Photo Credit: Brendan Stephen

West Kaua’i

Today: Cloudy and windy. Numerous showers and isolated thunderstorms in the morning, then showers likely and scattered thunderstorms in the afternoon. Some thunderstorms may produce gusty winds in the afternoon. Locally heavy rainfall possible. Highs around 84 near the shore to around 74 above 3000 feet. South winds 15 to 30 mph. Chance of rain 70 percent.

Tonight: Windy. Cloudy with showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 65 to 70 near the shore to around 56 above 3000 feet. Southwest winds 15 to 30 mph. Chance of rain 90 percent.

Wednesday: Partly sunny. Scattered showers in the morning. Highs around 82 near the shore to around 73 above 3000 feet. West winds 10 to 15 mph. Chance of rain 50 percent.

South Kaua’i

Today: Cloudy and breezy. Numerous showers and isolated thunderstorms in the morning, then showers likely and scattered thunderstorms in the afternoon. Some thunderstorms may produce gusty winds in the afternoon. Locally heavy rainfall possible. Highs around 82. South winds 15 to 25 mph. Chance of rain 70 percent.

Tonight: Breezy. Cloudy with showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 63 to 69. Southwest winds 15 to 25 mph. Chance of rain 90 percent.

Wednesday: Partly sunny. Scattered showers in the morning. Highs around 81. West winds 10 to 15 mph. Chance of rain 40 percent.

Kaua’i Mountains

Today: Cloudy and windy. Numerous showers and isolated thunderstorms in the morning, then showers and scattered thunderstorms in the afternoon. Some thunderstorms may produce gusty winds in the afternoon. Locally heavy rainfall possible. Highs around 77 in the valleys to around 66 above 4000 feet. South winds 10 to 30 mph. Chance of rain 90 percent.

Tonight: Windy. Cloudy with showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows around 59 in the valleys to around 51 above 4000 feet. Southwest winds 15 to 30 mph. Chance of rain 90 percent.

Wednesday: Mostly cloudy. Breezy. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 76 in the valleys to around 65 above 4000 feet. West winds 10 to 20 mph. Chance of rain 50 percent.

North Kaua’i

Today: Cloudy and breezy. Numerous showers and isolated thunderstorms in the morning, then showers likely and scattered thunderstorms in the afternoon. Some thunderstorms may produce gusty winds in the afternoon. Locally heavy rainfall possible. Highs 77 to 83. South winds 10 to 25 mph. Chance of rain 70 percent.

Tonight: Breezy. Cloudy with showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 59 to 68. Southwest winds 15 to 25 mph. Chance of rain 90 percent.

Wednesday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 76 to 82. West winds 10 to 15 mph. Chance of rain 50 percent.

East Kaua’i

Today: Cloudy and breezy. Numerous showers and isolated thunderstorms in the morning, then showers likely and scattered thunderstorms in the afternoon. Some thunderstorms may produce gusty winds in the afternoon. Locally heavy rainfall possible. Highs 75 to 81. South winds 15 to 25 mph. Chance of rain 70 percent.

Tonight: Breezy. Cloudy with showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 57 to 70. Southwest winds 15 to 25 mph. Chance of rain 90 percent.

Wednesday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 74 to 80. West winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A vigorous front will move over the islands later today into Wednesday, bringing the potential for impactful weather. Main impacts with the front will be strong and gusty south to southwest winds, heavy rainfall leading to localized flooding, and the potential for strong thunderstorms. The front is expected to stall and weaken near the Big Island on Thursday, then dissipate by Friday. Winds and shower chances will rapidly diminish after the front passes, with light and variable winds on Thursday. Light to moderate east to southeast winds will develop on Friday, then continue into next weekend, delivering just a few showers.

