West Kaua’i

Today: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 80 to 87 near the shore to around 72 above 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with 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: Mostly cloudy. 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.

South Kaua’i

Today: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 79 to 86. East winds 10 to 15 mph. Chance of rain 40 percent.

Tonight: Mostly cloudy with numerous showers. Lows around 69. East winds around 10 mph. Chance of rain 60 percent.

Tuesday: Mostly cloudy with scattered showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 78 to 85. Southeast winds around 10 mph. Chance of rain 50 percent.

Kaua’i Mountains

Today: Partly sunny with scattered showers. Highs 74 to 79 in the valleys to around 64 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with numerous showers. 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 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 90 percent.

North Kaua’i

Today: Mostly sunny in the morning then becoming partly sunny. Scattered showers. Highs 74 to 83. East winds around 10 mph. Chance of rain 50 percent.

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

Tuesday: Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 75 to 83. South winds up to 10 mph. Chance of rain 70 percent.

East Kaua’i

Today: Partly sunny with scattered showers. Highs 71 to 83. East winds around 10 mph. Chance of rain 50 percent.

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

Tuesday: Mostly cloudy with scattered showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 72 to 84. Southeast winds up to 10 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Light to moderate trade winds today with some passing showers will give way to a wetter pattern Tuesday. A deepening low to the far northwest of the state will sink southward over the next several days. Southerly winds will help to bring increased showers across the state. The threat for heavy rain and isolated thunderstorms will likely increase over Kauai Tuesday through Wednesday.

Discussion

While there have been widespread modifications to the forecast, the general philosophy remains the same. Starting late tonight, the chance for some heavier showers starts to approach the islands. While some of the guidance suggests the possibility of thunderstorms near Kauai during Wednesday's pre-dawn hours, the chances are low, so have kept the start of thunderstorms after daybreak Wednesday. Some modifications of the areas of thunderstorms have been made so that the areas follow the latest National Blend of Models (NBM) guidance which factors in multiple high resolution and ensemble model guidance.
Trade winds will be a little weaker today than yesterday, so expect some localized land and sea breezes to set up, which in turn means a few interior showers this afternoon. The more stable airmass over the islands should limit rainfall totals.
A surface low pressure system, coupled with its associated upper level low, is north-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 another round of 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 will likely mean that Kauai is most likely to experience the heavy showers and possible thunderstorms that will be associated with the system. However southerly winds on the east side of the system will bring increased moisture over all the islands. There is good agreement with these models that precipitable water (PW) will exceed 1.5 inches, which is well above normal for this time of year. The models are also in good agreement with 500 mb temperatures around Kauai and Oahu reaching -10C, which is normal for this time of year. Despite upper level temperatures near normal, the well above normal PW will help to fuel heavier showers.
With this event, we expect a large gradient in rainfall between the western end and the eastern end of the state. The Island of Kauai will be the closest to the low pressure system and should see the greatest rainfall from this event. Heavy rain will be the main concern and a Flood Watch may be needed for Kauai around the Tuesday through Wednesday time frame. With the latest QPF ensemble guidance coming into better agreement, the main moisture axis will likely stay just west of Kauai. Nonetheless, it would only take a slight shift more eastward for Kauai to be under the main moisture band. Even if the main band of rain stays just west, heavy downpours with isolated thunderstorms should be expected over Kauai during this time. For rest of the state, scattered showers with some moderate rainfall totals will be possible with higher rainfall amounts likely around Oahu. By Friday, the upper level low should lift further north and away from the state which should bring the return of stable conditions, although trade winds may not return until later in the weekend.

Aviation

Moderate trades will weaken slightly today. Low cigs and SHRA expected for mauka and windward locations. Winds will be light enough to support isol sea/land breeze development that will bring a few SHRA to some leeward and interior areas during the afternoon hours and clearing of these areas overnight. MVFR conds will be possible in any SHRA, otherwise VFR should prevail.
No AIRMETs currently in effect.

Marine

Moderate to locally fresh easterly trade winds will ease later today and veer southeast tonight into Tuesday as a high centered far northeast of Hawaii weakens and a surface trough roughly 700 nm northwest of Kauai deepens into a low. On Tuesday, the low will is expected to stall roughly 400 nm northwest of Kauai, further veering moderate winds out of the south across most waters while moderate to fresh southeast winds develop around the Big Island and Maui through Thursday. The low will also bring a chance of heavy showers and thunderstorms, mainly over the western end of the island chain. The low will lift northward on Friday, allowing winds to back out of the southeast statewide.
Small surf is expected along north shores through today. A tiny, medium period northwest swell will fill in 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 near April average through today. A small, long period south-southwest swell is expected to arrive Tuesday, pushing surf along south facing shores above summer average. The swell will hold through Wednesday, then decline Thursday and Friday.
Small surf along east facing shores will be near current heights through today, then drop on Tuesday into Wednesday as southerly winds develop.

HFO Watches/Warnings/Advisories

None.