Photo Credit: Brendan Stephen

West Kaua’i

Rest Of Today: Mostly sunny in the morning then becoming mostly cloudy. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 81 near the shore to around 72 above 3000 feet. Southeast winds up to 15 mph shifting to the south in the afternoon. Chance of rain 40 percent.

Tonight: Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 64 near the shore to around 58 above 3000 feet. South winds 10 to 15 mph. Chance of rain 90 percent.

Monday: Breezy. Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs around 84 near the shore to around 72 above 3000 feet. Southwest winds 10 to 20 mph. Chance of rain 90 percent.

South Kaua’i

Rest Of Today: Mostly sunny early in the morning then becoming mostly cloudy. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 80. Southeast winds up to 15 mph shifting to the south in the afternoon. Chance of rain 30 percent.

Tonight: Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 64. Southwest winds 10 to 15 mph. Chance of rain 90 percent.

Monday: Breezy. Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs around 81. Southwest winds 10 to 25 mph. Chance of rain 90 percent.

Kaua’i Mountains

Rest Of Today: Mostly sunny early in the morning then becoming partly sunny. Breezy. Scattered showers in the afternoon. Highs 71 to 76 in the valleys to around 63 above 4000 feet. South winds up to 20 mph. Chance of rain 50 percent.

Tonight: Breezy. Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 57 to 62 in the valleys to around 55 above 4000 feet. South winds up to 20 mph. Chance of rain 90 percent.

Monday: Windy. Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 71 to 78 in the valleys to around 62 above 4000 feet. Southwest winds up to 25 mph increasing to 15 to 30 mph in the afternoon. Chance of rain 90 percent.

North Kaua’i

Rest Of Today: Mostly sunny in the morning then becoming mostly cloudy. Breezy. Scattered showers in the afternoon. Highs 72 to 81. South winds up to 20 mph. Chance of rain 50 percent.

Tonight: Breezy. Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 59 to 65. South winds up to 20 mph. Chance of rain 90 percent.

Monday: Breezy. Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 74 to 83. Southeast winds up to 20 mph shifting to the southwest 10 to 20 mph in the afternoon. Chance of rain 90 percent.

East Kaua’i

Rest Of Today: Partly sunny with isolated showers. Highs 72 to 81. South winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 20 percent.

Tonight: Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 55 to 66. Southwest winds 10 to 15 mph. Chance of rain 90 percent.

Monday: Breezy. Cloudy with occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 74 to 82. Southwest winds 10 to 25 mph. Chance of rain 90 percent.

Detailed Forecast

Synopsis

Deep tropical moisture being drawn northward ahead of an approaching cold front and an upper disturbance will lead to an increasing threat of heavy rainfall tonight through the first half of the week. Strengthening southerly winds, periods of heavy rainfall, and thunderstorms will accompany this front as it moves through from west to east. The active pattern will continue through the second half of the week as another system approaches and moves into the area.

Discussion

Guidance remains in good agreement and shows a strong pulse aloft approaching the region today, then closing off into an upper low a few hundred miles north-northwest of the islands tonight through Monday night. Strong surface cyclogenesis with a gale low forming along the attendant cold front is shown as the system advances toward the islands today. The low-level flow will gradually respond locally and begin to strengthen as it veers out of the south.
Deep tropical moisture being drawn northward ahead of the approaching system combined with lowering upper heights and increasing instability will lead to a period of widespread rainfall with embedded thunderstorms later today through the first half of the week. In addition to the increasing threat of flash flooding where the heavy rainfall becomes focused during this period, embedded thunderstorms and gusty south-southwest winds (damaging downsloping winds possible Monday through Tuesday) are anticipated. For timing, these threats will be most likely over the western end of the state this evening into Monday, then over the rest of the state Monday through Tuesday.
The front will gradually dissipate near Maui County Tuesday night into Wednesday. Winds will continue to veer out of the west- southwest, but ease slightly during this time. Lingering shower activity will continue into Wednesday, but rainfall intensity should trend down during the day as the front dissipates and the upper support lifts northward and opens up.
Models remain consistent in that the front during the first half of the week will be quickly followed by another system beginning on Thursday. Although it remains too early to discern many details regarding this second system, indications are that another round of heavy showers, thunderstorms and stronger winds will return to the islands from Thursday onward.

Aviation

A nearby surface ridge is supporting light to moderate SE-S winds, with limited cloud/shower coverage seen near Kauai and Oahu. Increased clouds and showers are noted over windward portions of the Big Island and E Maui, where persistent MVFR VIS/CIG in SHRA have led to the issuance of AIRMET SIERRA for mountain obscuration.
Some decrease in shower coverage is anticipated over windward Maui/Big Island later today, as the low-level flow becomes increasingly southerly and gradually strengthens. Daytime heating will lead to increased clouds over interior areas statewide this afternoon with ISOL MVFR CIG, while an approaching front will bring increasing rain chances later in the day to Kauai. The front will likely bring increased rain chances to Oahu by tonight, and potentially Maui County by early Tuesday. The front will bring widespread MVFR VIS/CIG in SHRA and ISOL IFR VIS/CIG in +SHRA, with leeward areas likely seeing the greatest coverage.

Marine

The southwest edge of the high pressure ridge is moving over the Hawaiian Islands today as a low pressure system and cold front approaches the region from the northwest. The Small Craft Advisory near some of the Big Island waters was cancelled today as wind speeds diminish. Unstable showers will likely develop ahead of the frontal band potentially affecting waters surrounding Kauai later this afternoon and Oahu waters by tonight.
A surface low will deepen along the frontal boundary just northwest to north of Kauai today, causing winds to veer from a southerly to southwesterly direction over the western islands. These southerly winds spreading statewide by Monday along with increasing shower activity and possible thunderstorms as the cold front marches down the island chain. The front is expected to weaken and stall near Maui on Wednesday, with winds easing and shifting westerly. Uncertainty increases Thursday and Friday, but a surface low may develop near the islands, raising chances for gusty winds and thunderstorms.
The current north-northwest (330 degree) swell will continue to slowly decline today. The next moderate northwest (310-320 degree) swell is due to arrive Monday, and may push north shore surf to the advisory levels from Tuesday into Wednesday. Forecast confidence decreases later in the week due to uncertainty in the location and intensity of a surface low developing near Hawaii.
East shore surf will decline today with small surf through the first half of the week as the local winds veer from a more southerly to southwesterly direction. Small surf will also continue on south shores this week, and conditions may become rough as southerly to southwesterly winds increase on Monday and Tuesday.

HFO Watches/Warnings/Advisories

Flood Watch from this evening through Tuesday afternoon for Niihau, Kauai Leeward, Kauai Mountains, Waianae Coast, Oahu North Shore, Olomana, Central Oahu, Waianae Mountains, Lanai Mauka, Kahoolawe, Maui Windward West, Maui Leeward West, Haleakala Summit, Kona, Kohala, Big Island Interior, Kauai North, Kauai East, Kauai South, East Honolulu, Honolulu Metro, Ewa Plain, Koolau Windward, Koolau Leeward, Molokai, Lanai Windward, Lanai Leeward, Lanai South, 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.

Data Courtesy of NOAA.gov