West Kaua’i

Tonight: Partly cloudy with isolated showers. Lows 70 to 75 near the shore to around 63 above 3000 feet. Northeast winds up to 10 mph. Chance of rain 20 percent.

Saturday: Partly sunny with scattered showers. Locally heavy rainfall possible in the afternoon. Highs 85 to 91 near the shore to around 77 above 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Saturday Night: Partly cloudy with isolated showers. Lows 70 to 75 near the shore to around 63 above 3000 feet. East winds up to 10 mph. Chance of rain 20 percent.

South Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 71 to 76. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Saturday: Partly sunny with scattered showers. Locally heavy rainfall possible in the afternoon. Highs 80 to 89. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Saturday Night: Mostly cloudy with scattered showers. Lows 71 to 76. East winds 10 to 15 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Mostly cloudy with numerous showers. Lows 66 to 71 in the valleys to around 60 above 4000 feet. East winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 70 percent.

Saturday: Mostly cloudy. Frequent showers in the morning, then numerous showers in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 75 to 84 in the valleys to around 67 above 4000 feet. East winds 10 to 15 mph. Chance of rain 80 percent.

Saturday Night: Mostly cloudy. Frequent showers in the evening, then numerous showers after midnight. Locally heavy rainfall possible. Lows 65 to 71 in the valleys to around 60 above 4000 feet. East winds up to 15 mph. Chance of rain 90 percent.

North Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 66 to 74. East winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain 70 percent.

Saturday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 77 to 87. East winds up to 15 mph. Chance of rain 70 percent.

Saturday Night: Mostly cloudy with numerous showers. Locally heavy rainfall possible. Lows 66 to 75. East winds 10 to 15 mph. Chance of rain 70 percent.

East Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 65 to 77. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Saturday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 73 to 86. East winds around 10 mph. Chance of rain 70 percent.

Saturday Night: Mostly cloudy with numerous showers. Locally heavy rainfall possible. Lows 65 to 77. East winds 10 to 15 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

Rain chances increase over the weekend in response to increasing moisture and instability across the island chain, with a focus over the western end of the state and the leeward slopes of the Big Island. Heading into early next week, a more typical stable trade wind pattern will return with limited rainfall favoring windward and mauka areas.

Discussion

The short term forecast focuses on the increasing instability and moisture across the state through the weekend. Current satellite and radar imagery show clouds and a few showers moving into windward and mauka areas on the trades, along with clouds and showers building over leeward Haleakala and the leeward Big Island slopes. Some of these leeward showers have been pretty hefty, with radar estimated rainfall rates exceeding an inch per hour at times.
Upper air soundings show that the low level temperature inversion eroded overnight at Hilo, hinting at the increase in low to mid level instability accompanying the 700 mb trough that is progressing northwestward across the eastern end of the state this afternoon. The low level inversion at Lihue has also lifted and weakened on this afternoon's sounding, likely due to the approach of the 700 mb trough from the southeast and a weak mid level low edging in from the north. In addition to this instability, CIMSS MIMIC Layer Precipitable Water satellite imagery shows a large area of precipitable water values between 1.4 to 1.6 inches moving across the island chain from the east this afternoon. Model guidance indicates that this large plume of above average moisture will stick around through the weekend, with a concentration near the western end of the state. With the instability provided by both the 700 mb trough and the proximity of the weak mid level low and the presence of abundant moisture, a wetter trade wind pattern is expected through the weekend, with clouds and enhanced showers favoring windward and mauka areas across the western end of the state, and leeward slopes of the Big Island and Haleakala each afternoon. Isolated thunderstorms near Kauai cannot be completely ruled out as the instability increases with the mid level low through the weekend.
Early next week, mid level ridging will build over the state and a moderate to locally breezy stable tradewind pattern will return. Then global model guidance suggests a weak mid level shortwave trough will sweep by the western end of the state on Wednesday, potentially helping to briefly enhance trade wind showers across the western end of the state.
Model solutions begin to diverge towards the end of the forecast period, but current guidance shows a low level trough/low approaching from the east that may disrupt the trade winds, causing local winds to weaken and shift out of the east-northeast for the latter part of the week.

Aviation

High pressure far to the north will drive moderate to locally fresh trade winds across the state. AIRMET Tango is in effect for tempo moderate turbulence below 070 across all leeward areas. This AIRMET may be able to be dropped Saturday morning as winds weaken slightly.
VFR prevails statewide late this afternoon. An area of cloud and showers is forecast to move in from the east later tonight and will likely bring periods of MVFR conditions along windward slopes. Will monitor for AIRMET Sierra conditions, but should expect isolated mountain obscurations.

Marine

High pressure will remain nearly stationary far northeast of the state through the middle of next week. Fresh to locally strong trades will persist through tonight then drop slightly as a weak low-level trough moves through the state from the east through the weekend. The Small Craft Advisory (SCA) for the windier waters and channels around Maui County and the Big Island has been extended through tonight. The SCA can likely be dropped by Saturday morning as trade winds continue to gradually decline to gentle to locally fresh through the forecast period.
A mixed southerly swell will provide average surf along south facing shores through early Saturday. A new small, long-period southwest swell will fill in slowly Saturday, peak over the weekend, then decline slowly through early next week.
Surf along east facing shores will continue to remain choppy due to the fresh to locally strong trade winds before declining slightly over the weekend. Surf along north facing shores will remain nearly flat through the forecast period.
Peak high tides associated with the lunar cycle could lead to minor coastal flooding along shorelines and low-lying coastal areas next week Monday through Wednesday during the daily peak tide each afternoon.

Fire weather

Fire weather concerns will be low this weekend as a result of a wetter pattern. Trade winds will remain moderate to locally breezy the next couple of days with a higher frequency of showers. Drier weather is expected to return early next week. This may increase the fire weather threat over those leeward areas that may not receive the full benefit of significant weekend rain.

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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