West Kaua’i

Tonight: Mostly clear. Isolated showers in the evening. Lows around 72 near the shore to around 62 above 3000 feet. Light winds. Chance of rain 20 percent.

Columbus Day: Partly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 87 near the shore to around 75 above 3000 feet. Light winds. Chance of rain 40 percent.

Monday Night: Partly cloudy. Isolated showers in the evening. Lows around 73 near the shore to around 61 above 3000 feet. Light winds. Chance of rain 20 percent.

South Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening. Lows around 72. Light winds. Chance of rain 20 percent.

Columbus Day: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 82 to 90. Light winds becoming east up to 10 mph in the afternoon. Chance of rain 40 percent.

Monday Night: Partly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows around 73. Northeast winds around 10 mph. Chance of rain 30 percent.

Kaua’i Mountains

Tonight: Partly cloudy. Scattered showers in the evening. Lows 64 to 70 in the valleys to around 58 above 4000 feet. Light winds. Chance of rain 40 percent.

Columbus Day: Partly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 75 to 83 in the valleys to around 67 above 4000 feet. Light winds. Chance of rain 50 percent.

Monday Night: Partly cloudy with scattered showers. Lows 63 to 70 in the valleys to around 59 above 4000 feet. Light winds becoming east up to 10 mph after midnight. Chance of rain 40 percent.

North Kaua’i

Tonight: Partly cloudy with isolated showers. Lows 65 to 73. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Columbus Day: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 78 to 86. East winds up to 10 mph. Chance of rain 40 percent.

Monday Night: Partly cloudy with scattered showers. Lows 65 to 73. East winds up to 10 mph. Chance of rain 40 percent.

East Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows 64 to 76. Light winds. Chance of rain 20 percent.

Columbus Day: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 76 to 88. Northeast winds up to 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy with scattered showers. Lows 64 to 77. Northeast winds around 10 mph. Chance of rain 40 percent.

Detailed Forecast

Synopsis

An upper-level low centered several hundred miles north of the state will continue to move south through midweek, then weaken and slide off to the west. This will bring a chance for heavier showers and a few thunderstorms from Wednesday into Friday evening. Relatively light southeast surface winds continue through Tuesday, with stronger trades kicking in Wednesday into lasting into next weekend.

Discussion

Another afternoon of sea breezes and isolated showers mainly in leeward areas is well underway. That pattern will generally continue through Tuesday. The largest area of showers as of 3 PM HST was over southwest Kauai. In addition, a band of higher clouds, associated with the entrance region of an upper level jet, was moving SW to NE.
A strong upper level low and its surface reflection will continue to move south through Wednesday afternoon, reaching the western islands (it will be centered roughly overhead) before sliding off to the west and weakening late this week and into next weekend. This will bring cooler air aloft, increasing instability and the chance for stronger showers and thunderstorms, especially Wednesday through Friday evening. We have gone ahead and introduced thunderstorms into the forecast as confidence is high that they will occur. The most likely area is the western part of the state, but we can't rule out storms in the east at this time.
The other significant change this coming week will be the surface winds. Relatively light southeast winds will give way to easterly trades Wednesday. Wind speeds will increase, especially in the usual topographically driven areas straights. Small craft advisories may be needed beginning Thursday.

Aviation

Light E to ESE winds are prevailing across the state, which has resulted in localized sea breezes during the day. Low clouds and showers remain limited overall, though some low clouds and showers will remain possible through early evening over island interiors and select leeward locations. VFR flight conditions are expected to prevail at most locations. Meanwhile, an upper jet is producing a band of high clouds streaming over the central and eastern islands. This jet is expected to weaken and push east of the area through tonight, so anticipate that high cloud cover will diminish by early Monday.
Another day of light background winds and daytime sea breezes is expected on Monday, increasing interior clouds and showers during the day. Overnight, land breezes are forecast to diminish cloud cover over the islands.
There are currently no AIRMETs in effect, and none are anticipated at this time.

Marine

A trough north of the area will settle in the proximity of the islands the next couple of days. Troughing will likely lay across the far west waters and, as energy from the north dives south into the trough, the chances for mid to late week western water isolated to scattered thunderstorms will increase. Slightly strengthening surface high pressure centered around a thousand nautical miles northeast of the state will produce a tight enough downstream pressure gradient by Wednesday to result in more water-wide moderate trade winds.
Long period forerunners from an approaching northwest (320 degree) swell reached the offshore 51001 and 51101 buoys late this morning. This has a moderate size, long period swell reaching Kauai and Oahu this evening, Maui around Midnight Monday. Swell will fill in around all islands during the overnight hours and produce advisory level surf along many north and west-facing shores of the smaller islands by late Monday morning. A High Surf Advisory will be in effect from Monday morning through Tuesday afternoon to account for over double head high surf that should peak along many north and west-facing shores early Tuesday. Surf will begin to gradually fall from Tuesday night through Thursday.
A series of very small, medium period southern swell impulses coming from a couple of fast-moving southern gales passing south of New Zealand will pass through this week.
East-facing shore wind wave chop will be small the next couple of days. More rough surf will occur later this week as trades build over and upwind of the islands.
Water levels will continue to gradually lower through Monday. There may be some minor flooding within low laying areas during Monday's high tide.

Fire weather

Light winds and isolated showers will help maintain fire weather below critical thresholds for the next several days. Inversion heights will range from 6,000 to 8,000 feet.

HFO Watches/Warnings/Advisories

High Surf Advisory from 6 AM Monday to 6 PM HST Tuesday for Niihau, Kauai Leeward, 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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