Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Mostly sunny in the morning then becoming partly sunny. Breezy. Scattered showers. Highs around 83 near the shore to around 67 above 3000 feet. East winds up to 25 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 66 near the shore to around 55 above 3000 feet. Northeast winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain 50 percent.

Monday: 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 68 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

South Kaua’i

Today: Breezy. Mostly cloudy with numerous showers. Highs 74 to 81. Northeast winds 15 to 25 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows around 66. Northeast winds 10 to 15 mph. Chance of rain 60 percent.

Monday: Partly sunny. Numerous showers in the morning, then scattered showers in the afternoon. Highs 75 to 82. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Kaua’i Mountains

Today: Breezy. Mostly cloudy with showers. Highs 69 to 78 in the valleys to around 59 above 4000 feet. East winds 10 to 25 mph. Chance of rain 90 percent.

Tonight: Breezy. Mostly cloudy with numerous showers. Lows around 63 in the valleys to around 53 above 4000 feet. East winds 10 to 20 mph. Chance of rain 70 percent.

Monday: Mostly cloudy. Showers in the morning, then numerous showers in the afternoon. Highs 70 to 78 in the valleys to around 60 above 4000 feet. East winds 10 to 15 mph. Chance of rain 90 percent.

North Kaua’i

Today: Breezy. Mostly cloudy with numerous showers. Highs 72 to 82. East winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 60 to 69. East winds 10 to 15 mph. Chance of rain 60 percent.

Monday: Partly sunny. Numerous showers in the morning, then scattered showers in the afternoon. Highs 73 to 83. East winds 10 to 15 mph. Chance of rain 70 percent.

East Kaua’i

Today: Breezy. Mostly cloudy with numerous showers. Highs 68 to 78. Northeast winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with numerous showers. Lows 60 to 69. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Monday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 69 to 79. East winds 10 to 15 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

Surface high pressure far north of the state is weakening as it moves east. This will result in the gradual weakening of trade winds through Tuesday. Periodic bands of showers moving in from the east will focus rainfall over windward exposures, with occasional showers passing into leeward areas. Low pressure is expected to develop several hundred miles northwest of the state Wednesday. This scenario would veer easterlies more southerly and increase humidity. The potential exists for more numerous showers and isolated thunderstorms that could produce locally heavy rain Thursday and Friday over the western half of the state.

Discussion

The large surface high positioned about 900 miles north northeast of Oahu is beginning to drift east southeast and weaken. The result has been the subtle weakening of our local winds. Overnight wind gusts over exposed terrain prone to higher gusts on both Big Island and around Maui County have been steadily falling into the 20 to 30 mph range. Therefore, the Wind Advisory for the northern and southeastern regions of Big Island and windward areas of Maui County has been cancelled. The subtropical jet streaming across the state is maintaining a near 10 kft inversion and likely creating enough upper level diffluence to generate some lift within a fairly saturated boundary layer. Diffuse bands of moisture are producing scattered offshore showers that are filling in across the smaller islands this morning. A slug of higher mid level moisture just east of the island chain will likely have windward Big Island getting into the higher rainfall action later this morning. Gusty trades will periodically carry showers over into the leeward areas of most islands.
A near-stationary mid and upper level low northwest of the region and its associated trough extending north of the islands will continue to maintain an elevated subsidence inversion the next few days. The region will remain under these relatively lower mid to upper level heights through Tuesday. Periodic pockets of higher moisture and subsequent showers will ride in on the lower level easterly steering flow. The jet passing overhead, in addition to this deeper mid layer moisture, will help support early week partly to mostly cloudy skies for most lower elevations, mainly overcast and wetter windward/upslope higher terrain conditions. While not receiving as much accumulated rain, downstream leeward locations will experience frequent downpours from windward spillover showers. Organized showers may produce brief periods of strong gusty conditions with their passage.
As the upper low drifts east and in the vicinity of Kauai late Monday or early Tuesday, there will be increased chances for heavier rain or isolated offshore thunderstorms north of the island. Trades will decline further Tuesday as low pressure at the surface and aloft starts to develop several hundred miles northwest of Kauai. Trades will begin to veer southeasterly Tuesday night and decreasing stability could trigger some heavier showers over more windward slopes. Upper-level troughing northwest of the state will dig southeastward toward the islands going into Wednesday. Forecast confidence is slightly increasing beyond Wednesday as both ensemble and deterministic model solutions are in better agreement on the development and evolution of a late week deep cut off low northwest of the state. Late week winds will veer southerly and tap into more moisture- rich equatorial air. Thursday and Friday's rain may become locally heavy, especially across the western half of the state, as southern convergence bands draw up a 50% higher precipitable water air mass (than a couple of days prior).

Aviation

Strong high pressure north of the state will drive breezy trade winds today and tonight. Bands of low clouds and showers will bring widespread MVFR cigs/vsbys to windward areas of the smaller islands through late this morning, with improving conditions expected this afternoon. Widespread MVFR conditions will develop over windward Big Island later this morning and hold on through the remainder of the day before improving tonight. Showers will reach leeward sections of the smaller islands frequently through the morning hours, with coverage diminishing this afternoon. Some brief MVFR conditions will be possible in leeward areas through the morning hours, but predominantly VFR conditions will prevail.
AIRMET Sierra remains in effect for mountain obscuration over windward sections of Kauai, Oahu, Molokai and Maui. Little change is expected through the morning hours, with improvement expected this afternoon. AIRMET Sierra will likely need to be expanded to include the Big Island later this morning.
AIRMET Tango remains in effect for moderate low level turbulence over and downwind of the terrain of all islands. Little change is expected during the next 24 hours.

Marine

A strong surface high pressure north of the islands continues to drive strong to near gale-force easterly trade winds across the coastal waters. This high will weaken as it slowly drifts south then east over the next few days. The Gale Warning for the typically windier waters of Maui County and around the Big Island has been downgraded to a Small Craft Advisory (SCA) as the winds begin to taper off. An SCA remains in effect for all coastal waters through the day due to a combination of strong winds and seas. As winds and seas slowly decline through the day, the SCA may be pulled back to typical windy waters around Maui County and the Big Island tonight. A surface trough will set up west of the state around Tuesday with winds weakening and veering southeasterly across the islands, leading to southeast veering and lighter winds Tuesday and Wednesday. The SCA will likely drop for all zones coinciding with the declining winds and seas.
East facing shores will continue to see elevated short period rough surf as the strong trade wind generated seas continue to impact the islands. A High Surf Advisory (HSA) has been extended till 6 pm today for all exposed east facing shores. As trade winds gradually ease near and upstream of the state, the rough surf along east and exposed north facing shores will gradually decline.
Surf along north facing shores will remain elevated due to areas exposed to the east wrap, as well as small northerly reinforcements. A small medium period northwest swell is expected to fill in Monday night and hold through Wednesday. The second half of the week guidance suggests a low developing north of the state that could aim a moderate short to medium period north- northwest swell reaching the islands by Thursday night and Friday.
For southern shores, small pulses of medium period south-southeast swell and long period southwest swell will move through over the next several days.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST this evening for Olomana, Maui Windward West, Kauai East, Kauai South, Koolau Windward, Molokai Windward, Molokai Southeast, Windward Haleakala, Big Island Southeast, Big Island East, Big Island North.
Small Craft Advisory until 6 PM HST this evening for all Hawaiian waters,

Data Courtesy of NOAA.gov