West Kaua’i

Tonight: Partly cloudy with isolated showers. Lows 64 to 69 near the shore to around 57 above 3000 feet. East winds around 10 mph. Chance of rain 20 percent.

Saturday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 78 to 85 near the shore to around 71 above 3000 feet. East winds around 10 mph. Chance of rain 50 percent.

Saturday Night: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows around 67 near the shore to around 58 above 3000 feet. East winds around 10 mph. Chance of rain 50 percent.

South Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows around 67. East winds 10 to 15 mph. Chance of rain 60 percent.

Saturday: Partly sunny in the morning then becoming mostly sunny. Scattered showers. Highs 77 to 84. East winds 10 to 15 mph. Chance of rain 50 percent.

Saturday Night: Mostly cloudy. Scattered showers and slight chance of thunderstorms in the evening, then numerous showers after midnight. Lows around 67. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 57 to 67 in the valleys to around 55 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Saturday: Partly sunny. Numerous showers in the morning, then scattered showers in the afternoon. Highs 71 to 77 in the valleys to around 62 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Saturday Night: Breezy. Mostly cloudy with numerous showers and slight chance of thunderstorms. Lows 58 to 67 in the valleys to around 56 above 4000 feet. East winds 10 to 20 mph. Chance of rain 70 percent.

North Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 60 to 67. East winds 10 to 15 mph. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Highs 73 to 81. East winds 10 to 15 mph. Chance of rain 50 percent.

Saturday Night: Mostly cloudy with occasional showers and slight chance of thunderstorms. Lows 61 to 68. East winds 10 to 15 mph. Chance of rain 90 percent.

East Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 61 to 70. East winds around 10 mph. Chance of rain 70 percent.

Saturday: Partly sunny. Numerous showers in the morning, then scattered showers in the afternoon. Highs 69 to 82. East winds around 10 mph. Chance of rain 70 percent.

Saturday Night: Mostly cloudy. Numerous showers and slight chance of thunderstorms in the evening, then occasional showers and slight chance of thunderstorms after midnight. Lows 61 to 71. East winds around 10 mph. Chance of rain 90 percent.

Detailed Forecast

Synopsis

Declining trade winds will focus showers over windward areas tonight. Winds will weaken and shift out of the east to southeast late tonight and Saturday as moisture near the state increases and the atmosphere destabilizes. Showers, some potentially heavy, will increase over windward slopes, while leeward areas experience periods of afternoon showers through at least Sunday. Trade winds will return Tuesday or Wednesday.

Discussion

After a breezy several days, trade winds are on a steady decline this afternoon while the atmosphere is gradually destabilizing. Trades are dropping as surface high pressure to the northeast is pushed away from the region and the local pressure gradient relaxes due to surface low pressure developing along a front about 1,000 miles northwest of Kauai. A weak low aloft is already parked over the islands, and as the upper-level trough generating the surface low approaches, the atmosphere is gradually destabilizing. This has not resulted in any significant increase in shower activity today. However, a shallow band of moisture just east of Maui and the Big Island should bring some boost in shower coverage for windward areas on those islands before sundown, and a few showers could flare up across leeward Big Island into the evening.
Chances for showers, some heavy, increase tonight into Sunday as trade winds decrease further and shift out of the east to southeast. The low to the northwest of Kauai will strengthen as the high to the northeast of Hawaii weakens, causing the decline and shift in the winds, and a broad upper-level trough will stretch across the region, creating some instability. Meanwhile, a mid-level trough, currently just south of the Big Island, will drift over the state. Convergent east-southeasterly flow on the mid-level trough's northern flank will produce an increase in moisture, raising precipitable water, from its current value of around an inch, to 1.3 to 1.5 inches. Guidance has been showing the deepest moisture developing just north of the islands this weekend. However, there will be enough moisture and instability to increase shower frequency and intensity over windward slopes of all islands, and given the weakened surface flow, afternoon sea breezes will produce clouds and spotty showers across sheltered leeward terrain. Some showers may be heavy, and an isolated rumble of thunderstorm cannot be ruled out. The ingredients still do not push confidence high enough for the issuance of a Flood Watch, though the situation will need monitoring.
The GFS and ECMWF show the chances for rainfall declining by late Sunday or Monday. The upper-level trough over the region will weaken, and the mid-level trough will lift northward, lowering chances for rainfall. The main difference between the models is the strength of the east-southeast winds, which are stronger in the GFS due to a more rapid weakening of the nearby surface low.
A gradual return to a more stable trade wind flow is expected Tuesday or Wednesday.

Aviation

Locally breezy trade winds will continue to gradually weaken through tomorrow and shift out of the east-southeast. This pattern will allow isolated to scattered showers to ride into windward areas on the trades, and localized sea breezes, and possibly island plumes (wind-convergence downstream of island terrain), to bring isolated showers to leeward areas during the afternoon hours through tomorrow. Brief MVFR conditions will be possible in showers. Otherwise, VFR conditions are generally expected to prevail.
No AIRMETs are in effect.

Marine

High pressure to the northeast will continue to weaken and drift east later today into tomorrow while simultaneously a surface low and trough will drift towards the islands from the west. This will lead to a decrease in trade wind speeds into the moderate to fresh range and winds will veer to a more east-southeast direction through the weekend. A few isolated thunderstorms will be possible tomorrow into Sunday due to the proximity of the low. The low is then expected to drift west by Tuesday, allowing trade winds to return to a more typical east-northeast direction and increasing in speed to fresh to strong from Tuesday through the second half of the week.
A moderate west northwest (300-320) swell will continue to build and peak tonight before quickly dropping tomorrow. Buoy data and webcams have suggested this swell has come in larger than expected, and a High Surf Advisory is now in place for Kauai, Oahu, Molokai, and Maui until 6 AM tomorrow. This swell will then gradually fade over the weekend. A small to moderate, medium period north northwest swell (340-350) may overlap the fading west northwest swell Sunday night into Monday. In the long range, a series of small, long period northwest swell look to keep surf elevated through much of next week.
Surf along east-facing shores will remain rough and choppy but may ease a couple of notches later today and Saturday, and then again early next week as winds fluctuate in strength.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 AM HST Saturday for Niihau, Waianae Coast, Oahu North Shore, Maui Windward West, Kauai North, Molokai Windward, Molokai North, Molokai West, Maui Central Valley North, Windward Haleakala.

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