West Kaua’i

Tonight: Mostly clear. Isolated showers in the evening. Lows around 65 near the shore to around 55 above 3000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Saturday: Mostly sunny with isolated showers in the morning, then partly sunny in the afternoon. Highs around 81 near the shore to around 70 above 3000 feet. East winds up to 10 mph shifting to the southwest in the afternoon. Chance of rain 20 percent.

Saturday Night: Partly cloudy with isolated showers. Lows around 66 near the shore to around 56 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

South Kaua’i

Tonight: Partly cloudy. Breezy. Isolated showers in the evening. Lows around 65. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Saturday: Mostly sunny. Breezy. Isolated showers in the morning. Highs 77 to 82. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Saturday Night: Breezy. Partly cloudy with isolated showers. Lows around 66. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Kaua’i Mountains

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North Kaua’i

Tonight: Partly cloudy with isolated showers. Lows 59 to 67. East winds up to 15 mph. Chance of rain 20 percent.

Saturday: Mostly sunny. Isolated showers in the morning. Highs 72 to 80. East winds up to 15 mph. Chance of rain 20 percent.

Saturday Night: Partly cloudy with isolated showers. Lows 59 to 68. East winds up to 15 mph becoming around 10 mph after midnight. Chance of rain 20 percent.

East Kaua’i

Tonight: Partly cloudy with isolated showers. Lows 59 to 70. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Saturday: Mostly sunny. Isolated showers in the morning. Highs 70 to 80. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Saturday Night: Partly cloudy with isolated showers. Lows 60 to 70. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Moderate trades focus showers windward and mauka through the weekend. A strengthening low then brings potential for heavy rain to the islands from Tuesday night onward.

Discussion

Moderate trades focus showers windward and mauka, and a zonally oriented subtropical jet funnels abundant high clouds across area skies. High clouds likely clear out during Saturday as the STJ sags south then return again Sunday as subtle ridging forces it northward once again. Benign stable trade wind weather through Monday.
Attention then turns to flooding potential beginning late Tuesday or Wednesday and persisting for an uncertain period of time. This event will represent yet another flavor of low pressure evolution distinct from the two recent events. Overall, confidence in details (including timing/duration/location) is below normal owing to the dependence of the upper air pattern on the interaction of multiple relatively small-scale shortwaves with the larger scale flow.
Late this weekend, a mid-latitude trough will cross the dateline and will serve as the waveguide for a quick-moving shortwave that will undergo a deepening sequence as it passes north of the islands early Tuesday. This will help establish surface pressure falls that will weaken the wind field while eliciting a southerly flow response aloft that will draw abundant tropical moisture northward. Shortwave ridging naturally follows on Tuesday night suggesting a more muted response for any initial wave of rain during this time. PWATs around 2″, and still rising, will likely be established by Wednesday morning. Upper air evolution then nearly repeats itself on Wednesday as another quick-moving shortwave deepens as it approaches the base of a trailing mid- latitude trough. There is strong consensus that phasing aloft will produce a more robust surface response Wednesday into Thursday. As surface low pressure develops NW of the islands, the ambient wind field ramps up, and with plenty of moisture already in place, confidence increases in the Wed-Thurs time frame as a target for flooding potential.
Model consensus breaks down thereafter. The GFS allows Wednesday's deepening trough to cutoff NW of the islands resulting in multiple periods of forcing as shortwaves interact with lingering moisture and numerous waves of rain each representing a potential flooding threat. The EC likewise allows moisture to remain in place for a prolonged period of time, but is considerably more progressive with the parent low and suggests perhaps a wet pattern, but a low magnitude one, after Thursday. It is worth noting that with moisture remaining in place for the foreseeable future, flash flood potential will likely have to be assessed regularly through mid-April as any trough intruding from the mid-latitudes will have potential to cause trouble.

Aviation

The trade wind pattern and VFR conditions will continue into Saturday. Temporary MVFR conditions are possible in windward locations in any showers that develop creating lowered visibilities and ceilings.
Overall, turbulence has weakened, so AIRMET Tango has been canceled.

Marine

Moderate to locally fresh east-northeasterly trade winds will remain in place through the weekend. Winds will then begin to veer east to southeast by Monday, then become more southerly ahead of a front approaching from the northwest and a trough moving up from the southwest. This surface front and trough will be driven by an upper level trough, which could allow for isolated thunderstorms over the offshore and coastal waters through much of next week. A Small Craft Advisory remains in effect for the typically windier waters around Maui County and the Big Island through tonight.
Surf along north-facing shores will remain tiny to small through the weekend as a short-period northeast swell fades and small northwest pulses fill in. Tonight into Saturday, small, medium- period west-northwest (310 degree) energy will fill in and on Saturday night into Sunday, small, medium-period northwest (330-340 degree) energy will also fill in. Both of these small swells will bump up surf slightly for north and west facing shores. Then by Monday night through Wednesday, a moderate medium- period west-northwest (310 degree) swell will fill in and increase surf to just below advisory levels.
Along south facing shores, expect minor pulses from the South Pacific to provide periods of small surf through the weekend until a larger long-period south swell fills in Monday and lasts through at least midweek. In addition, strengthening southerly winds may lead to increasingly choppy conditions by midweek.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

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Data Courtesy of NOAA.gov