West Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows around 68 near the shore to around 58 above 3000 feet. South winds up to 10 mph shifting to the northeast after midnight. Chance of rain 50 percent.

Friday: Mostly cloudy with numerous showers. Highs 78 to 85 near the shore to around 71 above 3000 feet. East winds up to 10 mph shifting to the south in the afternoon. Chance of rain 70 percent.

Friday Night: Mostly cloudy with scattered showers. Lows around 67 near the shore to around 57 above 3000 feet. Light winds. Chance of rain 30 percent.

South Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows around 68. West winds 10 to 15 mph. Chance of rain 30 percent.

Friday: Partly sunny. Scattered showers in the morning, then numerous showers in the afternoon. Highs 78 to 84. Southwest winds up to 10 mph. Chance of rain 60 percent.

Friday Night: Mostly cloudy with scattered showers. Lows around 68. Northeast winds around 10 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Partly cloudy. Breezy. Isolated showers in the evening, then scattered showers after midnight. Lows 58 to 68 in the valleys to around 56 above 4000 feet. Southwest winds up to 20 mph. Chance of rain 50 percent.

Friday: Partly sunny with numerous showers in the morning, then cloudy with occasional showers in the afternoon. Highs 71 to 78 in the valleys to around 63 above 4000 feet. North winds up to 10 mph. Chance of rain 80 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 57 to 67 in the valleys to around 56 above 4000 feet. Light winds. Chance of rain 50 percent.

North Kaua’i

Tonight: Partly cloudy with isolated showers. Lows 60 to 68. South winds up to 15 mph. Chance of rain 20 percent.

Friday: Mostly cloudy. Scattered showers in the morning, then numerous showers in the afternoon. Highs 73 to 80. Southeast winds up to 10 mph shifting to the north in the afternoon. Chance of rain 70 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 61 to 68. East winds up to 10 mph. Chance of rain 50 percent.

East Kaua’i

Tonight: Partly cloudy with isolated showers. Lows 61 to 71. Southwest winds up to 15 mph. Chance of rain 20 percent.

Friday: Mostly sunny with isolated showers in the morning, then mostly cloudy with numerous showers in the afternoon. Highs 72 to 83. Southwest winds up to 10 mph shifting to the west in the afternoon. Chance of rain 70 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 61 to 71. North winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Moderate south to southwest winds ahead of an approaching front will produce an increase in modest shower activity tonight. The weakening front will stall near Kauai on Friday, bringing increased rainfall for the western end of the island chain and decreasing winds over most areas. East to southeast winds will briefly strengthen Friday night and Saturday, and another front will bring the potential for unsettled weather early next week.

Discussion

The subtropical ridge is being pushed down the island chain by an approaching front, leading to increased southerly winds, especially around Kauai. Low-level moisture has remained below an inch, and an eroding ridge aloft has maintained stable conditions, resulting in scant rainfall today. The best odds for any rainfall through the remainder of the afternoon will be across sheltered slopes of the Big Island and Maui, though little accumulation is expected. Due to the on-again off-again nature of the recent eruptive activity at Halemaumau, the extent of volcanic haze (vog) is somewhat uncertain, but a handful of air quality sensors from the Big Island to Oahu have been detecting significant concentrations.
The front will weaken on its approach tonight, leading to some decrease in southerly winds. The ridge aloft over the islands will erode, allowing a broad area of convergence, currently south of Oahu, to produce some showers in the southerly flow. The most likely target for this rainfall is Oahu, though Kauai to Maui has some chance of showers. The southerly winds should lead to diminishing vog concentration on Oahu with little change on the Big Island and Maui.
The weakening front will stall near Kauai on Friday. The latest deterministic runs of the GFS and ECMWF and their ensemble guidance keep the bulk of the rainfall just off the north shore of Kauai, but an inch or more of precipitation is possible in isolated downpours over northern and interior Kauai. A few heavy heavy showers with less rainfall potential are expected on Oahu. For the rest of the island chain, easing south to southeast winds will produce little rainfall and persistent vog. Surface high pressure passing north of the state Friday night will usher in weak east to southeast trade winds, causing moisture along the dissipated front to lift north of Kauai.
On Saturday, the surface high will build as it moves to the northeast, and its associated subtropical ridge will be just north of Kauai, causing the strengthening winds to be out of the east-southeast. An area of moisture within this flow will likely brush the Big Island, leading to increased showers on windward and southeast slopes. Elsewhere, a ridge aloft will produce stable and rather dry conditions.
Winds are expected to shift out of the southeast and south on Sunday as the next approaching front pushes the subtropical ridge over the islands. The above mentioned moisture that brushes the Big Island Saturday will be drawn over the western end of the island chain on Sunday, bringing increased rainfall chances for mainly Kauai and Oahu. The ridge aloft will erode, leading to some developing instability that could trigger a few heavy showers.
Unsettled conditions are likely early next week. A strong west Pacific jet stream will produce troughing aloft near the state by Monday as the front nears. The GFS and ECMWF are suggesting that the area of moisture that will be drawn over the western islands may spread and be a source for heavy showers and possible thunderstorms. The front will likely dissipate on Tuesday, but lingering moisture and troughing aloft could maintain unsettled conditions.
Forecast confidence is not high heading into the middle of next week, since the ECMWF and GFS are handling an upper-level trough and associated instability differently. However, trade winds are favored.

Aviation

A ridge will remain near Maui and the Big Island as a front approaches from the NW, leading to light to moderate S-SW winds, especially over Kauai and Oahu. Winds will likely remain light enough over the Big Island to allow land and sea breezes to prevail. An area of consolidating clouds and showers is expected to move over Kauai and Oahu (and potentially portions of Maui County) from the S overnight and early Friday, leading to ISOL MVFR VIS/CIG in SHRA, mainly over leeward areas. When the front stalls near Kauai on Friday, shower coverage may increase to the point that AIRMET Sierra may be needed for mountain obscuration, especially along N and E sections. Until then, no AIRMETs are expected.

Marine

Light to moderate southwesterlies ease tonight as low pressure tracks north of the area. Winds then back to ESE for the weekend when an SCA may be needed for the typical windier zones of Maui County and the Big Island.
The nearshore PacIOOS buoy at Hanalei has trended down considerably during the last 24 hours and is now reporting at 7 feet at 15 seconds, the bulk of which is NW swell energy. A similar trend is noted at the Waimea buoy. Both support allowing the High Surf Advisory (HSA) for N facing shores to expire. The HSA for W facing shores of the Big Island, where small long period swell is still hanging on, has been extended through tonight.
The next swell in the sequence is modeled to be a large NW (300-310) medium period swell that will build Friday night into Saturday boosting surf well into the High Surf Advisory range. Taken at face value, modeling suggests warning-level surf late Friday into Saturday. However, close proximity of the parent low likely leads to a component of shorter period fresh swell emanating out of the same NW quadrant. This in turn spreads energy across the short to medium period bands and leads to higher confidence in surf falling into the High Surf Advisory range for N and W facing shores during this time. An SCA for seas exceeding 10 ft will also likely be needed by Friday night over northern and western waters. Deep low pressure presently situated just southeast of Russia's Kamchatka Peninsula will deliver a long period swell next week with forerunners arriving as early as Sunday night. This large, long period NW (310) swell is modeled to peak Monday night and hold through Tuesday before gradually declining through mid- week.
Surf along east and south facing shores will remain small through the week, though a modest uptick in waves on the south shore is expected to hold into the weekend.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 AM HST Friday for Kona and Kohala.

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