West Kaua’i

Tonight: Occasional showers and slight chance of thunderstorms. Lows around 69 near the shore to around 60 above 3000 feet. Light winds. Chance of rain near 100 percent.

Washingtons Birthday: Occasional showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Highs 79 to 85 near the shore to around 69 above 3000 feet. Light winds becoming north up to 10 mph in the afternoon. Chance of rain near 100 percent.

Monday Night: Partly cloudy. Lows around 68 near the shore to around 57 above 3000 feet. Northeast winds around 10 mph.

South Kaua’i

Tonight: Occasional showers and slight chance of thunderstorms. Lows around 70. West winds around 10 mph in the evening becoming light. Chance of rain near 100 percent.

Washingtons Birthday: Occasional showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Highs 76 to 84. Light winds becoming northeast up to 15 mph in the afternoon. Chance of rain near 100 percent.

Monday Night: Mostly cloudy with isolated showers. Lows around 68. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Kaua’i Mountains

Tonight: Occasional showers and slight chance of thunderstorms. Lows 62 to 68 in the valleys to around 59 above 4000 feet. Light winds. Chance of rain near 100 percent.

Washingtons Birthday: Occasional showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Highs 70 to 79 in the valleys to around 62 above 4000 feet. Light winds becoming north up to 10 mph in the afternoon. Chance of rain near 100 percent.

Monday Night: Mostly cloudy with scattered showers. Lows 60 to 67 in the valleys to around 56 above 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

North Kaua’i

Tonight: Occasional showers and slight chance of thunderstorms. Lows 63 to 70. South winds up to 10 mph in the evening becoming light. Chance of rain near 100 percent.

Washingtons Birthday: Occasional showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Highs 74 to 81. Light winds becoming north around 10 mph in the afternoon. Chance of rain near 100 percent.

Monday Night: Mostly cloudy with isolated showers. Lows 62 to 69. Northeast winds around 10 mph. Chance of rain 20 percent.

East Kaua’i

Tonight: Occasional showers and slight chance of thunderstorms. Lows 63 to 72. Southwest winds up to 10 mph. Chance of rain near 100 percent.

Washingtons Birthday: Occasional showers and slight chance of thunderstorms in the morning, then scattered showers in the afternoon. Highs 71 to 82. Light winds becoming north up to 15 mph in the afternoon. Chance of rain near 100 percent.

Monday Night: Mostly cloudy with isolated showers. Lows 62 to 71. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Showers over Kauai may produce nuisance flooding today through Monday with showers increasing over Oahu during Monday. Elsewhere and otherwise, quiet weather prevails through the period as light to moderate trades return on Tuesday.

