Photo Credit: Brendan Stephen

West Kaua’i

Today: Partly sunny with scattered showers until late afternoon, then mostly sunny late in the afternoon. Highs 75 to 82 near the shore to around 67 above 3000 feet. Northwest winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly clear. Lows around 63 near the shore to around 54 above 3000 feet. Northwest winds 10 to 15 mph.

Saturday: Mostly sunny with isolated showers. Highs 75 to 82 near the shore to around 68 above 3000 feet. Northwest winds up to 15 mph. Chance of rain 20 percent.

South Kaua’i

Today: Mostly sunny with scattered showers until late afternoon, then sunny late in the afternoon. Highs 75 to 83. West winds 10 to 15 mph. Chance of rain 40 percent.

Tonight: Mostly clear. Lows around 63. West winds up to 10 mph in the evening becoming light.

Saturday: Mostly sunny with isolated showers. Highs 75 to 82. Light winds. Chance of rain 20 percent.

Kaua’i Mountains

Today: Partly sunny with scattered showers until late afternoon, then mostly sunny late in the afternoon. Highs 69 to 76 in the valleys to around 61 above 4000 feet. West winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly clear. Lows 53 to 62 in the valleys to around 51 above 4000 feet. West winds up to 15 mph.

Saturday: Mostly sunny with isolated showers. Highs 68 to 75 in the valleys to around 60 above 4000 feet. Northwest winds up to 10 mph. Chance of rain 20 percent.

North Kaua’i

Today: Breezy. Partly sunny with scattered showers until late afternoon, then mostly sunny late in the afternoon. Highs 69 to 78. West winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly clear. Lows 57 to 64. West winds up to 15 mph increasing to 10 to 15 mph after midnight.

Saturday: Mostly sunny with isolated showers. Highs 69 to 77. Northwest winds around 10 mph. Chance of rain 20 percent.

East Kaua’i

Today: Mostly sunny with scattered showers until late afternoon, then mostly sunny late in the afternoon. Highs 71 to 83. Northwest winds up to 15 mph. Chance of rain 40 percent.

Tonight: Mostly clear. Lows 56 to 67. Northwest winds up to 10 mph.

Saturday: Mostly sunny with isolated showers. Highs 68 to 80. Light winds becoming north up to 10 mph in the afternoon. Chance of rain 20 percent.

Detailed Forecast

Synopsis

The main rain shield leading a front passing into the central islands has shifted over eastern Maui and Big Island early this morning. This shield still contains pockets of heavy showers and isolated thunderstorms and will shift east over Big Island this morning. There still be intense rainfall rates within the strongest cells that are capable of producing localized flash flooding. Strong westerlies are also still possible as this area of rain and the trailing cold front passes across Big Island through the day. Conditions will gradually across the state from west-to-east as cooler, drier air fills in behind the front this weekend. Cooler and breezy trade wind weather is expected through the first half of next week.

Discussion

The eastern half of the state and surrounding coastal waters remains within the warm sector of a 989 mb storm force low that is positioned about 570 miles north of Lihue, Kauai, at 4 AM HST this morning. Moderate layer southwesterly winds still persist over the region as evidenced by local island radar VADs. Communities just downstream of north and east-facing terrain will still experience downslope accelerations, in addition to any passing convective downdrafts, will produce breezy conditions. The wind threat will remain unchanged for primarily Big Island today. The High Wind Warning (HWW) remains in effect for Haleakala and the Big Island through the afternoon. Veering weakening winds to the west northwest over the western two thirds of the state will lower HWW down to a Wind Advisory for the remaining lower elevation zones and islands of Maui County. The cold front is passing across the western third of the state (as of this writing). The front is expected to fall apart over the eastern third of the state (or around Big Island) in response to weakened forcing and the influx of a drier, post frontal air mass.
Due to the progressive nature of the rain, the main near term thunderstorm threat will be from localized wind gusts up to and in excess of 60 mph across Big Island this morning. Recent rain episodes of rain rates of 1 to 4″ per hour rates over Oahu induced the flooding of many roads and low-lying areas while producing some minor flooding along various streams yesterday. The latest deterministic, global scale guidance indicates that the greatest mid-level instability will continue shifting east Big Island during the day. Higher resolution modeling propagates the highest QPF quickly eastward onto Big Island by sunrise, holding it up there through the day. Thus, convection is still expected to migrate eastward and will refocus heavy rain over more leeward areas of eastern Maui and Big Island for the remainder of the morning.
High resolution model guidance is depicting that the heaviest rain activity will shift east of Maui and onto Big Island this morning, but the larger scale models paint the picture of the greatest mid layer moisture/tightest instability gradient not moving, or remaining anchored over east Maui and vicinity the next few hours. These inconsistencies hold onto higher chances of a slowing line near or over Big Island that could create a prolonged flooding threat into the early afternoon. This outcome would delay the onset of heavy rain over Big Island and the snow over the summits until later this morning and afternoon. The instability gradient shifts over the Big Island late today as low pressure pulls away and overall forcing diminishes their flash flood threat.
Pleasant, dry post-frontal air filters in over Kauai and Oahu going into the weekend as trade winds return.

