Photo Credit: Braden Jarvis

West Kaua’i

Today: Partly sunny with scattered showers and isolated thunderstorms. Highs around 77 near the shore to around 65 above 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Partly cloudy with isolated showers. Lows around 66 near the shore to around 55 above 3000 feet. East winds up to 15 mph shifting to the northeast after midnight. Chance of rain 20 percent.

Saturday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 78 near the shore to around 66 above 3000 feet. East winds around 10 mph. Chance of rain 50 percent.

South Kaua’i

Today: Breezy. Mostly cloudy with showers likely and isolated thunderstorms. Locally heavy rainfall possible. Highs around 76. East winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with showers likely. Lows 62 to 67. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Highs around 77. East winds 10 to 15 mph. Chance of rain 50 percent.

Kaua’i Mountains

Today: Breezy. Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 64 to 72 in the valleys to around 59 above 4000 feet. East winds 10 to 20 mph decreasing to up to 20 mph in the afternoon. Chance of rain near 100 percent.

Tonight: Mostly cloudy with occasional showers. Lows 55 to 60 in the valleys to around 49 above 4000 feet. East winds 10 to 15 mph. Chance of rain 80 percent.

Saturday: Mostly cloudy with numerous showers. Highs 65 to 73 in the valleys to around 60 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

North Kaua’i

Today: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 69 to 76. East winds up to 15 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy with occasional showers. Lows 58 to 67. East winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 80 percent.

Saturday: Partly sunny. Showers likely in the morning, then scattered showers in the afternoon. Highs 70 to 77. East winds 10 to 15 mph. Chance of rain 70 percent.

East Kaua’i

Today: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 69 to 76. East winds 10 to 15 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy with occasional showers. Lows 56 to 68. East winds 10 to 15 mph. Chance of rain 80 percent.

Saturday: Mostly cloudy. Showers likely in the morning, then scattered showers in the afternoon. Highs 70 to 77. East winds 10 to 15 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

Wet trade wind weather will continue today, with locally heavy showers and a slight chance of thunderstorms. Today's breezy trade winds will diminish, becoming light and variable later Saturday and Sunday. The lighter winds will bring much fewer showers over the weekend, although a few localized heavy showers could form in the afternoons. A slow-moving and large low pressure system will develop north of the islands early next week, sending several cold fronts over the islands. Occasionally gusty southwest to west winds will prevail, shifting the rainfall focus to leeward areas. A cool and dry air mass will follow the fronts for the second half of next week.

Discussion

Wet and locally windy trade wind weather continues today, but changes are expected over the weekend, with additional changes anticipated by early next week as prevailing trade winds shift to a westerly direction for several days.
In the short term, a complex low aloft lies directly over the island chain, supporting a somewhat unstable atmosphere. Early morning sounding from PHTO showed a moist and unstable profile, with PWAT near 1.3″ and a lack of a subsidence inversion. At PHLI, PWAT has increased to 1.3″ overnight as the profiles has also become moist and unstable. Satellite and radar numerous showers over windward waters with a few embedded thunderstorms. Moisture is deep enough to bring a wintry mix of precipitation to the upper slopes and summits of the Big Island, and a Winter Storm Warning is posted as yet another surge of deep moisture is poised to move ashore into tonight. A 1035 mb surface high centered about 1000 miles NNE of the islands continues to support locally windy conditions, with trade winds especially gusty in and near showers. Little significant change is expected today as trade winds slowly ease from the recent peak.
The high will re-organize northward today, then move E while weakening this weekend. In its place a complex, large and relatively slow-moving low pressure system will develop N of the islands, then remain in place for the first half of next week. Whereas surface pressures N of the islands have been hovering in the upper 1030 mb range for the last 2 weeks or so, next week pressures will be 30-50 mb lower as several low pressure systems develop and pass N of the islands.
This significant pattern change will lead to diminishing trade wind speeds through tonight, remaining locally breezy today. Light and variable winds will develop Saturday and Sunday as a front now about 1100 miles NW of the islands moves closer. The low aloft will linger today, then weaken and move E Saturday and Sunday. End result appears to be a fairly quiet and much drier weekend weather pattern as a land/sea breeze pattern develops within a gradually stabilizing island atmosphere. The best chance for showers will shift to interior areas in the afternoons as sea breezes converge.
The first in what appears to be several fronts will move over the islands from the NW on Monday, potentially briefly stalling before a second and third front moves through Tuesday and Wednesday. The atmosphere will be relatively stable as the fronts pass, thus widespread heavy rainfall is not anticipated. With the surface ridge well SE of the islands and deep low pressure passing to the N, SW to W winds will prevail, at times locally gusty (especially over windward areas), with forecast details hinging on the strength and proximity of the passing lows. Latest guidance indicates that the strongest low will pass N of the islands on Wednesday, potentially making it the windiest day. These winds will ensure that any rainfall that does occur will focus over leeward areas. After the passage of this low, high pressure building NW of the islands looks to deliver a dry, cool and stable air mass with NW to N winds on Thursday. Thereafter, forecast uncertainty increases dramatically.

