West Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening. Lows 71 to 76 near the shore to around 62 above 3000 feet. Northeast winds up to 10 mph. Chance of rain 20 percent.

Friday: Mostly sunny in the morning, then mostly cloudy with scattered showers in the afternoon. Highs 86 to 92 near the shore to around 77 above 3000 feet. East winds up to 10 mph. Chance of rain 30 percent.

Friday Night: Mostly cloudy in the evening then becoming partly cloudy. Lows 70 to 75 near the shore to around 61 above 3000 feet. Northeast winds up to 10 mph.

South Kaua’i

Tonight: Partly cloudy with isolated showers. Lows around 74. Northeast winds around 10 mph. Chance of rain 20 percent.

Friday: Mostly sunny. Isolated showers in the morning. Highs 82 to 91. East winds around 10 mph. Chance of rain 20 percent.

Friday Night: Mostly cloudy in the evening then becoming partly cloudy. Isolated showers. Lows around 74. Northeast winds around 10 mph. Chance of rain 20 percent.

Kaua’i Mountains

Tonight: Mostly cloudy with scattered showers. Lows 65 to 71 in the valleys to around 60 above 4000 feet. East winds around 10 mph. Chance of rain 50 percent.

Friday: Mostly cloudy with scattered showers. Highs 76 to 85 in the valleys to around 68 above 4000 feet. East winds around 10 mph. Chance of rain 40 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 65 to 71 in the valleys to around 59 above 4000 feet. East winds around 10 mph. Chance of rain 40 percent.

North Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows 66 to 75. East winds up to 10 mph. Chance of rain 20 percent.

Friday: Partly sunny with isolated showers. Highs 79 to 88. East winds up to 10 mph. Chance of rain 20 percent.

Friday Night: Mostly cloudy in the evening then becoming partly cloudy. Isolated showers. Lows 65 to 74. East winds up to 10 mph. Chance of rain 20 percent.

East Kaua’i

Tonight: Partly cloudy with isolated showers in the evening, then mostly cloudy with scattered showers after midnight. Lows 65 to 78. East winds around 10 mph. Chance of rain 40 percent.

Friday: Partly sunny with isolated showers. Highs 74 to 88. East winds around 10 mph. Chance of rain 20 percent.

Friday Night: Mostly cloudy with isolated showers. Lows 65 to 77. East winds around 10 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Light to moderate trade winds will prevail through the middle of next week as the subtropical high remains weakened and displaced southeast from its typical middle summer position. Stable and somewhat dry conditions will produce minimal windward rainfall and isolated showers over leeward terrain each afternoon.

Discussion

The stable trade wind flow is on a gradual decline today. The trades are easing slightly as an unseasonably strong low in the North Pacific weakens the subtropical high and displaces it southeast from its normal middle summer position. A mid-level ridge parked overhead is maintaining very stable conditions, though a diffuse area of higher moisture managed to lift the inversion up to around 7,000 to 8,000 feet today. Even with the slight uptick in moisture, only light showers were been observed along windward slopes, mostly notably on Kauai where a few sites measured up to a quarter of an inch of rainfall during the past 24 hours. The weaker trades resulted in more widespread leeward sea breezes and increased cloud cover over leeward terrain this afternoon, but this has only resulted in isolated showers on the Kona and Kau slopes of the Big Island.
Light to moderate trade winds along with stable and somewhat dry conditions will persist over most areas through the middle of next week with only minor day to day fluctuations. The GFS and ECMWF have been showing an area of deeper tropical moisture grazing the Big Island on Sunday. If this does occur, the likely result would be increased shower activity on the Kona and Kau slopes Sunday afternoon. In addition, a compact upper-level drifting just northwest of the islands early next week could lead to an uptick in shower activity on Kauai. Otherwise, expect limited windward rainfall and clouds and a few afternoon showers along leeward terrain.
Long term guidance shows increased odds for rebuilding trade winds next Wednesday or Thursday.

Aviation

Light to moderate trade winds with local land and sea breezes can be expected over the next few days. Clouds and showers will focus mostly over windward and mountain areas, and few brief showers are possible over leeward areas during the afternoons. MVFR conditions are expected to accompany any showers, overall VFR conditions will prevail.
No AIRMETs are in effect and none are expected.

Marine

A weak trough embedded in the broader ridge north of the islands will keep gentle to locally fresh trades blowing through the weekend and into early next week. Trade wind speeds might increase back to more typical levels by the middle of next week.
A small, long period south swell continues to slowly fill in this afternoon. South swell energy will peak this evening into Friday with surf heights along south facing shores near the summer averages before declining over the weekend. A smaller portion of this long period south swell energy will wrap around into west facing shores. Surf heights will return to background levels by early next week along south facing shores.
A brief gale force low developing north of Hawaii will generate some small, short period swell that will arrive late this weekend and bring a slight uptick in surf along north facing shores. This swell will be overshadowed by some longer period energy out of the northwest generated by a storm force low developing near Kamchatka that will arrive around September 2nd. This swell could produce surf along north facing shores near the September average during its peak. For the month of September, the average surf heights along north facing shores is 4 to 6 feet (Goddard- Caldwell Database).
Below average surf expected to continue along east facing shores due to gentle to moderate trade winds persisting through early next week.
Peak monthly high tides combined with water levels that are running higher than predicted may lead to minor flooding along the shoreline and in low-lying coastal areas toward the end of next week. Coastal flooding may be possible around the daily peak tide, which will be during the afternoon hours.

Fire weather

Lighter trade winds will remain well below the critical fire weather threshold, even though high stability and somewhat dry conditions are expected to prevail through early next week. The inversion will likely be in the 5,500 to 7,500 foot range.

HFO Watches/Warnings/Advisories

None.