to 5 inches across interior and northern portions of Kauai in the last 6 hours. This persistent heavy rainfall has lead to flash flooding, particularly near the Hanalei River Bridge, where Emergency Management has reported flooding. High resolution models are indicating that showers will continue to fall over interior Kauai this morning which may further exasperate flooding issues and will later develop over the interior portions of the other islands by the afternoon as daytime heating induces sea breezes. Any of these showers have the potential to produce localized heavy downpours, and instability may be high enough to allow more thunderstorms to develop. In addition, snow is expected across the high elevation summits of the Big Island and the Winter Weather Advisory has been extended through 6 AM Saturday for elevations above 11,000 feet.
At the surface, a strong high pressure system to far north and weakening surface trough retreating to west will bring light to moderate southeast winds of the state today. As mentioned earlier, this will allow sea breezes to develop and bring clouds and showers to leeward and interior areas across the island chain by the afternoon and then land breezes will help to push some of this activity offshore overnight. In addition, showers caught up in the southeast flow will periodically move into windward areas, particularly during the overnight hours, and occasionally pass over into leeward areas.
Over the weekend, moderate easterly trades will return as the surface trough pushes further away and dissipates, allowing the high pressure system to the north to take more control of the local winds. Aloft, we'll lose some of the instability as the upper low opens into a trough as it weakens, but showers are expected to stick around as moisture lingers. As a result, expect a showery tradewind pattern through the weekend, with showers occasionally spilling over to leeward areas.
A new, potent storm system will disrupt the tradewind flow much of next week bringing light to moderate southeasterly winds back to the area late Monday into Tuesday and another round of potentially significant rainfall Tuesday through Thursday.

Aviation

Land breezes and some moisture shadowing within unstable ESE flow is maintaining mostly quiet conditions from Maui through Oahu this morning. Showers and thunderstorms mainly reside offshore. Exceptions are Kauai where heavy interior showers have been frequent as well as the SE facing slopes of the Big Island. Similar to yesterday, diurnal heating will allow moisture to mix inland leading to widespread moderate and isolated heavy showers over island interiors. MVFR will be common during this time with isolated pockets of IFR within heavier showers. A few thunderstorms will also be possible.
AIRMET Sierra remains in effect for E thru S slopes of Kauai. AIRMET Sierra may be needed for some or all remaining islands this afternoon.

Marine

Light to moderate east-southeast winds will persist today due to a broad surface trough located west of Kauai. Over the western end of the islands, the light winds will allow a land and sea breeze pattern to continue near the coasts. For the eastern end, light to moderate east to southeast breezes, with localized fresh pockets, will likely persist, particularly around the Big Island. Expect spotty heavy showers and thunderstorms to continue today due to an upper disturbance in the area. A return to moderate easterly trades is expected over the weekend as high pressure builds to the northeast. Guidance continues to show the trades becoming disrupted and shifting to be out of the south early next week as deep low pressure sets up nearby to the west.
Surf along exposed north and west facing shores peaked overnight and will steadily lower through the remainder of the week, with the swell direction shifting from northwest to north. On Sunday, surf will dip below average, then remain so well into next week. Guidance does depict a gale forming on Monday near the Kurils, then tracking northeast to the western Aleutians by Wednesday. If this materializes, Hawaii could experience an uptick in the northwest swell next weekend.
Surf along east facing shores will remain below the April average through early next week due to the lack of trades locally and upstream.
Surf along south facing shores will remain small this weekend, as a background south swell moves through. A larger pulse of south-southwest swell may arrive early next week.

HFO Watches/Warnings/Advisories

Winter Weather Advisory until 6 AM HST Saturday for Big Island Summits.