West Kaua’i

Tonight: Mostly clear. Lows around 70 near the shore to around 59 above 3000 feet. Northeast winds up to 15 mph.

Saturday: Sunny. Isolated showers in the afternoon. Highs 83 to 88 near the shore to around 75 above 3000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Saturday Night: Mostly clear with scattered showers. Lows 68 to 73 near the shore to around 59 above 3000 feet. East winds up to 15 mph becoming around 10 mph after midnight. Chance of rain 50 percent.

South Kaua’i

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Isolated showers. Lows around 71. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Saturday: Mostly sunny with isolated showers. Highs 79 to 87. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Saturday Night: Partly cloudy with scattered showers. Lows around 71. Northeast winds 10 to 15 mph. Chance of rain 40 percent.

Kaua’i Mountains

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Isolated showers. Lows 62 to 68 in the valleys to around 57 above 4000 feet. East winds up to 10 mph. Chance of rain 20 percent.

Saturday: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs 72 to 81 in the valleys to around 65 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Saturday Night: Breezy. Partly cloudy with numerous showers. Lows 61 to 68 in the valleys to around 58 above 4000 feet. East winds up to 15 mph increasing to 10 to 20 mph after midnight. Chance of rain 70 percent.

North Kaua’i

Tonight: Partly cloudy with isolated showers. Lows 63 to 72. Southeast winds up to 10 mph. Chance of rain 20 percent.

Saturday: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs 75 to 85. East winds up to 10 mph. Chance of rain 40 percent.

Saturday Night: Partly cloudy with scattered showers. Lows 63 to 72. East winds up to 10 mph. Chance of rain 50 percent.

East Kaua’i

Tonight: Mostly cloudy with isolated showers. Lows 62 to 75. Northeast winds up to 15 mph. Chance of rain 20 percent.

Saturday: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs 72 to 84. Northeast winds up to 10 mph shifting to the east in the afternoon. Chance of rain 50 percent.

Saturday Night: Partly cloudy with scattered showers. Lows 63 to 75. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Locally breezy trades continue into the weekend during which time drier weather is anticipated. There is a chance for a brief period of wetter weather during the middle of next week.

Discussion

A drier pattern has already established over the western end of the island chain where partly cloudy skies and limited shower activity prevail. Visible imagery suggests a narrow ribbon of even drier air lurking immediately upstream as evidenced by nearly cloud free skies east of the islands. As this drier airmass advances into the area, it will cause steady showers over Windward Big Island & Maui to gradually diminish as the night progresses. The smaller islands will likewise remain mostly dry through the night. Locally breezy trades and a fairly typical trade wind pattern then prevail through at least Monday.
Global guidance in good agreement this afternoon that a rather warm closed low will develop in situ over the islands during the early portion of next week. Attendant adjustment to the low-level wind field will cause emerging SE winds in the mid-levels to advect deep tropical moisture over the area with PWATs modeled to be in excess of 1.5″ as early as Monday while ESE trades prevail at the surface. PWATs eventually modeled to reach or exceed 2″ areawide by late Tuesday or Wednesday. Forecast soundings and model cross sections indicate rather dry and stable low-levels inititally which suggests potential for some higher based light/stratiform rain operating independently of trade wind showers below during the initial moisture surge. However, as moisture deepens during mid-week, the potential will exist for any moisture embedded in the trades to capitalize on existing convective instability and also tap into ample mid/upper level moisture suggesting a brief window for moderate to heavy, but still progressive, trade wind showers. The best opportunity for such shower activity likely comes as the aforementioned closed low deamplifies and lifts NE out of the area bringing with it a brief hit of dynamic forcing Tues into Wed. Worth noting that the GFS has demonstrated a tendency to spin up a spurious low out of grid scale convection SE of the Big Island. Subsequent output of heavy indicating associated heavy rain and strong wind field are likely unreliable for now. Potential for flooding in this scenario appears limited at this time but bears monitoring in subsequent forecast cycles.

Aviation

Breezy trade winds will begin to gradually ease tonight and into tomorrow. Showers are continuing to impact windward and mountainous areas, especially over the Big Island. MVFR conditions prevail over windward sections of the Big Island, with brief isolated IFR conditions possible through tonight. Occasional MVFR conditions are possible over windward and mountainous areas elsewhere across the state. Otherwise, VFR will prevail.
AIRMET Sierra remains in effect for windward Big Island. This will likely continue through tonight due to upstream clouds and showers.
AIRMET Tango remains in effect for moderate low-level turbulence over and downwind of island terrain below 10,000 feet. This is expected to remain at least through the evening, and will likely be adjusted tomorrow due to winds gradually easing across the state.

Marine

Strong high pressure to the NE will weaken and move SE over the weekend. Moderate to strong easterly trade winds will decrease to locally strong this weekend as the high weakens. A Small Craft Advisory (SCA) is in effect through this afternoon for both gusty winds and high seas. Western marine zones will fall off tonight with zones from the Kaiwi Channel eastward remaining in the SCA overnight Saturday morning. A SCA will be in effect for the traditionally windy waters surrounding Maui County and Big Island until 6 AM Sunday morning.
The forecast becomes uncertain next week as some guidance indicates a mid level trough moving over the islands from the southeast. This mid level trough, if actually occurs as depicted, could create a surface trough by Monday night, disrupt the trade winds, and allow for thunderstorms to develop across the local waters. Other model guidance suggests a weaker mid level trough with trade winds continuing through the first half of next week. Additionally, a cold front could approach the NW offshore waters by Tuesday evening.
Large short period northeast swell generated by a strong trade fetch will be on the gradual decline through the weekend as a result of weakening local and upstream trades. Large surf for east shores is possible by the middle of next week depending on the evolution of a potential surface trough and approaching cold front.
A small, medium period northwest swell will fill in late Saturday afternoon and provide a small boost to north and west-facing shores through the weekend. Small medium to long period west- northwest swells will then keep surf from going flat early next week. A moderate long period west northwest swell is possible next week and could bring advisory level surf to north and west shores by the middle of next week.
For south shores, small background southerly swell will remain through the weekend and into early next week.

Fire weather

The inversion slopes from around 7kft at Lihue to 10kft at Hilo per this afternoon's upper air observations. The inversion will fall to around 5 or 6kft by Saturday as drier air infiltrates the area and the resident band of showers over Windward Big Island sags south and diminishes. Seasonably cool temperatures will maintain elevated RH values even as dewpoints tumble a bit heading into Saturday. The potential exists for a brief period of wetter weather during the middle of next week.

HFO Watches/Warnings/Advisories

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

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