Discussion

Flow out of the southerly quadrant prevails across the state this afternoon. Trades have veered to ESE over the eastern end of the state as governing high pressure is weakened and forced eastward in advance of progressive mid-latitude troughing. Partly cloudy skies prevail from Molokai through Big Island with little in the way of shower activity expected through tonight owing to a stable environment and lack of upstream moisture. Further west, winds have veered to southerly over Kauai and Oahu in response to the approaching cold front associated with said mid-latitude trough. This has allowed a narrow ribbon of moisture characterized by PWAT values of nearly 2″ and dewpoints around 70 to become established over the western end of the state. Cloud tops within the shower band over/west of Kauai are observed to generally be around 12kft. Rain rates remain in check, generally a half inch per hour or less with isolated rates of up to 1″ per hour. Likewise, rainfall totals have been limited despite steady ongoing showers with only Puu Opae on Leeward Kauai reporting more than an inch total in the last 6 hours. Model cross sections actually indicate low static stability through the lowest 20kft or so suggesting greater potential for vertical growth of showers within any pockets of deeper moisture or stronger forcing. Shallower moisture depth over Oahu has generally led to extensive low clouds and transient pockets of light showers.
The most organized shower activity is expected to remain anchored to the mid-level moisture gradient remains positioned immediately west of Kauai, but still close enough to allow waves of showers along its eastern periphery to spread over the island. This will continue through Monday as prefrontal convergence strengthens, reaching a maximum around midday Monday when a period of more organized heavy rain capable of producing nuisance type flooding will be possible. Generally expecting the next 24 hours or so to be wet over Kauai, particularly south and west-facing slopes with mostly dry conditions elsewhere. Limited showers embedded within southerly flow may spread over Leeward Oahu overnight.
Tricky forecast for Oahu on Monday as the moisture axis advances toward the island. Anticyclonic flow associated with building trades will act to rapidly erode the band of showers as it tracks east. At the same time, the local gradient should be light enough to allow afternoon sea breeze convergence to capitalize on resident low-level moisture. One way or another, expect afternoon showers over a portion of the island during Monday afternoon, greater coverage conditional on the shower band holding together. A few showers may be heavy, particularly over the NE half of the island where sea breeze convergence will be maximized.
Low-level convergence is then lost altogether as moderate trades build on Tuesday and the moisture boundary aligns increasingly parallel with the flow. Extended guidance in general agreement that the remainder of the forecast period from Tuesday onward will be quiet and stable, though differ on the latitude of the trade wind belt. Confidence is high that the next several days will be characterized by light to moderate stable trades over the eastern half of the state, but a weaker gradient over Oahu and Kauai may allow light trades to give way to afternoon sea breezes/overnight land breezes on certain days.

Aviation

A cold front northwest of the main Hawaiian islands is pushing a ridge over the western islands this afternoon, resulting in light winds across the state. The background flow is expected to turn southerly as the ridge is pushed southward down the island chain through tomorrow. A nearly stationary pre-frontal convergence band remains over Kauai this afternoon, bringing clouds and showers to the island. AIRMET Sierra remains in effect for mountain obscuration, and that is expected to remain in place into tonight.
An earlier cloud band that had been over Oahu has thinned significantly since the pre-dawn hours, and the AIRMET Sierra was cancelled as a result. It is possible that additional clouds and showers will form overhead later this evening or overnight which would necessitate a re-issuance.
For most of tonight, the islands of Maui County and the Big Island will continue to experience VFR conditions.

Marine

Light to moderate southeast flow will prevail across the eastern half of the island chain through Monday. Winds around Kauai and Oahu today will veer more southwesterly this evening and overnight as a frontal system approaches the region from the northwest. Expect moderate northerly winds behind the front as it moves across Kauai late tonight into Monday. Enough instability and moisture will exist along and ahead of the front to bring widespread showers and a slight chance of thunderstorms to the waters surrounding Kauai and Oahu late tonight through Monday. The front is expected to stall and dissipate near Oahu Monday night into Tuesday. Moderate east-southeast winds will return statewide on Tuesday and continue through the remainder of the week.
The current northwest swell will continue to slowly lower into this evening, with surf heights well below High Surf Advisory (HSA) levels. Deep low pressure, currently situated just southeast of Russia's Kamchatka Peninsula, is the source for a large long- period northwest (310-320) swell that will impact the local waters during the first part of this week. Long-period forerunners are expected to arrive tonight, followed by building swell through Monday night. Surf will rise to HSA heights by Monday afternoon for north and west facing shores and perhaps approach warning criteria as it peaks Monday night through Tuesday. The HSA for this event will likely be issued overnight for a Monday start time, though the details may change as we see the swell appear later today at the NDBC buoys (51001 and 51101) far northwest of Kauai. A Small Craft Advisory for hazardous seas will be needed for waters exposed to this swell, likely by Monday night. The swell will slowly subside Tuesday night through the end of the week.
Moderate southeast flow will maintain small chop along east facing shores, while background energy out of the southerly quadrant keeps small surf in place for south facing shores.

HFO Watches/Warnings/Advisories

None.

Kauai Now Weather is brought to you by Blue Hawaiian Helicopters.