West Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Lows 69 to 74 near the shore to around 61 above 3000 feet. Light winds.

Friday: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs 83 to 89 near the shore to around 75 above 3000 feet. Northwest winds up to 10 mph. Chance of rain 40 percent.

Friday Night: Partly cloudy. Isolated showers in the evening. Lows around 71 near the shore to around 61 above 3000 feet. North winds around 10 mph. Chance of rain 20 percent.

South Kaua’i

Tonight: Mostly cloudy. Isolated showers in the evening. Lows around 72. Northeast winds around 10 mph in the evening becoming light. Chance of rain 20 percent.

Friday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 81 to 88. Light winds. Chance of rain 40 percent.

Friday Night: Partly cloudy. Isolated showers in the evening. Lows around 71. Light winds becoming north around 10 mph after midnight. Chance of rain 20 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 63 to 69 in the valleys to around 59 above 4000 feet. Light winds. Chance of rain 50 percent.

Friday: Mostly sunny in the morning then becoming partly sunny. Scattered showers. Highs 74 to 82 in the valleys to around 67 above 4000 feet. Light winds becoming northeast up to 10 mph in the afternoon. Chance of rain 50 percent.

Friday Night: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 62 to 69 in the valleys to around 58 above 4000 feet. North winds around 10 mph. Chance of rain 40 percent.

North Kaua’i

Tonight: Mostly cloudy with isolated showers. Lows 65 to 72. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Friday: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs 76 to 85. Light winds becoming north around 10 mph in the afternoon. Chance of rain 30 percent.

Friday Night: Partly cloudy with isolated showers. Lows 64 to 73. North winds around 10 mph. Chance of rain 20 percent.

East Kaua’i

Tonight: Mostly cloudy with isolated showers. Lows 64 to 75. Light winds. Chance of rain 20 percent.

Friday: Mostly sunny in the morning then becoming partly sunny. Scattered showers. Highs 75 to 86. North winds up to 10 mph. Chance of rain 50 percent.

Friday Night: Partly cloudy with isolated showers. Lows 63 to 75. North winds around 10 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Trade winds will weaken through Friday as a front approaches the islands from the northwest. The front will move north to south across the islands chain this weekend into early next week, bringing increased rainfall that will be primarily focused along windward slopes. Breezy to strong trade winds will follow the front, and will continue through much of next week.

Discussion

This afternoon, a surface low pressure system is centered roughly 1100 miles north of Kauai with a trailing cold front that extends from this low southwestward into the West Pacific. Meanwhile, high pressure is located far east-northeast of the main Hawaiian Islands, with ridging east of the state. Observations from around the state, and a late morning scatterometer pass, showed light to moderate east to slightly east-southeast trades prevailing today. Afternoon 00z soundings from Lihue and Hilo showed stable conditions with a strong inversion near 5,000 feet. Latest satellite and radar imagery depicts only a few light showers embedded within the prevailing flow along windward locations, as well as typical afternoon cloud buildups along the Kona slopes of the Big Island.
Over the next 24 to 36 hours, the aforementioned low far north of the state is expected to quickly move eastward across the North Pacific. As a result, the trailing cold front will progress east- southeastward as well, advancing towards Kauai. As this occurs, the light to moderate trade wind flow across the islands will become light and variable, setting up a daytime sea breeze and nighttime land breeze regime. Stable conditions will remain in place in advance of the front's arrival in Hawaii, so mostly dry conditions are expected through Friday. Any showers that do form will be light, similar to those observed today.
The cold front is expected to arrive and begin moving down the island chain this weekend. The GFS continues to offer a quicker moving band of showers with the front, arriving by Saturday morning at Kauai and reaching the Big Island by Sunday. The ECMWF is quite a bit slower, depicting the front at Kauai by Saturday afternoon and reaching the Big Island later Sunday or early Monday. With this afternoon's forecast package, have trended the higher rain chances a little earlier than the previous forecast, but the overall forecast philosophy remains in tact. Strong trade winds overrunning the front will bring mostly windward showers, but some showers may also blow over island terrain to reach leeward areas of the smaller islands as well. As previous discussions have mentioned, there is little to no upper-level support with this system, which should limit rainfall rates. The relative quick-moving nature of the front through the island chain will serve as an additional limiting factor for flooding concerns.
Breezy to strong trade winds will fill in behind the front. These stronger trades will continue through much of next week as a strong high becomes established far north of the state and strengthens through mid to late week. Conditions will remain stable, with scattered low clouds and windward and mauka showers.

Aviation

Moderate easterly trade winds will decrease and shift out of the southeast overnight, eventually placing much of the island chain in a light and variable wind regime downwind of the Big Island. Daytime sea breezes will develop on Friday as light background winds shift out of the north around Kauai and Oahu. The atmosphere will remain stable, and aside from isolated MVFR ceilings over interior terrain, VFR conditions will prevail.

Marine

Moderate trades weaken into Friday in response to low pressure advancing north of the Hawaiian Islands. Winds become light to variable over area waters during Friday. Moderate NNE trades build from W to E Friday night into Saturday becoming strong with the passage of a cold front during the weekend. Strong trades continue into next week. Small Craft Advisories will likely be needed during this time.
Current NNW (340-350) swell stubbornly hovers near its peak values at press time this afternoon. Surf numbers have therefore been nudged upward for N and W facing shores to accommodate the most recent observations. This swell will fade heading into Friday. A deep low is expected to develop N of the islands Friday into Saturday. This low will generate a large short to medium period NNE (010-020 degree) swell late this weekend that will likely exceed the High Surf Advisory threshold for N facing shores. This fetch will also favor harbor surges in N facing harbors such as Kahului and Hilo on Sunday.
Surf remains small for E shores through the near term, eventually catching some of the aforementioned NNE swell. Short period chop and NEly fresh swell emanating from upstream trades will bring elevated surf to E shores behind the cold front and continuing into next week. S shores remain small through the remainder of this week. A small long period SSW swell will boost surf Friday into the weekend.
Peak monthly tides combined with water levels that are running higher than predicted will lead to minor flooding along the shoreline and in low-lying coastal areas. The greatest chance for coastal flooding will occur during the daily peak high tide during the early morning hours and along N facing shores exposed to the current NNW swell.

Fire weather

With winds weakening for the next couple of days, critical fire weather concerns are not expected. Additionally, increasing moisture is expected with a frontal passage this weekend. Trades are forecast to return and become breezy to strong throughout the week next week, though it may be difficult to reach critical RH thresholds.

HFO Watches/Warnings/Advisories

None.

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