West Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows around 69 near the shore to around 59 above 3000 feet. Light winds becoming northeast up to 10 mph after midnight. Chance of rain 50 percent.

Tuesday: Mostly sunny with isolated showers. Highs 80 to 86 near the shore to around 73 above 3000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Tuesday Night: Mostly clear. Isolated showers in the evening. Lows 66 to 71 near the shore to around 59 above 3000 feet. East winds up to 10 mph. Chance of rain 20 percent.

South Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows around 69. Northeast winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 78 to 85. East winds 10 to 15 mph. Chance of rain 40 percent.

Tuesday Night: Partly cloudy. Isolated showers after midnight. Lows around 68. East winds around 10 mph in the evening becoming light. Chance of rain 20 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 59 to 69 in the valleys to around 58 above 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Tuesday: Partly sunny with scattered showers. Highs 73 to 79 in the valleys to around 64 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Tuesday Night: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 59 to 69 in the valleys to around 57 above 4000 feet. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 40 percent.

North Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 62 to 70. East winds up to 10 mph. Chance of rain 60 percent.

Tuesday: Partly sunny in the morning then becoming mostly sunny. Scattered showers. Highs 75 to 83. East winds up to 15 mph. Chance of rain 50 percent.

Tuesday Night: Partly cloudy with isolated showers. Lows 62 to 69. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

East Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 62 to 72. Northeast winds around 10 mph. Chance of rain 60 percent.

Tuesday: Partly sunny with scattered showers. Highs 72 to 83. East winds around 10 mph. Chance of rain 50 percent.

Tuesday Night: Partly cloudy with isolated showers. Lows 62 to 71. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Detailed Forecast

Synopsis

A prefrontal convergent boundary over Oahu followed by a weak cold front that will move towards Kauai this evening will bring showers, some heavy, to the western end of the state through tomorrow. Then moderate ESE trades will resume through the rest of the week, with clouds and showers favoring windward and mauka areas. Another cold front will approach this weekend and weaken the trades, allowing land and sea breezes to expand in coverage across the state.

Discussion

Current radar and satellite imagery show a band of showers along a prefrontal convergent boundary moving across Oahu and towards Maui County this afternoon. In the wake of this boundary, Kauai has cleared out for the afternoon, even as a weak cold front about 100 miles to its northwest continues to approach.
General consensus from model guidance has the prefrontal convergent boundary falling apart this evening. Even though this boundary will become less defined, it will still bring showers, some potentially heavy with precipitable water values near 2 inches, to Oahu and portions of Maui County through tonight before the axis of moisture retreats back to the northwest tomorrow morning. In addition, thunderstorm potential will remain low, but still present, mainly along the prefrontal convergent boundary where the best combination of moisture, lift and instability continue to line up. Meanwhile, model guidance shows the weak cold front approaching Kauai this evening and dissipating…possibly before it even reaches the area. Both of these surface features (though mostly the former) and their attendant moisture will help to maintain rain chances across the western half of the state through tomorrow.
By Wednesday, the surface ridge to the northeast of the state will take control and moderate ESE trade winds will return and persist through the rest of the week. As such, showers will once again focus over windward coasts and slopes, with some localized sea breeze activity bringing clouds and a few showers to leeward locations. Mid-level ridging will build over the state from the west during the latter part of the week, increasing stability and suppressing shower activity. Another shallow cold front will approach the islands from the northwest this weekend, weakening the surface ridge and producing lighter southeast winds across the region. With these lighter winds, land and sea breeze development may become more prevalent.

Aviation

A NE-SW oriented band of broken to overcast layered clouds over Oahu and Molokai has embedded towering cumulus with cloud tops near 30 kft, with potential for isolated thunderstorms through this evening, with tops greater than 35 kft. The forward motion of the cloud band is expected to slow overnight while it gradually weakens near Oahu, Molokai and potentially Lanai and Maui. Widespread MVFR VIS/CIG, (and isolated IFR CIG) in SHRA can be expected within the band, and AIRMET Sierra is in effect for mountain obscuration on Oahu and Molokai.
A surface high will move rapidly E to the N of the islands overnight, supporting a developing light to moderate E trade wind flow that will likely push remnant moisture into windward slopes on Tuesday.

Marine

A front just northwest of Kauai will continue to move eastward this evening then dissipated around Oahu tonight. The band of showers ahead of the front will continue to produce frequent showers around the waters of Oahu tonight with an outside chance of an isolated thunderstorm overnight. As this front dissipates tonight, light to moderate east to east-southeast winds will return statewide on Tuesday and continue through the remainder of the week as a ridge of high pressure lingers just north of the state.
A series of west-northwest to northwest (300-320 degree) swells are expected this week with the first of the series building this afternoon and peaking tonight. The High Surf Advisory (HSA) has been expanded to include exposed shores of Maui County and the Big Island as this swell continues to make its way down the island chain. In addition, a Small Craft Advisory (SCA) has been issued for hazardous seas for waters exposed to these large swells. Another reinforcing swell is expected on Wednesday, which should maintain advisory level surf along north and west shores through the middle of the week and possibly into Thursday.
Due to the lack of strong trades, small surf is expected along east facing shores this week. A slight increase is possible Tuesday through the second half of the week due to the light to moderate east to east-southeast trades. Surf along south facing shores should see a small boost on Wednesday as a small south swell fills in and should produce some rideable surf throughout the second half of the week. Select exposures could also see some small wrap from the large west-northwest swells over the next several days.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST Wednesday for Niihau, Kauai Leeward, Waianae Coast, Oahu North Shore, Maui Windward West, Kauai North, Molokai Windward, Molokai North, Molokai West, Windward Haleakala.
High Surf Advisory from 6 AM Tuesday to 6 PM HST Wednesday for Kona, Kohala, Big Island South.
Small Craft Advisory until 6 PM HST Tuesday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters.

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