West Kaua’i

Tonight: Mostly cloudy with scattered showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 71 near the shore to around 62 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Saturday: Partly sunny with scattered showers. Highs 82 to 88 near the shore to around 76 above 3000 feet. East winds up to 15 mph shifting to the southeast in the afternoon. Chance of rain 40 percent.

Saturday Night: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows around 72 near the shore to around 63 above 3000 feet. East winds up to 10 mph. Chance of rain 50 percent.

South Kaua’i

Tonight: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 71. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Highs 77 to 87. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Saturday Night: Mostly cloudy with scattered showers. Lows around 72. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 66 in the valleys to around 59 above 4000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Saturday: Mostly cloudy. Scattered showers in the morning, then numerous showers in the afternoon. Highs 76 to 81 in the valleys to around 67 above 4000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Saturday Night: Mostly cloudy with numerous showers. Lows around 68 in the valleys to around 61 above 4000 feet. East winds up to 10 mph. Chance of rain 70 percent.

North Kaua’i

Tonight: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 63 to 72. Southeast winds up to 15 mph. Chance of rain 70 percent.

Saturday: Mostly cloudy with scattered showers. Highs 71 to 84. East winds up to 15 mph. Chance of rain 50 percent.

Saturday Night: Mostly cloudy with scattered showers. Lows 64 to 74. East winds up to 15 mph. Chance of rain 50 percent.

East Kaua’i

Tonight: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 66 to 73. East winds up to 15 mph. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Highs 76 to 84. Northeast winds up to 15 mph shifting to the east in the afternoon. Chance of rain 50 percent.

Saturday Night: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 67 to 74. East winds up to 10 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

An upper level low is bringing unstable conditions, heavy rain, and thunderstorms to parts of the state through tonight. A flood watch is in effect for central and eastern islands through 6 AM HST Saturday. This low will slowly move away from the state Saturday into Sunday, diminishing the chance for heavy rain. Wet trades will build back in Sunday through Tuesday, becoming more typical trades thereafter. This pattern should then last into next weekend.

Discussion

Showers and thunderstorms continue over the region, with the largest concentration of thunderstorms over Oahu and the near shore waters west of Oahu as of 3 PM HST. Heavy rain was occurring, and gages showed rapid rises on numerous streams across Oahu. A flood warning has been issued. Please exercise caution during this warning as water may flow over roads, and may be deeper than if first appears. Winds were gusting 25 to 35 mph near the stronger showers and thunderstorms, but otherwise winds were averaging 5 to 15 mph.
An upper level low centered near Kauai was moving slowly southeast this afternoon. This low is expected to gradually weaken and move east and then northeast tonight. The cold air aloft associated with this low will help provide unstable conditions through tonight, and therefore we have extended the flood watch for Oahu and Maui County, and added Big Island. Kauai seems to be too far west now to have a significant threat of flooding, so they were removed from the watch. Where the watch remains, it goes through 6 AM HST Saturday.
As the low moves away from the islands Saturday into Sunday, the atmosphere will become more stable, and even though lots of low level moisture will remain with us through Tuesday, the chance of flooding will decrease. Trade winds will return Sunday after another day of southeast winds Saturday. Once the trades return, they are expected to remain over the state through the end of next week. Initially, the increased moisture from this current system will bring wet trades Sunday through Tuesday. After Tuesday, more typical trade wind showers should be with us through the end of the week.

Aviation

Surface trough near Kauai this afternoon will result in gentle to moderate trade to ESE winds (90 to 120 degrees) across the state. Upper low tracking over the western half of Hawaii will destabilize the atmosphere producing scattered heavy showers and thunderstorms across Oahu through the rest of the afternoon, with isolated coverage across Maui County and the Big Island. Guidance forecast most showers decreasing in coverage and intensity overnight.
SIGMET Zulu is posted for thunderstorms across Oahu and adjacent waters this afternoon. AIRMET Tango is posted for tempo moderate turbulence all islands elsewhere from 150 to FL380 due to the upper low. AIRMET Sierra is in effect for tempo mountain obscurations above 2000 feet for entire area of Oahu and Maui County. The upper low is expected to weaken tonight through Saturday.

Marine

This afternoon, an upper low and its associated surface trough near Kauai is continuing to support unstable conditions across the western and central coastal waters. Thunderstorms capable of gusty winds, torrential downpours, extensive lightning, and possibly small hail will remain possible into the evening hours, primarily from the Kauai Channel eastward. Meanwhile, a late morning advanced scatterometer pass showed that background flow has decreased below Small Craft Advisory (SCA) levels, so the SCA that was previously in effect has been cancelled. Background winds will remain slightly east southeasterly this weekend at moderate to fresh levels. By Saturday, the upper disturbance will be moving northeast away from the state, with stability slowly returning to the region. As a result, expect decreasing thunderstorms and heavy showers across the waters. A more typical easterly trade wind patter will return next week.
A small, medium period north northwest (320-330 degree) swell that peaked last night will continue to slowly decline this evening into tonight. Surf along north and west facing shores will follow suit, easing during the weekend.
A mix of small, medium to long period south swells will maintain small surf along south-facing shores this weekend. A series of gale lows passing south of New Zealand should send small south swell energy into the southern waters through next week. Surf should lift to summer averages around Sunday and hold through Wednesday. Moderate to fresh trades will maintain rough and choppy surf along east-facing shores.
Tides will peak around 2.5 to 3.0 ft MLLW this weekend. Combined with a small south swell bump, water levels will peak near or above 3.0 ft as early as tomorrow. Minor overwash of low lying coastal areas will be possible during times of high tide.

HFO Watches/Warnings/Advisories

Flood Watch through late tonight for Big Island East, Big Island Interior, Big Island North, Big Island South, Big Island Southeast, Big Island Summits, Central Oahu, East Honolulu, Ewa Plain, Haleakala Summit, Honolulu Metro, Kahoolawe, Kipahulu, Kohala, Kona, Koolau Leeward, Koolau Windward, Lanai Leeward, Lanai Mauka, Lanai South, Lanai Windward, Maui Central Valley North, Maui Central Valley South, Maui Leeward West, Maui Windward West, Molokai Leeward South, Molokai North, Molokai Southeast, Molokai West, Molokai Windward, Oahu North Shore, Olomana, South Haleakala, South Maui/Upcountry, Waianae Coast, Waianae Mountains, Windward Haleakala.

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Data Courtesy of NOAA.gov