West Kaua’i

Today: Cloudy in the morning then becoming partly sunny. Breezy. Isolated showers. Highs 84 to 89 near the shore to around 73 above 3000 feet. East winds up to 20 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 68 to 73 near the shore to around 60 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Friday: Mostly sunny. Isolated showers in the morning. Highs 84 to 90 near the shore to around 74 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

South Kaua’i

Today: Cloudy and breezy. Scattered showers in the morning, then isolated showers in the afternoon. Highs 78 to 86. Northeast winds 15 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 70. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Friday: Partly sunny with scattered showers in the morning, then mostly sunny in the afternoon. Highs 80 to 88. Northeast winds 10 to 15 mph. Chance of rain 40 percent.

Kaua’i Mountains

Today: Showers in the morning, then scattered showers in the afternoon. Highs 72 to 83 in the valleys to 61 to 66 above 4000 feet. East winds up to 15 mph. Chance of rain near 100 percent.

Tonight: Cloudy. Showers in the evening, then numerous showers after midnight. Lows 64 to 69 in the valleys to around 58 above 4000 feet. East winds up to 15 mph. Chance of rain 90 percent.

Friday: Mostly cloudy with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 74 to 84 in the valleys to 63 to 68 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

North Kaua’i

Today: Cloudy with scattered showers. Highs 77 to 85. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 64 to 72. East winds up to 15 mph shifting to the southeast after midnight. Chance of rain 70 percent.

Friday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 78 to 86. East winds up to 15 mph. Chance of rain 40 percent.

East Kaua’i

Today: Cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 73 to 84. East winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 65 to 75. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Friday: Mostly cloudy with scattered showers in the morning, then mostly sunny in the afternoon. Highs 75 to 86. East winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A weakly unstable cloud band associated with the remnants of an old East Pacific cold front are drifting through the islands this morning. An upper level disturbance west of Kauai will help to enhance shower activity through the early morning hours across the islands. A general troughing pattern at lower and upper levels will keep periods of showers in the forecast mainly in the overnight to early morning hours through Monday morning with more stable trends during the day. More stable trends develop from Monday onward as a high pressure ridge builds in over the state. Easterly trade winds will prevail in the moderate to breezy range today with a slight decrease in wind speeds to more moderate levels from Friday through early next week.

Discussion

An interesting picture in our weather pattern starts with satellite imagery this morning. A combination of low and upper level forcing will keep wet trends in the forecast across the state this morning. In the lower levels, a long cloud band associated with an old East Pacific frontal boundary drifts westward into the islands on the trade winds. This band will likely clear the island chain just before noon HST today with a notable decrease in daytime shower trends. In the upper levels, a weak upper low remains in place just west of Kauai, some upper level divergence and lingering instability over the islands will help to lift cloud heights over the region into Friday. Shower trends will tend to favor the typical windward and mountain areas in the overnight to early morning hours.
The latest weather forecast guidance showers a general upper level troughing pattern over the state into Monday morning. Starting with the upper low currently west of Kauai, this low will begin to drift northeastward and weaken through Friday. However a cyclonic general troughing pattern remains in place, which will keep brief passing showers in the overnight to early morning windward and mountain, and brief showers over island interior sections during the day are in the forecast through the weekend. On Monday this upper level pattern changes to a more stable environment, as an upper level ridge moves over the state and the temperature inversion heights drop to around 5,000 to 6,000 feet through much of next week.
In our wind forecast guidance, we see moderate to breezy easterly trades will continue into the overnight hours. These easterly trade winds decrease slightly into the light to moderate range starting on Friday as the high pressure ridge north of the state weakens in response to a migrating low pressure system slowly drifting across the North Pacific. Some expansion of daytime sea breezes along terrain sheltered western island slopes should be expected during this time period. This means a hybrid trade wind sea breeze weather regime will take hold over each island with brief afternoon showers possible over island interior sections this weekend due to the lingering troughing environment and low level wind convergence bands forming along trade wind and sea breeze boundaries.

Aviation

A long cloud band currently drifting westward through the islands will keep showers in the forecast, mainly over windward and mountain areas. Brief MVFR conditions are possible in showers this morning. Expect This cloud band to drift west of most islands later this morning with improving weather trends. Overnight to early morning hours will bring a return to brief periods of MVFR conditions in showers, mainly over windward mountain slopes.
Around 10:25 PM HST (8:25Z) on Wednesday night our local doppler radar detected a plume of volcanic ash quickly rising above the Halemaumau Crater at the Kilauea volcano. This ash plume swiftly rose to around 18,000 feet elevation and the easterly trade winds kept the ash cloud fairly localized to areas near the crater and immediately southwest through west of Halemaumau. The latest radar imagery shows this ash plume has decreased in intensity and the plume height has fallen to around the 8,000 foot level.
AIRMET Tango remains in effect for low level moderate turbulence over and immediately south through west of island mountains due to moderate to breezy trade winds. This AIRMET will likely be cancelled later tonight as the trade wind speeds decrease.

Marine

Moderate to locally strong trades will gradually ease today, becoming gentle to fresh by Friday in response to weakening high pressure northeast of the state. Gentle to fresh trades prevail Friday into early next week then restrengthen as the seasonal high redevelops. Small Craft Advisory has been extended through 6 am Friday as locally strong winds hold through the day and into the evening hours.
Tiny to small, long period southwest swell will hold today maintaining small surf along favored exposures. Forerunners from a moderate, long period south southwest swell originating from an extratropical cyclone east of New Zealand will gradually fill in tonight into Friday. This swell will peak over the weekend near or above the High Surf Advisory threshold before slowly fading early next week.
Choppy surf along east facing shores will hold into the weekend even though locally induced tradewind swell will be on the decline. This is thanks to a low that developed near the California coast earlier this week which is sending a moderate, medium period northeast swell later today and should peak Friday before fading Saturday. Surf along north facing shores will remain small.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Friday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.