West Kaua’i

Tonight: Mostly clear. Scattered showers after midnight. Lows around 74 near the shore to around 64 above 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Saturday: Sunny with isolated showers. Highs 85 to 90 near the shore to around 76 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

Saturday Night: Mostly clear with isolated showers. Lows around 73 near the shore to around 64 above 3000 feet. Northeast winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain 20 percent.

South Kaua’i

Tonight: Breezy. Partly cloudy with isolated showers in the evening, then mostly cloudy with scattered showers after midnight. Lows 70 to 77. Northeast winds 15 to 20 mph. Chance of rain 50 percent.

Saturday: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs 79 to 89. Northeast winds 15 to 20 mph. Chance of rain 50 percent.

Saturday Night: Mostly cloudy with scattered showers. Lows 70 to 76. Northeast winds around 15 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows around 68 in the valleys to around 61 above 4000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Saturday: Breezy. Partly sunny with scattered showers. Highs 75 to 82 in the valleys to around 68 above 4000 feet. Northeast winds 10 to 15 mph becoming up to 20 mph in the afternoon. Chance of rain 50 percent.

Saturday Night: Breezy. Mostly cloudy with scattered showers. Lows around 67 in the valleys to around 61 above 4000 feet. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

North Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows 64 to 75. East winds up to 15 mph. Chance of rain 50 percent.

Saturday: Mostly sunny in the morning then becoming partly sunny. Scattered showers. Highs 71 to 86. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

Saturday Night: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 63 to 75. East winds 10 to 15 mph. Chance of rain 50 percent.

East Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 66 to 76. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Saturday: Partly sunny with scattered showers. Highs 78 to 85. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Saturday Night: Mostly cloudy with scattered showers. Lows 64 to 76. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Moderate to breezy trades will gradually strengthen through early next week, then weaken over the latter half. Brief pulses of enhanced showers will roll through on the trades during this time period, with showers favoring windward and mauka areas in the overnight to early morning hours.

Discussion

Radar and satellite imagery show isolated to scattered showers moving into mainly windward and mauka areas of the state this afternoon, with shower coverage increasing over the eastern half of the state as a plume of deeper moisture moves in on the weakly convergent trade wind flow. This uptick in windward shower activity will work its way up the island chain and persist into the evening hours before drier air fills in on Saturday.
Surface high pressure to the north will build through the weekend and strengthen the trade wind flow to breezy levels through early next week. Additional batches of increased moisture will continue to filter in on the trades over the next several days and bring brief upticks in windward and mauka shower activity, particularly during the overnight to early morning hours. Overall, these showers may produce brief moderate downpours at times, but they will move through rather quickly, so flooding is not expected to be an issue.
Both aloft and at the surface, broad and weak troughing will gradually develop over the Central Pacific by midweek, weakening the pressure gradient and therefore decreasing the trade wind speeds. Otherwise, periodic showers will continue to filter into windward and mauka areas on the weakening trade wind flow.
By next weekend, models are hinting at more substantial instability developing aloft and a larger area of deep tropical moisture being drawn up from the southeast, possibly leading to the potential for widespread heavy showers. However, the window with the greatest potential for any impacts is beyond the forecast period and with the model discrepancies that far out, forecast confidence is on the lower side for that period.

Aviation

Gusty trade winds will continue today into tomorrow. Shower coverage will increase from east to west during the next 24 hours, initially favoring windward portions of the Big Island and Maui, then spreading toward Oahu and Kauai tonight into tomorrow. Brief MVFR ceilings and visibility are expected in passing showers, with VFR conditions prevailing between showers.
AIRMET Sierra may be needed tonight as increasing shower coverage may bring renewed mountain obscuration.
AIRMET Tango remains for moderate low level turbulence downwind of island terrain.

Marine

Surface high pressure far north of the islands will strengthen as it drifts eastward through Saturday. Reinforcing high pressure will build far north of Hawaii on Sunday and will settle far northeast of the islands through early next week. Fresh to strong trade winds will prevail during this time, and the Small Craft Advisory (SCA) for the typically windy waters around Big Island and Maui may need to be expanded to additional waters this weekend. Trades are expected to ease slightly by Wednesday.
A medium-period north-northwest swell will peak overnight, producing north shore surf around seasonal average. This swell has been slow to arrive today, with measurements at NOAA buoy 51001 recently coming in at 5 to 6 feet at 11 to 12 seconds. This swell will slowly decline and shift more northerly during weekend as a smaller overlapping northwest swell moves through the area. Both swells will lower on Monday and fade on Tuesday. Another small northwest swell is possible next Thursday or Friday.
Long period south swell will peak through this evening, producing south shore surf slightly above seasonal average. This swell will gradually decline this weekend, and surf along south- facing shores will below average through the coming work week.
Surf along east-facing shores will gradually increase over the next couple of days as trade winds strengthen over the region. In addition, expect some wrapping north-northwest swell to reach exposed areas through at least Saturday. Rough trade wind swell will produce east shore surf above seasonal average Sunday through Tuesday, followed by a decline Wednesday.

Fire weather

Surface high pressure far north of the islands will produce moderate to breezy trade winds for the next few days, with gradual strengthening expected through early next week. Between batches of showers rolling through on the trades, RH levels may approach critical fire threshold ranges, particularly Saturday afternoon. Otherwise, RH levels should help to maintain low to moderate fire weather concerns.
Inversion heights should increase to around 10,000 feet through tonight as the current batch of deeper moisture rolls through, then drop to 5,000 to 7,000 feet on Saturday.

HFO Watches/Warnings/Advisories

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

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Data Courtesy of NOAA.gov