West Kaua’i

Tonight: Breezy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 68 near the shore to around 60 above 3000 feet. East winds 10 to 20 mph. Chance of rain near 100 percent.

Thursday: Windy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 80 near the shore to around 68 above 3000 feet. Southeast winds 10 to 25 mph shifting to the south 10 to 35 mph in the afternoon. Gusts up to 55 mph. Chance of rain near 100 percent.

Thursday Night: Windy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 66 near the shore to around 58 above 3000 feet. Southwest winds up to 30 mph with gusts to 55 mph becoming west up to 10 mph after midnight. Chance of rain near 100 percent.

South Kaua’i

Tonight: Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 68. East winds 10 to 15 mph. Chance of rain near 100 percent.

Thursday: Windy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 74 to 80. Southeast winds 10 to 20 mph shifting to the south 15 to 30 mph in the afternoon. Chance of rain near 100 percent.

Thursday Night: Windy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 66. Southwest winds 10 to 30 mph. Chance of rain near 100 percent.

Kaua’i Mountains

Tonight: Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 62 to 68 in the valleys to around 58 above 4000 feet. Southeast winds up to 15 mph shifting to the east after midnight. Chance of rain near 100 percent.

Thursday: Very windy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 72 to 77 in the valleys to around 62 above 4000 feet. Southeast winds 10 to 35 mph shifting to the south up to 40 mph in the afternoon. Gusts up to 60 mph. Chance of rain near 100 percent.

Thursday Night: Windy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 63 in the valleys to around 56 above 4000 feet. South winds up to 35 mph becoming southwest up to 15 mph after midnight. Gusts up to 60 mph. Chance of rain near 100 percent.

North Kaua’i

Tonight: Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 63 to 70. Southeast winds up to 15 mph. Chance of rain near 100 percent.

Thursday: Windy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 72 to 80. Southeast winds up to 20 mph increasing to up to 30 mph in the afternoon. Gusts up to 60 mph. Chance of rain near 100 percent.

Thursday Night: Breezy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 60 to 68. South winds up to 20 mph shifting to the southwest up to 10 mph after midnight. Gusts up to 60 mph. Chance of rain near 100 percent.

East Kaua’i

Tonight: Breezy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 63 to 71. Southeast winds up to 10 mph shifting to the east up to 20 mph after midnight. Chance of rain near 100 percent.

Thursday: Windy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 68 to 79. Southeast winds 10 to 20 mph shifting to the south 10 to 30 mph in the afternoon. Chance of rain near 100 percent.

Thursday Night: Windy. Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 62 to 70. South winds up to 30 mph becoming southwest up to 10 mph after midnight. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

Low pressure west of the islands will draw tropical moisture northward leading to periods of heavy rainfall, thunderstorms, and an increased flash flooding risk. Strong and gusty south to southeasterly winds are also expected Thursday and Thursday night. Unsettled conditions linger into next week.

Discussion

Notable moisture gradient in place over the islands as PWATs on this afternoon's Hilo sounding hover around 2″ while Lihue is closer to 1″. Southerly flow aloft will lift this moisture across the remainder of the forecast area tonight. At press time, water vapor imagery shows a potent mid-latitude shortwave anchoring the base of a longwave trough that extends NE toward Alaska. Pressure falls in advance of this feature have allowed a trough to develop west of the islands causing winds across the state to veer to SE. As tonight progresses, a slug of moisture advection will lift through the area bringing widespread showers, some of which will be heavy. At the same time, cyclogenesis can be expected within the upstream surface trough which will substantially ramp up low-level convergence within the trough as well as SSE winds. A deep, mature jet-front system develops in the right entrance of upper jet resulting in increasing intensification of showers and thunderstorms west of Kauai during the day Thursday. As the upper low advances east, this line of intense rainfall will sweep eastward reaching Kauai late Thursday evening, Oahu early Friday, and then Maui County and the Big Island during the second half of Friday. Southwesterly moisture advection aloft combined with onshore flow in the lower levels may also prove favorable for persistent heavy rainfall over the SE slopes of the Big Island on Fri. Confidence in intense and significant rainfall capable of producing considerable flash flooding increases with westward extent, meaning it is the highest for Kauai. Cyclonic flow aloft looks to barely squeak in over the middle of the state suggesting moderate-high confidence for Oahu and then decreasing confidence for Maui County and the Big Island.
Going forward, the moisture band left behind by this system will gradually become less active Friday night through Saturday as upper ridging builds overhead and subsidence increases. On Sunday, the southern periphery of another mid-latitude wave will graze the area coincident with increasing right entrance jet support. It appears increasingly likely that a band of moderate to heavy rain will develop over the islands late Sunday into Monday. Heavy rain potentially necessitating a Flood Watch for a portion of the island chain is not out of the question during this time period.
The High Wind Watch for favored portions of Kauai and Oahu will remain in place this afternoon as downsloping potential over less traditional downslope-prone areas remains unchanged. The watch will be converted to High Wind Warning or a Wind Advisory tonight.
The Flood Watch remains upgraded one category to “considerable” mainly due to expectations for particularly heavy rainfall within the eastward-advancing frontal rain band late Thursday through Friday. As noted above, confidence decreases further eastward.
The Winter Wx Advisory for the Big Island summits remains a bit tenuous, as expected, but remains in decent shape for now. A Wind Advisory has been issued for the BI Summits and Haleakala Summit on Maui beginning Thursday afternoon.

