Photo Credit: Jason Weingardt

West Kaua’i

Today: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 83 near the shore to around 72 above 3000 feet. Southeast winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 69 near the shore to around 60 above 3000 feet. East winds up to 10 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy with scattered showers. Highs around 81 near the shore to around 70 above 3000 feet. Southeast winds up to 10 mph shifting to the southwest in the afternoon. Chance of rain 50 percent.

South Kaua’i

Today: Mostly sunny with isolated showers. Highs around 82. Light winds. Chance of rain 20 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 68. Light winds. Chance of rain 40 percent.

Thursday: Partly sunny with scattered showers. Highs 79 to 84. South winds up to 10 mph. Chance of rain 40 percent.

Kaua’i Mountains

Today: Partly sunny. Scattered showers in the morning, then showers likely in the afternoon. Highs 76 to 81 in the valleys to around 65 above 4000 feet. South winds up to 10 mph. Chance of rain 60 percent.

Tonight: Mostly cloudy with numerous showers. Lows 61 to 67 in the valleys to around 57 above 4000 feet. South winds up to 15 mph. Chance of rain 60 percent.

Thursday: Mostly cloudy with scattered showers. Highs 75 to 80 in the valleys to around 65 above 4000 feet. Southwest winds up to 10 mph. Chance of rain 50 percent.

North Kaua’i

Today: Partly sunny with scattered showers. Highs 75 to 83. South winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 62 to 70. South winds up to 15 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy with scattered showers. Highs 73 to 82. South winds up to 10 mph shifting to the west in the afternoon. Chance of rain 50 percent.

East Kaua’i

Today: Mostly sunny with isolated showers. Highs 74 to 84. South winds up to 10 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy with scattered showers. Lows 62 to 72. Southwest winds around 10 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy with scattered showers. Highs 75 to 85. South winds 10 to 15 mph shifting to the southwest in the afternoon. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Southeast winds will weaken over the smaller islands giving rise to a land and sea breeze pattern. Afternoon clouds and shower development are likely, especially Thursday and Friday afternoon. There is a slight chance of thunderstorms over interior and upslope portions of the Big Island this afternoon.

Discussion

The band of moisture that was slowly advancing toward the islands yesterday evening is being drawn NNW as mid-level winds veer to southerly, and will not reach the smaller islands. Further south, ESE winds will push moisture in the tail end of this band into Windward Maui & Big Island where showers will be common through tonight. A strong, zonal upper jet is seen on water vapor imagery nosing toward the forecast area early this morning. Strong cyclonic shear along the northern periphery of the jet is maintaining a low amplitude shortwave that will move through the area today. This will introduce some favorable background forcing and a slight reduction in stability that for existing showers, but will largely be inconsequential as mid-level capping is unlikely to budge. The potential exception will be interior and upslope portions of the Big Island where deepening moisture in the presence of increasingly favorable forcing for ascent warrant the addition of thunderstorms to this afternoon's forecast.
As the parent upper low lifts northeast, the attendant surface trough will be ushered eastward into the area, albeit in weakening form. This will serve to weaken the gradient, especially over the western half of the state where a land and sea breeze pattern may emerge as early as today and continue through Friday afternoon. The primary forecast uncertainty during this period will be heavy rainfall potential over island interiors each afternoon. For today, it is unlikely that any interior convection (over the smaller islands) can match the intensity of what developed over Windward Oahu overnight given that the environment will remain essentially unchanged. Given that the sea breeze can be highly sensitive to even very thin high clouds, both coverage and intensity may underachieve today. Potential appears much better on Thursday and Friday afternoons. First, the arrival of the aforementioned dissipating trough will be accompanied by improved mid-level moisture which should finally weaken existing capping. Second, high clouds are forecast to lift out of the forecast area which increases confidence in sea breeze development each day.
Looking ahead to early next week, models depict another round of troughing developing northwest of the islands. This would serve to maintain the background of southeasterly flow and increased potential for light winds over the smaller islands, with another increase in shower potential during the first half of next week.

Aviation

A frontal boundary and surface low to the northwest of the main Hawaiian island will maintain east to southeast flow over the area into the weekend. High clouds will continue to stream over the islands from this system today, with the thickest clouds over the western end of the island chain. Satellite shows a northwest to southeast oriented showery low cloud band making its way onto the Big Island early this morning, and it will likely reach the smaller islands later this morning boosting shower activity. Additional convergence shower bands are also possible.
AIRMET Sierra remains in effect for Oahu due to the clouds and showers obscuring the mountains at times. The Big Island slopes from Laupahoehoe to Cape Kumukahi to South Point was added to the AIRMET recently due to the clouds and showers making their way over the slopes. Expect these AIRMETs to remain in place through at least the morning hours, and its possible that additional areas will need to be added later today.

Marine

A storm low pressure system, roughly 500 nm northwest of the state, will gradually lift north today before slowly moving east and weakening Thursday through the rest of the week. A trailing trough from the low will slowly move over the state by Thursday and dissipate north of the area Friday. These features will keep winds gentle across the western half of the state and moderate to locally strong out of the east to southeast direction over the eastern half of the state through this afternoon. A Small Craft Advisory (SCA) is in effect for select areas of Maui County and the Big Island through this afternoon, but may need to be expanded for select windy zones around Maui and the Big Island. Thursday through Friday as the trough moves over the state and weakens, winds will ease below SCA levels. Moderate to fresh trades will briefly return over the weekend. A similar pattern may develop northwest of the area late in the weekend into early next week.
Surf is expected to remain below advisory levels on all shores through the weekend. A fetch of strong easterly winds just upstream of the state will produce moderate surf along east facing shores through today then gradually decline late tonight through Friday. The current short- period north swell will steadily decline today. A mix of small northwest swells will maintain modest surf along many exposed north and west facing shorelines into the weekend. A small, medium- period south (190 degrees) swell will slowly fade over the next couple of days.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST this evening for Maui County Windward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov