West Kaua’i

Tonight: Mostly cloudy with isolated showers in the evening, then mostly clear after midnight. Lows 64 to 69 near the shore to around 58 above 3000 feet. Light winds becoming northeast up to 15 mph after midnight. Chance of rain 20 percent.

Monday: Sunny and breezy. Highs 80 to 87 near the shore to around 73 above 3000 feet. East winds up to 20 mph.

Monday Night: Mostly clear. Breezy. Isolated showers after midnight. Lows 65 to 71 near the shore to around 58 above 3000 feet. Northeast winds 10 to 25 mph. Chance of rain 20 percent.

South Kaua’i

Tonight: Mostly clear. Isolated showers in the evening. Lows around 68. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Monday: Sunny and breezy. Highs 79 to 86. Northeast winds 10 to 25 mph increasing to 20 to 25 mph in the afternoon.

Monday Night: Breezy. Partly cloudy with isolated showers. Lows around 69. Northeast winds 15 to 25 mph. Chance of rain 20 percent.

Kaua’i Mountains

Tonight: Partly cloudy with isolated showers. Lows 57 to 67 in the valleys to around 56 above 4000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

Monday: Breezy. Mostly sunny with isolated showers. Highs 73 to 79 in the valleys to around 64 above 4000 feet. Northeast winds up to 20 mph shifting to the east in the afternoon. Chance of rain 20 percent.

Monday Night: Partly cloudy in the evening then becoming mostly cloudy. Breezy. Scattered showers. Lows 59 to 69 in the valleys to around 57 above 4000 feet. East winds up to 25 mph. Chance of rain 50 percent.

North Kaua’i

Tonight: Mostly clear. Isolated showers after midnight. Lows 61 to 69. Southeast winds up to 10 mph. Chance of rain 20 percent.

Monday: Sunny with isolated showers. Highs 74 to 83. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 20 percent.

Monday Night: Breezy. Partly cloudy with isolated showers. Lows 62 to 69. East winds 10 to 20 mph. Chance of rain 20 percent.

East Kaua’i

Tonight: Partly cloudy with isolated showers. Lows 62 to 72. Northeast winds up to 10 mph. Chance of rain 20 percent.

Monday: Mostly sunny. Breezy. Isolated showers in the afternoon. Highs 72 to 83. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Monday Night: Breezy. Partly cloudy with isolated showers. Lows 63 to 73. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Trade winds will strengthen and become breezy and gusty by Monday afternoon as strong high pressure sets up north of the area. Rather dry conditions will produce limited rainfall over windward areas and the Kona slopes of the Big Island, followed by an increase in windward showers on Tuesday. A weak disturbance aloft combined with increased moisture should produce a breezy and wet weather Tuesday night into Friday.

Discussion

A rather dry trade wind flow is strengthening across the island chain this afternoon. Trades are on the rise as a surface ridge about 100 miles north of Kauai lifts northward and stronger surface high pressure builds far northwest of the state. Very stable conditions remain in place, and with precipitable water holding below an inch, rainfall has been scant today, aside from a few afternoon showers over leeward Big Island slopes and leeward Kauai. This rather dry air mass has pushed dew points down into the seasonable range of the low to mid 60s, which has provided relief from yesterday's humidity. Volcanic haze (vog) may linger over Kauai as this afternoon but otherwise returning trades have now largely confined vog to the area downwind of Halemaumau Crater and leeward Big Island.
Trade winds will strengthen on Monday, becoming breezy and gusty in the afternoon and evening. The driver of the increased trades will be the above mentioned surface high, which will strengthen to 1032 mb as it moves to roughly 700 miles north of the state by afternoon. Localized wind gusts around 40 mph over and downwind of terrain could develop in the afternoon and evening as a ridge aloft maintains an inversion near 5000 ft. These stable conditions will also limit rainfall, with modest showers focused across windward slopes.
Trade winds will remain breezy and gusty on Tuesday, as chances for showers increase. The surface high will build further to an impressive 1035 mb roughly 900 miles north-northeast of the islands. This will maintain breezy trades, and although the ridge aloft and its resulting inversion will begin to erode, localized wind gusts of 40 mph or more will remain possible. Guidance is indicting increased precipitable water moving in from the east bringing a return of a more typical shower pattern over windward areas, especially for the Big Island and Maui County.
Shower frequency is expected increase within the breezy trade wind flow Tuesday night. A disturbance aloft will move overhead and produce modest instability that will likely eliminate the inversion. While details remains unclear, enhanced moisture in the trades will likely fuel increased, mainly windward, rainfall through Friday. Trades will decline a notch Thursday and Friday but will remain locally breezy.

Aviation

Moderate to breezy easterly trades will continue through the 24-h period, with clouds and brief showers primarily favoring windward coasts and slopes overnight through early Monday morning.
AIRMET TANGO for low-level turbulence downstream of island terrain is likely Monday afternoon.

Marine

Moderate trade winds currently in place are expected to strengthen significantly over the next few days. A weakening front currently northwest of the islands will dissipate while strong high pressure builds N of the islands. This will lead to fresh to near gale-force trade winds that will remain in place for the foreseeable future. A Small Craft Advisory is in effect beginning tomorrow at noon through Wednesday afternoon for all waters as winds strengthen, and seas increase to near 10 feet.
The current declining NW swell will be replaced by a new NW swell tomorrow which will build through the day and lead to advisory- level surf at its peak late tomorrow/early Tuesday. This NW swell will diminish Tuesday and Wednesday, with another NW swell around Friday likely leading to advisory-level surf again. A small S swell is possible tomorrow. Large and choppy surf along E facing shores is expected this week due to the strengthening trade winds.

HFO Watches/Warnings/Advisories

Small Craft Advisory from noon Monday to 6 PM HST Wednesday for all Hawaiian waters.

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