Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Cloudy. Scattered showers in the morning, then showers likely and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 84 near the shore to around 74 above 3000 feet. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 70 percent.

Tonight: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 67 near the shore to around 57 above 3000 feet. East winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain near 100 percent.

Friday: Cloudy with showers likely and isolated thunderstorms. Locally heavy rainfall possible. Highs around 84 near the shore to around 74 above 3000 feet. East winds up to 15 mph shifting to the southeast in the afternoon. Chance of rain 70 percent.

South Kaua’i

Today: Cloudy. Showers likely in the morning, then showers likely and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 79. East winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 67. Northeast winds up to 15 mph. Chance of rain near 100 percent.

Friday: Cloudy. Occasional showers and isolated thunderstorms in the morning, then showers likely and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs around 79. East winds 10 to 15 mph shifting to the southeast in the afternoon. Chance of rain 90 percent.

Kaua’i Mountains

Today: Cloudy. Showers likely in the morning, then occasional showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 77 in the valleys to around 66 above 4000 feet. Southeast winds up to 15 mph. Chance of rain 90 percent.

Tonight: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 64 in the valleys to around 53 above 4000 feet. Southeast winds up to 10 mph. Chance of rain near 100 percent.

Friday: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs around 78 in the valleys to around 67 above 4000 feet. Southeast winds up to 15 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Cloudy. Scattered showers in the morning, then showers likely and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 76 to 84. Southeast winds up to 15 mph. Chance of rain 70 percent.

Tonight: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 60 to 70. Southeast winds up to 15 mph. Chance of rain near 100 percent.

Friday: Cloudy. Showers likely and isolated thunderstorms in the morning, then occasional showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs 76 to 84. Southeast winds up to 15 mph. Chance of rain 90 percent.

East Kaua’i

Today: Cloudy. Showers likely in the morning, then showers likely and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 74 to 79. East winds up to 15 mph. Chance of rain 70 percent.

Tonight: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 63 to 70. Southeast winds up to 10 mph. Chance of rain near 100 percent.

Friday: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 75 to 81. Southeast winds up to 15 mph shifting to the south in the afternoon. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

A strong low pressure system northwest of the state will develop a broad rainband just west of Kauai County and surrounding waters. The proximity of this rainband will continue the threat of potential heavy showers and thunderstorms for Kauai County and possibly spread as far as Oahu through Friday. The rainband may shift west briefly Saturday before sweeping across the state from west to east Sunday through Monday.

Discussion

A 1000 mb storm low roughly 550 miles northwest of Kauai and a weak high pressure to far northeast continues to generate gentle southeast winds over Kauai and Oahu and more gentle to locally breezy east to southeast winds elsewhere. Satellite imagery shows high clouds are now streaming over most the island chain with some lower clouds streaming from the east and southeast along Big island and Maui. Radar shows scattered light to moderate showers have been steadily moving in along Big Island southeast and windward coasts overnight and along exposed southeast areas of Maui. CIMSS satellite precipitable water imagery shows a tongue enhanced moisture building over the state with precipitable water values ranging between 1 to 2 inches.
As the low continues to rapidly intensify today, hi res models continue to show strengthening breezy east to southeast winds and a broadening of the associated rainband. The Flood Watch for Kauai County will start at noon today and persist through Friday due to the proximity of this rainband and upper level dynamics that will likely produce several inches of rainfall along with the chance for isolated thunderstorms. If this rainband or low moves further east than model guidance, then there is a chance Oahu may also receive a few heavy showers. Elsewhere, chances of showers will increase for sheltered leeward and interior areas where seabreezes can develop this afternoon. Chance of showers will continue for exposed east and southeast areas of Maui and the Big Island as winds remain more east to southeasterly.
The rainband is expected to shift west Friday afternoon and Saturday as the low shifts north and weakens slightly. This may confine the threat of rainfall more to just Kauai during that period. The low is then expected to continue to slowly weaken while tracking northeast late Saturday through early next week. During this time winds will become more southerly over the western end of the state ahead of the associated band of moisture through Saturday night. West to northwesterly winds may fill in Sunday into early next week from west to east which may push the associated moisture band across the the state. Upper level dynamics may also be conducive for active showers within this band, especially as the converging winds interact with island topography.

Aviation

In the short term, the gradient between high pressure to the distant NE and deep-layer low pressure to the NW will support a low-level flow that is veered to the SE over the Kauai end of the island chain. Moderate to fresh E trade winds will continue over the Big Island, delivering periods of MVFR VIS/CIG in SHRA, and AIRMET Sierra is posted. VFR conditions will generally prevail elsewhere as limited showers are currently embedded within the low-level flow. Otherwise, latest observations and satellite images show increasing layered mid- and upper-level clouds moving over the islands from the SW, with the potential for light icing.
Increased moisture moving over the area from the S will likely impact Kauai and surrounding waters (and potentially Oahu) later today and tonight, bringing the potential for heavy showers and thunderstorms that would result in localized IFR conditions and widespread MVFR. Otherwise and elsewhere, limited windward showers this morning, with some afternoon clouds and a few showers over leeward and interior areas.

Marine

Moderate to strong east-southeast winds will steadily increase into the fresh to strong category for all waters today as a storm-force low evolves several hundred nautical miles northwest of the state. The east-southeast winds will peak later today, hold into Friday, then ease into the moderate to fresh category by Saturday as the system begins to lift northeastward. As this system passes the state far to the north Sunday into early next week, the winds will steadily veer out of the south to southwest and potentially return to the fresh category over the western end of the state by Sunday. Moderate to fresh northerly winds are possible Monday, with a return of a more typical trade wind pattern by midweek as high pressure builds to the north.
Surf along exposed north and west facing shores will steadily ease today as a medium-period north-northwest swell moves out. Expect small surf to persist through Saturday. An upward trend is possible later Sunday as a small long-period north-northwest swell arrives from a system across the far northwest Pacific. This upward trend will continue through the first half of next week as the aforementioned low nearby to the northwest lifts northeastward over the weekend. Heights could reach advisory levels Monday night through Tuesday night as this swell peaks. Thereafter, expect a gradual downward trend with the swell direction shifting out of the north-northwest by Thursday.
Surf along east facing shores will become rough as the winds peak today and tonight, but should steadily trend down over the weekend as the winds veer out of the south. Small surf will persist next week.
Surf along south facing shores remain small with mainly background south-southwest swell expected. The next long-period pulse is expected late Friday that should hold through the weekend before moving out. If the moderate to fresh south-southwest winds materialize as advertised later in the weekend, conditions may become rough and choppy along southerly exposures.

HFO Watches/Warnings/Advisories

Flood Watch from noon HST today through Friday afternoon for Niihau, Kauai,
Wind Advisory until 6 PM HST this evening for Big Island Summits.
Small Craft Advisory until 6 AM HST Friday for Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov