West Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening. Lows around 65 near the shore to around 53 above 3000 feet. Light winds. Chance of rain 20 percent.

Saturday: Mostly sunny. Isolated showers in the afternoon. Highs around 82 near the shore to around 69 above 3000 feet. Light winds. Chance of rain 20 percent.

Saturday Night: Partly cloudy with isolated showers. Lows around 66 near the shore to around 55 above 3000 feet. Light winds. Chance of rain 20 percent.

South Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening. Lows around 63. Northeast winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 20 percent.

Saturday: Mostly sunny. Highs 77 to 83. East winds up to 10 mph.

Saturday Night: Partly cloudy with isolated showers. Lows around 64. Light winds. Chance of rain 20 percent.

Kaua’i Mountains

Tonight: Partly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 57 to 62 in the valleys to around 51 above 4000 feet. Light winds. Chance of rain 40 percent.

Saturday: Mostly sunny in the morning then becoming partly sunny. Isolated showers. Highs 71 to 80 in the valleys to around 63 above 4000 feet. Light winds. Chance of rain 20 percent.

Saturday Night: Partly cloudy with isolated showers. Lows 59 to 64 in the valleys to around 53 above 4000 feet. Light winds becoming southwest up to 10 mph after midnight. Chance of rain 20 percent.

North Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening. Lows 57 to 66. Northeast winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Saturday: Mostly sunny. Isolated showers in the afternoon. Highs 74 to 80. East winds up to 10 mph. Chance of rain 20 percent.

Saturday Night: Partly cloudy with isolated showers. Lows 58 to 67. Light winds. Chance of rain 20 percent.

East Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening. Lows 58 to 68. Northeast winds up to 10 mph. Chance of rain 20 percent.

Saturday: Mostly sunny. Isolated showers in the afternoon. Highs 71 to 81. Light winds. Chance of rain 20 percent.

Saturday Night: Partly cloudy with isolated showers. Lows 59 to 68. Light winds. Chance of rain 20 percent.

Detailed Forecast

Synopsis

A weak cold front, moving across Maui late this afternoon, will stall and dissipate near the Big Island tonight through Saturday. This front will bring enhanced clouds and light to occasionally moderate showers to mainly windward portions of both Maui County and the Big Island. The next front will begin to approach the islands on Sunday and bring moderate southwest winds. These winds will peak on Monday, directly ahead of the front, and may become quite gusty at times. Expect some pre- frontal showers develop over the western half of the state on Sunday, but the main rainband with pockets of heavier rain should push down the chain on late Monday into early Tuesday. Cool and dry, breezy northwesterly to northerly winds to blow into the region after the front passes each island.

Discussion

Visible satellite loop from this afternoon showed a disorganized line of low clouds aligned northeast to southwest over the central islands. This line was trudging slowly southeast towards the Big Island. Rain amounts since midnight have been rather dismal with this system, topping out between 0.25″ and 0.50″ along windward and mauka sections of Oahu and Kauai with lower amounts leeward. Meanwhile, Molokai and Maui have so far registered only few hundredths of an inch. The weak front may actually dissipate before reaching the Big Island tonight and lose what little forcing it had to work with. Thus, had to scale back some of the precip chances for windward Big Island tonight into Saturday morning to account for this. Post frontal light to moderate northeasterly flow will dominate on Saturday across the smaller islands and bring limited cloud cover and showers to windward and mauka areas. For the Big Island, afternoon sea breeze development, combined with shallow residual moisture from the deceased front, will slightly enhance shower activity on the lower slopes of Mauna Kea and Mauna Loa.
Late this weekend, the next cold front will approach the state from the northwest. In response, southwest winds initially develop across the islands late Sunday, then quickly ramp up through Monday afternoon. Models have been consistent with these stronger winds, depicting speeds of sustained 20 to 30 mph with higher gusts. Stronger winds will likely occur over mountain ridgetops and in the lee of mountain ranges. If guidance maintains or trends up, a wind advisory would be need for select mountain zones and those zones to the northeast of the mountains. Big Island summit winds will likely become strong late Monday and may break advisory thresholds. In addition, a moderate to locally heavier band of rain appears to accompany the front. Though the ECMWF is slightly weaker with the upper level support and available moisture, both the GFS and ECMWF show generally half inch to inch total rainfall accumulation along south and west facing portions of the islands from Monday through Tuesday. As always, locally higher amounts are possible. The speedy nature of the front should mitigate the flood risk, but some minor flooding is not out of the question.
Tuesday into Wednesday, post frontal breezy north to northwest flow will advect in a drier and cooler airmass. Winds will continue to veer easterly on Thursday as transient surface high pressure moves north of the region. Long range models prog yet another system to affect the islands by next weekend, but details are still to hazy that far out into the extended range.

Aviation

A frontal boundary continues to advance through the island chain this afternoon and evening. Light showers will impact Oahu and Maui County through this evening. MVFR conditions are expected in and around any showers. The Big Island sites are largely expected to be spared of any rain activity. Winds will flow north to northeast through the evening as the front passes through, with localized sea breezes setting up by Saturday afternoon.
AIRMET Sierra remains in effect for tempo mountain obscuration above 1500 feet for Oahu and Maui County. Conditions are expected to improve overnight as the front moves through the islands.

Marine

A weakening front moving over Maui County this afternoon will stall and dissipate tonight. Moderate to locally fresh north to northeast winds along and north of the front will ease and shift out of the east overnight as surface high pressure moves north of the state. Meanwhile, gentle and variable winds persist around the Big Island. The lighter trade winds will give way to southeast winds Saturday night as a surface ridge is pushed over the island chain. South to southwest winds will develop on Sunday and will build to fresh to strong levels Sunday night and Monday as another, stronger front approaches. The front is expected to pass down the island chain Monday night and Tuesday, and a Small Craft Advisory (SCA) may be needed for some waters as winds peak Monday into Tuesday.
Large to extra-large surf will prevail into Saturday, with another round of elevated surf due early next week. A complex storm low far northwest of Hawaii generated overlapping west-northwest to northwest (295 to 320 degrees) swells that are currently affecting the islands. The first swell has peaked, and the second has been building through the day, resulting in total seas of 13 to 16 feet with significant swell period energy ranging between 11 and 17 seconds at the NOAA buoys northwest of Kauai. Resulting surf will peak tonight above High Surf Warning levels along north and west facing shores of Kauai, Oahu, and Molokai and at High Surf Advisory levels for north Maui and west Big Island. A Small Craft Advisory is also in place for waters exposed to the elevated seas. As the swell declines, the warnings and advisories will likely be dropped Saturday evening or Sunday morning. Another round of potentially larger northwest swell is expected late Monday through early Thursday.
A Coastal Flood Statement remains in effect through Monday. During this time, higher than predicted tides will produce minor flooding along vulnerable low-lying coastal infrastructure as well as some some beach erosion during peak high tides between midnight and sunrise. The elevated tides will also contribute to runup and beach erosion tonight along north and west shores exposed to the large swell.
Surf along east- and south-facing shores will remain small through the week.

HFO Watches/Warnings/Advisories

High Surf Warning until 6 PM HST Saturday for Niihau, Kauai Leeward, Waianae Coast, Oahu North Shore, Kauai North, Molokai Windward, Molokai North, Molokai West.
High Surf Advisory until 6 PM HST Saturday for Maui Windward West, Kona, Kohala, Maui Central Valley North, Windward Haleakala.
Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Alenuihaha Channel, Big Island Windward Waters.

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Data Courtesy of NOAA.gov