Aviation

Widespread heavy showers and strong thunderstorms associated with a cold front will continue to impact the eastern end of the state today. Expect localized strong turbulence, minimal visibility, and low ceilings during this period. While the threat of heavy showers and thunderstorms will diminish over the western portion of the state behind the front, periods of MVFR ceilings and visibilities in light to moderate showers, along with gusty westerly winds, will be possible, particularly over Kauai.
AIRMET Sierra is in effect for IFR conditions over Maui and the leeward side of the Big Island this morning, which may persist through a good portion of the day. Additionally, AIRMET Sierra remains in effect for temporary mountain obscurations above 2,000 feet over the western end of the state.
AIRMET Tango is posted for temporary moderate turbulence below 10,000 feet and from FL260 to FL400 across the entire state. Strong sustained winds and downsloping gusts are expected along the north and east sides of the islands, warranting an AIRMET for sustained winds of 30 knots or greater. Light icing is anticipated between 14,000 feet and FL240.

Marine

A cold front over the islands, associated with strong low pressure passing to the N, will continue to move E through tonight before stalling and dissipating near the Big Island Saturday. Impactful weather, in the form of strong thunderstorms, wind gusts near 50 kt, and zero visibility in areas of blinding rain will affect waters around Maui and the Big Island through at least the morning hours. A Gale Warning is posted for select waters around the Big Island due to frequent wind gusts well over 34 kt. A Small Craft Advisory is posted for remaining waters. T
Moderate to locally strong fresh W to NW winds with improving weather trends will spread from NW to SE over the chain over the next 24 hours. Moderate to locally strong NE trade winds will develop by Sunday as high pressure builds far NW of the islands.
A medium- to long-period N swell will persist today, then gradually diminish, and a High Surf Advisory is posted for N facing shores of nearly all islands. Additionally, the powerful low passing N of the islands has produced a shorter-period NW swell that will build today, and may maintain high surf along north facing shores into Saturday before diminishing. Locally generated wind waves out of the S and W will gradually diminish through tomorrow. Another N swell will arrive Sunday and peak Monday, producing high surf along N facing shores. A couple moderate sized WNW swells are also expected, with a smaller one this weekend, and a slightly larger one from Monday into Wednesday.

Fire weather

Critical fire conditions are not expected during the forecast period.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST this evening for Niihau, Oahu North Shore, Olomana, Maui Windward West, Kauai North, Kauai East, Koolau Windward, Molokai Windward, Molokai North, Maui Central Valley North, Windward Haleakala, Big Island East, Big Island North.
Flood Watch through this afternoon for Maui Windward West, Maui Leeward West, Haleakala Summit, Kona, Kohala, Big Island Interior, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala, Big Island South, Big Island Southeast, Big Island East, Big Island North.
High Wind Advisory through Noon today for Maui Windward West, Maui Leeward West, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala.
High Wind Warning until 6 PM HST this evening for Haleakala Summit.
High Wind Warning until noon HST today for Kona, Kohala, Big Island Interior, Big Island South, Big Island Southeast, Big Island East, Big Island North.
High Wind Warning until 6 AM HST Saturday for Big Island Summits.
Winter Storm Warning until 6 AM HST Saturday for Big Island Summits.
Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Maalaea Bay, Pailolo Channel, Big Island Windward Waters, Big Island Southeast Waters.
Gale Warning until noon HST today for Alenuihaha Channel, Big Island Leeward Waters.

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