West Kaua’i

Tonight: Partly cloudy. Breezy. Lows around 74 near the shore to around 64 above 3000 feet. East winds up to 20 mph.

Thursday: Mostly sunny. Highs 87 to 93 near the shore to around 80 above 3000 feet. East winds up to 15 mph.

Thursday Night: Partly cloudy. Lows 71 to 76 near the shore to around 64 above 3000 feet. East winds up to 15 mph.

South Kaua’i

Tonight: Breezy. Mostly cloudy with isolated showers. Lows around 75. East winds 10 to 20 mph. Chance of rain 20 percent.

Thursday: Partly sunny. Breezy. Isolated showers in the morning. Highs 82 to 91. East winds 10 to 20 mph. Chance of rain 20 percent.

Thursday Night: Mostly cloudy with isolated showers. Lows around 75. Northeast winds around 15 mph. Chance of rain 20 percent.

Kaua’i Mountains

Tonight: Breezy. Mostly cloudy with scattered showers. Lows 67 to 72 in the valleys to around 61 above 4000 feet. East winds 10 to 20 mph decreasing to up to 20 mph after midnight. Gusts up to 40 mph. Chance of rain 50 percent.

Thursday: Partly sunny. Breezy. Scattered showers in the morning, then isolated showers in the afternoon. Highs 76 to 86 in the valleys to around 69 above 4000 feet. East winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Thursday Night: Breezy. Mostly cloudy with scattered showers. Lows 66 to 72 in the valleys to around 60 above 4000 feet. East winds up to 20 mph. Chance of rain 50 percent.

North Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 67 to 76. East winds up to 15 mph. Chance of rain 40 percent.

Thursday: Partly sunny with isolated showers. Highs 79 to 90. East winds up to 15 mph. Chance of rain 20 percent.

Thursday Night: Mostly cloudy with isolated showers. Lows 67 to 76. East winds 10 to 15 mph. Chance of rain 20 percent.

East Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 67 to 78. East winds 10 to 15 mph. Chance of rain 50 percent.

Thursday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 75 to 88. East winds 10 to 15 mph. Chance of rain 40 percent.

Thursday Night: Mostly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 67 to 78. East winds 10 to 15 mph. Chance of rain 40 percent.

Detailed Forecast

Synopsis

Breezy to locally windy trade winds will continue through the weekend, then gradually ease early next week. Clouds and light showers will focus along windward and mauka areas, particularly overnight and during the early morning hours as a rather dry and stable airmass settles over the state and persists over the next several days.

Discussion

Surface high pressure far north of the state will continue to interact with a series of tropical cyclones passing well south of the island chain to tighten the local pressure gradient and bring breezy to locally windy trade winds across the state over the next couple of days. Not expecting to need to issue a Wind Advisory at this point, but some of the typically windier locations may briefly approach Wind Advisory criteria at times, so will continue to monitor. The upper air soundings from Hilo and Lihue both showed a strong inversion just below 5000 feet earlier this afternoon, indicative of the stable airmass overhead. These stable, breezy to locally windy trades will focus clouds and light showers over windward and mauka areas as batches of shallow moisture filter across the region, particularly during the overnight to early morning hours. The strengthening trades may also help to carry over light showers into some leeward areas at times.
As the tropical cyclones to our south weaken and move further away early next week, the local pressure gradient may relax some, allowing the trades to ease slightly. However, other than this slight decrease in wind speeds, a typical trade wind weather pattern should continue through Tuesday.

Aviation

A relatively typical summertime pattern will continue over the islands during the forecast period. Breezy to locally windy trade winds and slightly-below-normal moisture levels for July will make for generally VFR weather. There can still be a few patches of MVFR though, especially windward and mauka (N and E facing slopes). The trade winds will be strong enough to keep AIRMET TANGO in place for the foreseeable future. The slightly south-of- due-east component to the breezy trades off the surface today has resulted in numerous reports of LLWS around the PHOG airport. As the flow aloft backs just a bit, expecting conditions to be a little less favorable for LLWS Thu afternoon compared to today.

Marine

Tropical Storm Iona, and the remnants of Keli, continue to track to the west, far south of the main Hawaiian Islands. Combined with high pressure to the north-northwest of the islands, the pressure gradient over the islands continues to support Small Craft Advisory (SCA) for waters near Maui County and the Big Island. The SCA for waters near Kauai has been cancelled in light of ASCAT data showing weakening winds at that end of the state. Winds may increase as the week goes on, which may require the SCA to be expanded to most coastal waters by the weekend.
Aside from Tropical Storm Iona, more information can be found at WMO header WTPA21 PHFO and AWIPS header HFOTCMCP1, additional tropical cyclone development is possible far southeast of Hawaii in the next few days. Isolated thunderstorms along the northern periphery of these systems may effect the far southern portion of the offshore waters. While some short-period southeast swells may reach southern and southeastern shores of Hawaii, a much larger and unrelated south swell will dominate.
A multi-day buildup of surf for south facing shores is building in across the islands. Forerunners from the next much larger swell will start to fill in and peak late Thursday well over advisory levels and may near warning levels near its peak, then hold through Friday. The swell will gradually decline over the weekend and will be followed by another long period south swell early next week. As a result, a High Surf Advisory has been posted for the south facing shores of all islands through Friday, but the advisory may need to be extended in time or upgraded to a warning depending on the arrival of the swell.
East shore surf will trend up over the next few days as strengthening trade winds generate larger wind waves. Surf along east facing shores will rise to seasonal average late this evening or Thursday and hold into the weekend, followed by a possible decline early next week.
Flat summer conditions along north facing shores may be interrupted this weekend by a small long period northwest swell from Typhoon Krosa, currently in the western Pacific.

Fire weather

Both wind speeds and relative humidity values look to be borderline critical through the weekend as trade winds become breezy to locally windy and afternoon humidity values dip below 50 percent across some leeward areas. Currently, critical conditions, if reached, are not expected to persist long enough to warrant a Red Flag Warning, but will continue to monitor.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST Friday for Niihau, Kauai Leeward, Waianae Coast, Kahoolawe, Maui Leeward West, Kona, Kauai South, East Honolulu, Honolulu Metro, Ewa Plain, Molokai Southeast, Molokai Leeward South, Lanai Leeward, Lanai South, Maui Central Valley South, Kipahulu, South Maui/Upcountry, South Haleakala, Big Island South, Big Island Southeast.
Small Craft Advisory until 6 AM HST Friday for Kaiwi Channel, Maui County Windward Waters, Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.