Aviation

Numerous showers and isolated thunderstorms have developed this afternoon, leading to areas of MVFR and isolated IFR conditions in low visibility and/or clouds. These conditions are expected to continue through tonight as convection evolves in areal coverage and strength in ways that models cannot yet accurately describe. Winds will increase Thursday and especially Thursday night into Friday.
AIRMET Sierra is in effect for mountain obscuration on the southeast portions of the Big Island all areas on the remaining islands. AIRMET Zulu is in effect for icing over the entire area.

Marine

South to southeast winds are expected to steadily increase late into Thursday, as a low pressure system develops just west of the state. A Small Craft Advisory (SCA) is now in effect for the waters around the Big Island and the SCA will expand in coverage at 6PM to include the waters around Kauai, the Kauai Channel and southern Oahu as the winds continue to strengthen. Showers, some heavy, will be possible through the weekend as a result of the southerly winds bringing tropical moisture to the north over the islands. As the low pressure system further deepens Thursday into Friday, we will likely see a greater coverage of thunderstorms and winds are expected to increase to gale force across the coastal waters. A Gale Watch has been issued starting Thursday for all Hawaiian waters. Depending on how the low pressure system evolves, Gale force winds could be possible by Thursday night. Very rough boating conditions are expected late Thursday into Friday as seas quickly rise in response to the strong to gale force winds.
The current south-southwest swell that has peaked will slowly drop tonight through the rest of the week. A High Surf Advisory is in effect for all south facing shores of the Hawaiian Islands through 6AM.
Along north facing shores, a moderate long-period northwest (310-320 degree) swell has also peaked. However a smaller short- period northwest swell is expected over the weekend into early next week.
As the long-period south swell declines on Thursday, strong to gale force southerly winds are expected to develop and strengthen even further Thursday night into Friday. This increase of southerly winds will bring a rapid increase of short- period energy Thursday into Friday and will once again bring surf heights above the advisory thresholds for south facing shores. Large disorganized surf is expected along many south facing shores Thursday into Friday.
Surf along east facing shores could see an increase later this week due to the strengthening southeasterly winds especially for southeast exposures.

HFO Watches/Warnings/Advisories

Flood Watch through Friday afternoon for all Hawaii islands,
High Surf Advisory until 6 AM HST Thursday for Niihau, Kauai Leeward, Waianae Coast, Kahoolawe, Maui Leeward West, Kona, Kauai South, East Honolulu, Honolulu Metro, Ewa Plain, Molokai West, Molokai Leeward South, Lanai Leeward, Lanai South, Maui Central Valley South, South Maui/Upcountry, South Haleakala, Big Island South, Big Island Southeast.
High Wind Watch from Thursday morning through late Thursday night for Kauai Mountains, Oahu North Shore, Olomana, Central Oahu, Waianae Mountains, Kauai North, Koolau Windward, Koolau Leeward.
Winter Weather Advisory until 6 AM HST Friday for Big Island Summits.
Wind Advisory from 2 PM Thursday to 6 PM HST Friday for Big Island Summits.
Small Craft Advisory from 6 PM this evening to 6 AM HST Thursday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Leeward Waters.
Gale Watch from Thursday morning through late Thursday night for all Hawaiian waters,
Small Craft Advisory until 6 AM HST Thursday for Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Kauai Now Weather is brought to you by Blue Hawaiian Helicopters.

Check out their Kaua‘i Helicopter Tours today!

Data Courtesy of NOAA.gov