Aviation

Moderate to locally brisk trade winds will persist. Layered clouds and bands of showers will be steered towards north and east facing coasts and slopes. As of 2 am Friday, multi-radar imagery revealed a thick band of light to moderate showers extending in an arc across the waters just north and east of Oahu through Maui. This band is expected to press slowly westward throughout the day today. Meanwnile a second smaller band of showers is approaching Windward Big Island from the east- southeast. Isolated embedded thunderstorms and locally heavy downpours are associated with both of these bands.
AIRMET Sierra for mountain obscuration remains in effect for windward slopes of Oahu, Molokai, Maui, and the Big Island north of Hilo. Conditions are expected to perist troughout the day, today, and should spread across Kauai by early afternoon.
AIRMET Tango also remains in effect–for both low level and upper level turbulence. Low level turbulence is the result of terrain interference and is expected to persist through early evening and possibly beyond. Upper level turbulence is the result of a strong upper level low just to the northeast of the state, and will likely continue through at least twenty-four hours.
Light icing in layered clouds above 10 kft is expected from Oahu through northeast Big island. Conditions will persist through at least mid-afternooon, and may spread acoss the remainder of the state by early evening.

Marine

Trade winds will gradually ease during the next couple days as high pressure north of the state weakens and a front approaches from the west. Strong to near-gale force winds this morning will lower into the fresh to strong range this afternoon, then decline into the light to moderate range for Saturday. A Small Craft Advisory (SCA) remains in effect for all waters through 6 PM this evening. The SCA will likely need to be extended through tonight for portions of the marine area, before dropping below advisory levels everywhere on Saturday.
As the front edges closer Saturday night through Sunday night, light and variable winds will overspread the waters. Light to moderate southwest winds will develop on Monday, with moderate to locally strong southwest to west winds overspreading the waters Tuesday and Wednesday.
A High Surf Advisory remains in effect through 6 PM this evening for large and rough surf along east facing shores. East shore surf will steadily decline tonight through the middle of next week, with flat conditions expected by the end of the upcoming work week.
An incoming small long-period northwest swell will give a slight boost to north shore surf today, with minimal surf then expected Saturday through Tuesday. An extra-large long-period northwest swell could bring warning level surf to north and west facing shores Wednesday through Thursday.
A series of small south swells will keep south shore surf near the seasonal average during the next 7 days. Short-period and choppy surf conditions could develop during the early to middle part of next week if southwest and west winds pick up as advertised.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST this evening for East facing shores of Kauai Oahu Molokai Maui and the Big Island.
Winter Storm Warning until 6 AM HST Saturday for Big Island Summits.
Small Craft Advisory until 6 PM HST this evening for all Hawaiian waters.

Data Courtesy of NOAA.gov