Photo Credit: Jason Weingardt

West Kaua’i

Today: Breezy. Mostly cloudy with scattered showers and isolated thunderstorms. Highs 77 to 83 near the shore to around 67 above 3000 feet. Northeast winds up to 20 mph. Chance of rain 50 percent.

Tonight: Partly cloudy. Lows 61 to 66 near the shore to around 53 above 3000 feet. Northeast winds up to 15 mph.

Thursday: Mostly sunny. Highs around 77 near the shore to around 66 above 3000 feet. Northeast winds up to 15 mph.

South Kaua’i

Today: Breezy. Mostly cloudy with scattered showers and isolated thunderstorms. Highs 78 to 83. North winds up to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Breezy. Lows around 61. North winds 10 to 20 mph.

Thursday: Mostly sunny. Breezy. Highs 75 to 80. North winds up to 20 mph.

Kaua’i Mountains

Today: Breezy. Showers and isolated thunderstorms. Highs 67 to 76 in the valleys to around 61 above 4000 feet. North winds up to 20 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy with isolated showers. Lows 53 to 60 in the valleys to around 51 above 4000 feet. North winds up to 15 mph. Chance of rain 20 percent.

Thursday: Partly sunny with isolated showers. Highs 65 to 73 in the valleys to around 58 above 4000 feet. North winds up to 15 mph. Chance of rain 20 percent.

North Kaua’i

Today: Breezy. Showers and isolated thunderstorms in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 69 to 79. North winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain near 100 percent.

Tonight: Breezy. Mostly cloudy with isolated showers. Lows 56 to 65. North winds up to 20 mph. Chance of rain 20 percent.

Thursday: Mostly sunny with isolated showers. Highs 66 to 76. North winds up to 15 mph. Chance of rain 20 percent.

East Kaua’i

Today: Breezy. Mostly cloudy with showers likely and isolated thunderstorms. Highs 70 to 80. North winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 70 percent.

Tonight: Mostly cloudy with isolated showers. Lows 54 to 66. North winds 10 to 15 mph. Chance of rain 20 percent.

Thursday: Breezy. Mostly sunny with isolated showers. Highs 66 to 77. North winds 10 to 20 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

An approaching cold front will bring a period of showers as it advances through the state Wednesday through Thursday followed by much cooler temperatures and locally breezy northerly trade winds. The front will stall over the eastern half of the state on Thursday, which will keep showery weather over Maui and the Big Island through rest of the week. A deeper trough approaching the state will bring the potential for heavy rainfall this weekend.

Discussion

Developing northerly trades and instability associated with the approaching upper trough allowed diurnal convection to gain a solid foothold over northern Kauai this afternoon through early this evening. Rain rates of 2 to 4 inches per hour over the northern slopes produced rainfall totals of 3 to 8 inches per radar estimates and a peak 3-hour measurement of 5.59″ at the Hanalei gauge. Rainfall has since dissipated with a transition to northerly trade wind showers expected as the night progresses. In the meantime, the latest radar imagery makes clear there is sufficient instability and large scale forcing advancing across the state to maintain heavy showers and pockets of thunderstorms over land breeze boundaries. For the most part, convection will tend to focus offshore during the overnight hours, but immediate coastal areas will be at risk for an isolated, quasi stationary heavy shower or thunderstorm. In the near term, convection will focus over leeward coasts of Maui County where heavy rain will be beneficial but will also require monitoring for flood potential. Confidence is increasing in wet trade wind weather this weekend centered over Maui County and possibly Oahu or the Big Island as well. Previous discussion follows.
Previous Discussion. An approaching front just northwest of Kauai has brought increasing showers across the state and added instability as well. Some of these showers are producing locally heavy rainfall, especially on Kauai where a few isolated thunderstorms were seen just offshore this afternoon. For this evening and tonight, we should see shower intensity decrease this evening as the sun sets, but the approaching front will likely maintain scattered showers around the western half of the state (especially Kauai).
As we head into Wednesday, the upper level trough will move over the state and will bring a surface front along with it. With the increased moisture and added instability, locally heavy showers will be possible throughout the day Wednesday with a slight chance of thunderstorms in the afternoon. With the bulk of the upper level dynamics passing far north of the state, shower coverage will be rather disorganized with scattered showers possible all across the state. By Wednesday evening, the front should have pushed through the western half of the state with drier air and breezy northerly winds filling in behind the front. Dewpoints will get down into the 50s for select areas on Oahu and Kauai and when combined with the breezy northerly winds, temperatures are going to feel quite cold for Hawaii standards. With the front stalling somewhere near Maui or the Big Island, the shower coverage will shift to the eastern half of the state Wednesday night.
For Thursday and Friday, we are expecting the stalled front to remain over the eastern half of the state. Drier and cooler conditions are expected over the western half of the state, with showery weather continuing over the eastern half. With the northerly trade winds, showers will primarily be focused over windward and mountain areas during this time.
For the weekend, models are hinting at a deeper upper level trough approaching the state, which will greatly enhance the upper level instability. At the surface, high pressure building northwest of the state will likely bring strong northerly trade winds across select areas. Although it is still many days away, there will be potential for significant rainfall with training storms possible wherever the remnant frontal moisture ends up being. If this forecasts holds up, flash flooding will be possible over select areas this weekend. Stay tuned for updates.

Aviation

Increasing instability ahead of an approaching front will maintain spotty heavy showers with MVFR conditions from Kauai to Maui overnight. The front will reach Kauai around sunrise and produce widespread MVFR conditions along north facing slopes, along with fresh northerly winds. The front will move to Oahu during the day, while light winds and spotty heavy showers will persist elsewhere.
AIRMET Sierra for mountain obscuration is posted for northern Kauai and Lanai. Expect this AIRMET to be dropped by midnight, only to return Wednesday morning.

Marine

A weakened ridge just northeast of the islands will keep a light to moderate east to southeast winds over the islands tonight. A cold front moving into our coastal waters this evening near Kauai will continue to move eastward across the state through Thursday, producing periods of showers for all islands. A high pressure system will build in quickly behind the front with fresh to strong northeast trade winds blowing into the region after frontal passage. These stronger winds will likely bring Small Craft Advisory (SCA) for most channel zones lasting through the weekend.
A new long period XL northwest swell (310-320 degree) is expected to rapidly build surf heights to High Surf Warning (HSW) levels late tonight, peak on Wednesday, and will once again raise combined seas above SCA thresholds for most coastal waters through Thursday. As the swell progresses down the island chain, a HSW and SCA for Big Island waters may need to be added. These surf heights will gradually trend lower from Thursday into Friday. Another northwest swell is expected to potentially reach HSA levels for north and west facing shores by Sunday. Small surf remains in the forecast for south and east facing shores this week.

HFO Watches/Warnings/Advisories

High Surf Warning from midnight tonight to 6 AM HST Thursday for Niihau, Kauai Leeward, Waianae Coast, Oahu North Shore, Maui Windward West, Kauai North, Molokai Windward, Molokai North, Molokai West, Maui Central Valley North, Windward Haleakala.
Small Craft Advisory from midnight tonight to 6 AM HST Thursday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters.

Data Courtesy of NOAA.gov