Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Sunny and breezy. Isolated showers in the afternoon. Highs around 82 near the shore to around 70 above 3000 feet. East winds 10 to 25 mph with gusts to 45 mph. Chance of rain 20 percent.

Tonight: Mostly clear. Breezy. Isolated showers in the evening, then scattered showers after midnight. Lows around 68 near the shore to around 57 above 3000 feet. East winds 10 to 25 mph with gusts to 45 mph. Chance of rain 40 percent.

Thursday: Mostly sunny. Breezy. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 82 near the shore to around 70 above 3000 feet. East winds 10 to 25 mph with gusts to 45 mph. Chance of rain 50 percent.

South Kaua’i

Today: Mostly sunny. Windy. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 81. Northeast winds 20 to 30 mph. Chance of rain 30 percent.

Tonight: Windy. Mostly cloudy with scattered showers. Lows 65 to 70. Northeast winds 20 to 30 mph. Chance of rain 50 percent.

Thursday: Breezy. Partly sunny with scattered showers. Highs around 81. Northeast winds 20 to 25 mph. Chance of rain 50 percent.

Kaua’i Mountains

Today: Windy. Partly sunny with scattered showers. Highs 69 to 77 in the valleys to around 64 above 4000 feet. East winds 10 to 30 mph with gusts to 50 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Windy. Scattered showers in the evening, then numerous showers after midnight. Lows 58 to 63 in the valleys to around 51 above 4000 feet. East winds 10 to 35 mph. Chance of rain 70 percent.

Thursday: Partly sunny. Windy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 69 to 77 in the valleys to around 64 above 4000 feet. East winds 15 to 30 mph. Chance of rain 60 percent.

North Kaua’i

Today: Mostly sunny. Breezy. Scattered showers in the morning, then isolated showers in the afternoon. Highs 74 to 81. East winds 10 to 20 mph. Chance of rain 40 percent.

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Breezy. Scattered showers. Lows 60 to 69. East winds 10 to 25 mph with gusts to 45 mph. Chance of rain 50 percent.

Thursday: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs 74 to 81. East winds 15 to 25 mph. Chance of rain 50 percent.

East Kaua’i

Today: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs 74 to 81. Northeast winds 10 to 25 mph. Chance of rain 50 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows 58 to 72. Northeast winds 10 to 25 mph. Chance of rain 50 percent.

Thursday: Breezy. Mostly cloudy with numerous showers in the morning, then mostly sunny with scattered showers in the afternoon. Highs 74 to 81. East winds 15 to 25 mph. Chance of rain 60 percent.

Detailed Forecast

Synopsis

Trade winds will ramp up today, becoming breezy and gusty state- wide through Friday as strong surface high pressure passes by north of the area. Wind speeds will slowly ease this weekend as the ridge to the north weakens slightly, but remain locally strong through early next week. Showers will mainly focus along windward and mountain areas through the forecast period, with a brief shower spreading to leeward areas at times.

Discussion

Satellite and radar imagery have shown an increase in shower activity through the night. The 12Z soundings from Lihue and Hilo both reveal an inversion around 6-7 kft. The slightly higher inversion compared to yesterday morning and increasing trade winds have resulted in scattered low clouds and showers moving into windward areas early this morning, especially from Oahu eastward. Have adjusted PoPs up a bit through the early morning to reflect this. Expect shower activity to trend down by this afternoon as drier mid-level air moves in. Trade winds are also quite breezy this morning, with a few locations statewide already gusting in the mid-30s (mph).
Trades will strengthen significantly this afternoon through Thursday night as surface high pressure passing well north of the state becomes even stronger. During this time, 850 mb winds will increase to around 35 kt over the islands as a mid-level ridge builds and compresses the trade wind inversion. As a result, surface winds over and downwind of terrain will become strong and gusty, so a Wind Advisory has been issued starting at noon today for most areas and continuing through Friday afternoon. The robust surface high will build further as it drifts eastward Thursday night and Friday, with 850 mb winds possibly increasing under continued mid-level ridging. Therefore, the Wind Advisory will add in the summits of Maui and the Big Island this evening.
A drier air mass will settle over the region as the trade winds increase today. This would suggest a general decrease in shower activity over the next couple of days. However, any lingering moisture combined with good orographic forcing from the strong winds will keep a traditional trade wind shower pattern in the forecast, with some windward rainfall occasionally being pushed leeward. Models do hint at the possibility of some enhanced low- level moisture during the second half of the week, which could lead to somewhat greater shower coverage/intensity, especially over the eastern end of the state.
Looking ahead to the weekend, trades will likely drop off a bit, but remain breezy as the surface ridge north of the state begins to erode. With trades blowing, showers will favor windward slopes, but troughing at the mid levels could lead to increased instability with some shower enhancement. Available low-level moisture to fuel these showers remains uncertain with some model differences, but have trended the forecast a bit wetter for the weekend.

Aviation

Strong trade winds will prevail through at least tomorrow, warranting AIRMET Tango for tempo moderate, and isolated severe, low-level turbulence over and downwind of island terrain. A developing subsidence inversion feet will keep incoming low clouds confined to the lowest 8-10 thousand feet, limiting rainfall intensity. Interaction between the strong trades and island terrain has kept broken low cloud ceilings and light showers over windward Big Island through the night, and AIRMET Sierra remains posted for mountain obscuration. The forecast anticipates that these clouds will diminish later this morning. Otherwise, only isolated MVFR VIS/CIG in showers is expected, mainly over windward slopes and coasts as fast-moving and relatively light showers will be widely separated.

Marine

Strong- to gale-force easterly trade winds are expected today, which will continue through the second half of the week as high pressure passes to the north. Gales are most likely over the typically windier waters and channels from Maui County to the Big Island beginning today, but can't be ruled out over the Kaiwi Channel late tonight through Thursday. Seas will respond and become very rough over exposed waters, quickly building and exceeding the 10 ft advisory level over most waters by night (already exceeded at the Hilo PacIOOS buoy). Although winds and seas should drop slightly over the upcoming weekend as a weakness in the ridge develops far north-northwest of the area, rough boating conditions will continue.
Surf along north and west facing shores will hold steady today as a northwest swell that arrived Monday continues to move through. Swell heights at the offshore NDBC buoys 51101 and 51001 reflect this and are coming in around 5 feet with the energy centered around the 14 second band. As this swell begins to move out Thursday, a fresh north-northwest swell from a gale that developed west of the Date Line Sunday into Monday will arrive and build down the island chain. Observations from a few SOFAR drifter buoys in and around the fetch region associated with this system recently showed seas significantly higher than predicted earlier today. If this source comes in higher than expected locally, expect surf heights to reach advisory levels for exposed north and west facing shores as it peaks late Thursday. This swell will hold into Friday, then ease quickly into the weekend. A small long-period northwest swell from a developing hurricane-force low centered around 2400 nautical miles away quickly tracking northeastward is expected to fill in through the day Saturday and linger through Sunday.
Surf along east facing shores will quickly ramp up through the second half of the week as the strong trades expand over the state and upstream over the eastern Pacific. Heights will likely reach the advisory level by late Thursday for most easterly exposures.

HFO Watches/Warnings/Advisories

Wind Advisory from noon today to 6 PM HST Friday for most islands except portions of the Big Island.
Wind Advisory from 6 PM this evening to 6 PM HST Friday for Haleakala Summit, Big Island Summits.
Small Craft Advisory until 6 PM HST Friday for most Hawaiian Waters.
Gale Warning until 6 PM HST Friday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov