Photo Credit: Brendan Stephen

West Kaua’i

Rest Of Today: Mostly sunny with isolated showers. Highs around 83 near the shore to around 68 above 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows around 66 near the shore to around 56 above 3000 feet. Northeast winds up to 10 mph. Chance of rain 40 percent.

Tuesday: Mostly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 84 near the shore to around 69 above 3000 feet. East winds up to 10 mph. Chance of rain 50 percent.

South Kaua’i

Rest Of Today: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 75 to 82. East winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 66. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 76 to 83. East winds 10 to 15 mph. Chance of rain 50 percent.

Kaua’i Mountains

Rest Of Today: Mostly cloudy with showers likely in the morning, then mostly sunny with scattered 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 70 percent.

Tonight: Mostly cloudy with scattered showers. Lows 61 to 66 in the valleys to around 54 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 70 to 80 in the valleys to 58 to 63 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

North Kaua’i

Rest Of Today: Partly sunny in the morning then becoming mostly sunny. Scattered showers. Highs 73 to 83. East winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 60 to 69. East winds 10 to 15 mph decreasing to up to 10 mph after midnight. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 73 to 83. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

East Kaua’i

Rest Of Today: Mostly cloudy with showers likely in the morning, then mostly sunny with scattered showers in the afternoon. Highs 69 to 79. East winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with scattered showers. Lows 61 to 70. East winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday: Partly sunny. Numerous showers in the morning, then scattered showers in the afternoon. Highs 70 to 80. East winds 10 to 15 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

Surface high pressure positioned north of the islands continues to gradually weaken from its once strong state as it treks east. This will result in a couple days of relatively weaker trade winds. While statewide shower coverage and frequency will diminish the next couple of days, periodic bands of showers will continue to move in from the east focusing higher rainfall over windward exposures, with occasional showers reaching leeward communities. Low pressure is expected to develop several hundred miles northwest of the state Wednesday. This scenario would veer winds more southerly and increase humidity. The potential exists for more numerous showers and isolated thunderstorms that could produce locally heavy rain Thursday and Friday, primarily over the western half of the Hawaiian Islands.

Discussion

The 1034 mb surface high centered north northeast of the islands has slowly, yet significantly, weakened from its once massive 1050 mb strength the past several days. The result has been a gradual easing of the trades. Morning soundings continue to indicate a deep and moist resident boundary layer with a near 10 kft inversion base. Weak mid to upper level troughing within this moist 850-700 mb layer, along with lift provided by the subtropical jet streaming up from the southwest, will keep the regions's boundary (mixed) layer sufficiently deep enough to allow orographically-forced showers to reign. Expect short duration gusty winds to accompany showers, especially during the overnight hours. Trade flow will gradually lighten up the next couple of days as the high moves slowly east and slightly weakens. Easterlies will veer to the southeast by mid week as the northeast-to-southwest orientated surface ridge axis extends back over the islands. As weak mid-level ridging develops, these veering winds will help support a relatively drier pattern into mid week per a diminished lower layer trade push. The large scale pattern over the central North Pacific will be of strong high pressure developing to the distant northwest as a deep low cuts off south of it Wednesday afternoon or evening. Veering southeast to south mid week winds will likely strengthen to moderate to locally fresh magnitudes by Thursday but will not reek havoc (as was the case last week) due to the smaller islands falling within Big Island's shadow.
Upper-level troughing northwest of the state will dig southeastward toward the islands Wednesday. Increasing confidence that there will be a developing late week low forming in the base of the trough that will cut off from the flow northwest of the state. Model guidance is leaning toward the low slightly wobbling to the north northeast heading into the weekend. This slow northerly track of the low will place the state under a long duration wet pattern as deep southern convergence sets up. 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 was present a few days earlier). Next weekend is forecast to remain wet as the region stays under decent southern convergence banding advecting high precipitable waters up and over the island chain. Energy diving into the base of the main trough should initiate an eastward progression of the trough by next Monday. The attendant weak surface boundary associated with a retrograding upper trough may move across the state Monday and swing light winds around to the north through the day.

Aviation

Strong high pressure north of the state will slowly weaken as it shifts eastward over the next few days, causing trade winds to decrease. AIRMET Tango remains in effect for moderate low level turbulence over and downwind of terrain of all islands. Conditions are expected to end across most areas tonight.
Last evening, low clouds and showers favoring windward areas created spotty MVFR conditions, prompting a brief issuance of AIRMET Sierra for mountain obscuration. However, low cloud cover and showers have diminished this morning, allowing cancellation of this AIRMET. VFR conditions dominate at the TAF sites.

Marine

Surface high pressure northeast of the islands will drive fresh to locally strong trades over the coastal waters. The high will weaken as it slowly drifts east over the next couple of days resulting in the trades weakening. A Small Craft Advisory (SCA) has been scaled back to the typically windy waters around Maui County and the Big Island. A surface trough will develop west of the state around Tuesday with winds weakening further and veering southeasterly. These lighter winds will persist into Thursday before the southeasterly winds strengthen to fresh to locally strong speeds.
Surf along east facing shores will slowly trend down as the winds ease over the next few days. Surf along north facing shores will see the leftovers of the east wrap today along with a small, medium- period northwest swell filling during the evening hours today. This northwest swell is expected to peak Tuesday and hold through Wednesday. Guidance for the second half of the week shows a low developing north of the state that could aim a moderate short to medium period north- northwest swell towards the islands, arriving Friday although confidence remains low.
For south facing shores, a small long period southwest swell has peaked and will gradually decline through Tuesday. A series of small medium to long period south- southeast swells will move through over the week.

HFO Watches/Warnings/Advisories

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

Data Courtesy of NOAA.gov