West Kaua’i

Tonight: Partly cloudy. Breezy. Isolated showers in the evening, then scattered showers after midnight. Lows 66 to 71 near the shore to around 59 above 3000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Tuesday: Mostly sunny with isolated showers. Highs 81 to 87 near the shore to around 74 above 3000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Tuesday Night: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows around 68 near the shore to around 59 above 3000 feet. Light winds. Chance of rain 20 percent.

South Kaua’i

Tonight: Breezy. Mostly cloudy with scattered showers. Lows around 69. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 79 to 86. East winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows around 68. Northeast winds up to 10 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Mostly cloudy with scattered showers. Lows 60 to 68 in the valleys to around 57 above 4000 feet. East winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain 50 percent.

Tuesday: Mostly cloudy with scattered showers. Highs 74 to 80 in the valleys to around 64 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 59 to 68 in the valleys to around 57 above 4000 feet. Light winds. Chance of rain 50 percent.

North Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 63 to 70. East winds up to 15 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 75 to 83. Southeast winds up to 15 mph. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 62 to 69. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

East Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 63 to 73. East winds up to 15 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 72 to 84. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 62 to 72. Northeast winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Trade winds will ease into Tuesday, with passing clouds and showers favoring windward and mauka areas. Lighter winds are expected by the middle of the week with fewer windward showers and a greater chance for afternoon interior showers through the weekend. An upper level trough and deep atmospheric moisture will enhance shower activity on Thursday and Friday and bring the potential for thunderstorms and heavy rainfall.

Discussion

High pressure centered to the northeast of the state will maintain breezy trade winds across the region through tonight, continuing to focus incoming clouds and showers over windward and mauka areas. Water vapor imagery shows a weak upper-level low spinning over the state. Mid-level ridging will briefly build overhead Tuesday night into early Wednesday.
At the surface, a strong low pressure system to the far north will displace the ridge that's been anchored to our northeast to bring it closer to, or over, the island chain. The proximity of this ridge axis and the approach of a cold front from the northwest will bring lighter southeast winds across the area beginning Tuesday and for nighttime land and daytime sea breezes by Wednesday.
A shortwave trough will dive southeastward down the island chain Wednesday night through Thursday night before weakening and moving off the east Friday. The most favorable upper-level dynamics associated with the orientation of the shortwave trough, the coolest temperatures aloft (500 mb around -12C), and the deepest atmospheric moisture (precipitable water around 1.6 inches) all seem to line up best on Thursday and Friday. As such, heavy showers and a slight chance of thunderstorms have been added to the forecast across the state for Thursday and Friday (more of the focus will be on Big Island Friday).
Light winds will continue into the weekend as the mid-level shortwave weakens resulting in either a land/sea breeze pattern with more muted diurnal convection or a hybrid sea breeze pattern which will tend to focus showers over western slopes of each island. Gentle to moderate trades will move in around Sunday or early next week.

Aviation

Moderate to breezy trade winds will gradually ease tonight and Tuesday. AIRMET Tango for turbulence below 8,000 feet over and downwind of terrain remains in effect. Conditions are borderline, and the AIRMET will likely be dropped overnight. The trades will produce brief periods of MVFR conditions over windward terrain, and aside from spotty afternoon and evening showers on leeward slopes of the Big Island, VFR will prevail across leeward areas. A mid-level ridge will maintain overall stable conditions, though a lingering weak upper-level trough may trigger an isolated heavy shower on Big Island slopes during the afternoon hours.

Marine

High pressure centered far northeast of the state will maintain moderate to locally strong east to southeasterly winds through tonight. A Small Craft Advisory (SCA) remains in effect through 6 AM Tuesday for the eastern half marine zones. The SCA is to mainly account for the typically windier waters surrounding Maui County and Big Island within this east southeast flow pattern. Winds will begin to weaken Tuesday. Breezes will transition to light and variable across more western waters later in the week, while staying gentle to moderate east southeast across the state's eastern waters. This will be in response to the approach and eventual stalling of a front northwest of the islands later this week.
A small to moderate size, medium period northwest swell is filling around Kauai and Oahu today. Nearshore buoys are observing a 4 to 6 foot swell peaking in the 14 to 16 second bands this afternoon. This swell will result in slightly over head to double head high surf within the highest sets along many north and west-facing shores through the early Tuesday morning hours. The swell will decline from Tuesday morning into Wednesday. A larger moderate size north northwest swell will arrive Thursday. If this swell comes in as guidance is currently suggesting, surf will achieve High Surf Advisory heights along many of the north and west-facing shores of the smaller islands Thursday night and Friday.
East-facing shore surf will remain somewhat rough tonight due to recent strong east trades over and upstream of the islands. As winds fall off in the coming days, responding east chop will significantly drop off.
Surf along south-facing shores will remain small and mostly at background levels through Thursday. A series of small overlapping south swells could increase surf up to seasonal levels from Friday through the weekend.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Tuesday for Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

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