Discussion

A vigorous front will move SE over the islands later today into Wednesday, bringing the potential for impactful weather. Main impacts with the frontal passage will be strong and gusty south to southwest winds, heavy rainfall leading to localized flooding, and the potential for strong thunderstorms. The ongoing Flood Watch (FFA) for all islands except the Big Island continues, with the start time for Oahu and Maui County pushed back to tonight. Additionally, a Special Weather Statement (SPS) has been issued to highlight the potential for strong thunderstorms over Kauai and Oahu later today and tonight.
Current analyses show low pressure centered far NW of the islands and high pressure to the distant NNE, with a cold front about 600 miles NW of Kauai, and a trough about 200 miles out. The front is expected to merge with the trough later today as they both approach the islands from the NW. A sharpening trough aloft to the NW is sending broken to overcast mid- and high-level layered clouds over most islands from the W, leading to mostly cloudy skies, with some clearing currently over Oahu and Maui County expected to be short- lived. Increasing low-level S winds over Kauai have been observed overnight, with radar detecting a few small heavy rain cores, mainly over leeward waters. An area of showers that was earlier over parts of leeward Maui and adjacent waters is now weakening after forming along a northward-moving Big Island plume.
Latest guidance indicates limited pre-frontal shower development, with the bulk of the rainfall associated with the actual frontal passage. Timing of the most impactful portion of the front is late afternoon/evening for Kauai, late tonight/early Wednesday morning for Oahu, during the day Wednesday for Maui County, and Wednesday night for the Big Island. Strong S to SW winds will be especially gusty over windward portions of Kauai and Oahu, especially right ahead of the front. The boundary is expected to be fairly progressive, but rainfall rates with the heavier showers will bring the potential for flooding. With a negatively-tilted trough aloft to the NW providing strong uplift and 0-6 km shear values forecast to be ~40 kt near Kauai and Oahu, convection along the frontal boundary could become organized to the point of producing wind gusts to 50 mph and small hail, thus the aforementioned SPS.
The front is forecast to weaken and become increasingly shallow as it reaches the Big Island late Wednesday, but associated clouds and showers will persist into Thursday as increased moisture from the stalling front remains. With the front expected to be weakening, confidence is somewhat reduced in the potential for flash flooding, thus the FFA has not been expanded to include the Big Island.
Behind the front on the smaller islands, expect dry and stable conditions with rapidly diminishing winds becoming light and variable on Thursday. The front will dissipate just E of the Big Island on Thursday, with some of the residual moisture spreading westward over the islands on Friday as trade winds gradually return. Another low NW of the islands may keep winds veered to the SE this weekend, with an associated weak front that may arrive early next week, but model guidance has not consistently portrayed this solution.

Aviation

Southerly winds will gradually strengthen ahead of a cold front approaching Kauai from the northwest. Increasing instability and enhanced moisture in advance of the front will cause layered clouds with embedded heavy showers and thunderstorms to eventually spread over the extreme western end of the state today. This will likely result in widespread MVFR conditions with brief periods of IFR conditions developing on Kauai. Gusty winds may also accompany some of the deep convection. The islands east of Kauai will likely have an increase in mid- and high-level clouds with spotty showers starting late this morning. However, VFR conditions should prevail from Oahu to the Big Island into this afternoon.
As the front moves closer to Kauai this evening, expect the layered clouds, heavy downpours and thunderstorms to begin spreading across Oahu. The clouds and showers may become widespread over portions of Maui County later tonight or early Wednesday morning. A few showers are possibly over parts of the Big Island late tonight, with increasing showers expected on Wednesday.
No AIRMETs are currently in affect. However, increasing clouds and showers may result in the issuance of AIRMET Sierra for mountain obscuration om Kauai starting this morning. In addition, with the increasing southerly winds, AIRMET Tango may be required for low- level mechanical turbulence developing downwind of the western islands starting later today or tonight.

Marine

Fresh to strong southeast winds will shift out of the south and become strong over most waters today through Wednesday as a cold front advances down the island chain. Seas will respond quickly and become rough over exposed waters, especially in and around heavy showers and thunderstorms that develop along and ahead of the front. The leading edge of the line of showers and storms will enter the Kauai waters by noon today, then to the Oahu and Maui County waters tonight through Wednesday, before stalling and diminishing over the eastern end of the state Wednesday night through Thursday. Winds and seas will quickly drop off following the front through the day Wednesday. The background flow will shift out of the west-northwest Wednesday over the Kauai and Oahu waters, then quickly shift out of the north to northeast Thursday through Friday for all waters as weak high pressure builds to the north. The Small Craft Advisory has been expanded and now includes all waters extending west from the Kaiwi Channel and the waters around the Big Island.
Surf along north and west facing shores will pick up late Wednesday as a small to moderate west-northwest medium period swell arrives from low pressure centered around 1000 nm to the northwest. This source will peak Thursday, then ease Friday. A long-period northwest swell from a distant source currently located near the Kurils in the far northwest Pacific is forecast to arrive Friday night, build through the day Saturday, then peak Saturday night into Sunday before easing.
Surf along south facing shores will become rough and choppy tonight through Wednesday due to the aforementioned strong south winds preceding the cold front moving through. This will quickly lower by Thursday as the winds become light again. A long-period south swell associated with a typical system passing through our swell window in the southern Pacific will arrive Friday night, then peak over the weekend.
Surf along east facing shores will remain small through the week due to the lack of trades locally and upstream.

HFO Watches/Warnings/Advisories

Flood Watch through Wednesday afternoon for Niihau and Kauai.
Flood Watch from this evening through Wednesday afternoon for Oahu Molokai Maui Lanai and Kahoolawe.
Small Craft Advisory until 6 PM HST Wednesday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel, Big Island Windward Waters, Big Island Leeward Waters.

Data Courtesy of NOAA.gov