West Kaua’i

Tonight: Mostly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 68 to 73 near the shore to around 61 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Friday: Mostly sunny with isolated showers. Highs 83 to 90 near the shore to around 76 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

Friday Night: Partly cloudy. Lows around 69 near the shore to around 60 above 3000 feet. Northeast winds up to 10 mph.

South Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows around 72. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Friday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 81 to 89. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Friday Night: Partly cloudy with isolated showers. Lows around 71. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Kaua’i Mountains

Tonight: Scattered showers in the evening, then occasional showers after midnight. Lows 63 to 69 in the valleys to around 58 above 4000 feet. East winds up to 15 mph. Chance of rain near 100 percent.

Friday: Mostly cloudy with showers likely in the morning, then mostly sunny with scattered showers in the afternoon. Highs 75 to 82 in the valleys to around 67 above 4000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Friday Night: Partly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 62 to 68 in the valleys to around 57 above 4000 feet. Northeast winds around 10 mph. Chance of rain 40 percent.

North Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 64 to 72. East winds up to 15 mph shifting to the southeast after midnight. Chance of rain 70 percent.

Friday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 77 to 85. East winds up to 15 mph. Chance of rain 50 percent.

Friday Night: Partly cloudy with isolated showers. Lows 64 to 71. East winds around 10 mph. Chance of rain 20 percent.

East Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 64 to 75. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Friday: Mostly cloudy with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 74 to 85. East winds 10 to 15 mph. Chance of rain 50 percent.

Friday Night: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows 62 to 74. Northeast winds around 10 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

High pressure far to the northeast of the islands will weaken over the next few days. Breezy trades will become gentle to locally breezy as we head into the weekend. A trough developing west of the islands early next week should keep winds gentle to locally breezy, but the flow will turn toward the east-southeast. Fairly typical night and morning trade wind showers are expected. A few more interior afternoon showers are possible early next week as well.

Discussion

A relatively strong and large surface high, about 1034 mb, is nearly stationary about 1600 mi to the NE PHNL, driving breezy trade winds over our islands again today. The high is a few mb weaker than yesterday and thus our trades are just a notch slower. A band of showers on the trades moved through late last night and this morning and has cleared the islands, leaving us a somewhat drier airmass with fewer showers upstream. Both 00z soundings showed an inversion near 7800 feet, and PW just slightly above normal.
Mainly subtle summertime changes expected through the forecast period over the main Hawaiian Islands, and models are in good agreement. The high far to the NE will weaken as a wavy front stalls far to the N of the islands. This will weaken the local pressure gradient a bit, so trades will drop to background gentle speeds, only locally breezy in the typical windier areas by Fri. Trades will then continue at those speeds through the weekend. Starting about Mon, a surface trough will develop out near 170w near Lisianski and Laysan Islands, and this will turn our local background flow over the main Hawaiian Islands to the ESE. It will still be locally breezy, but many areas will be shadowed from the background winds by island terrain. The surface trough will move away to the NW by the middle of next week, and a new large surface high will build far to the NE of the islands once again, returning us to breezy trades by Thu.
As far as the sensible wx goes, a fairly typical pattern of summertime trade wind showers is expected for the next few days as modest mid-level troughing develops over the islands, and moisture levels remains fairly close to climatology for June. Light to moderate showers will favor windward and mauka sections during the nights and mornings. For Mon and Tue, we look to get into a hybrid pattern of windward and mauka showers favoring nights and mornings. There will also be a small chance for a couple of afternoon interior or leeward showers as well, as low level moisture ticks up a bit to slightly above climo. Depending on how much the background flow veers, it's possible portions of the smaller islands, like the south shore of Oahu, could even see a few showers developing in the convergence downwind of Maui County in the ESE flow, but much too early to include this explicitly in the forecast right now. A more typical trade wind shower distribution returns on Wed as low level moisture decreases slightly once again.

Aviation

Moderate to locally breezy trades will persist through the evening, then gradually ease into Friday as the surface ridge to the far northeast of the state weakens. Batches of moisture riding in on the trades will deliver brief showers to windward and mauka areas through the period. MVFR conditions will be possible in showers. Otherwise, VFR conditions will prevail.
AIRMET Tango is now in effect for moderate turbulence between 15,000 feet and 36,000 feet due to a jet streak on the eastern side of the base of an upper level trough moving overhead. This AIRMET will likely be needed overnight and into tomorrow morning.
AIRMET Tango remains in effect for low level moderate turbulence over and immediately south through west of island mountains due to moderate to breezy trade winds. This AIRMET will likely be cancelled later this afternoon or overnight as the trade wind speeds decrease.

Marine

Moderate to locally strong trades will gradually ease tonight, becoming gentle to fresh by Friday in response to weakening high pressure northeast of the state. Gentle to fresh trades prevail Friday into early next week then restrengthen as the seasonal high redevelops. Additionally, winds could veer slightly east- southeast early next week in response to a potential surface trough developing west of the islands. Small Craft Advisory is in effect through 6 am Friday as locally strong winds hold through the afternoon and into the evening hours.
Tiny to small, long period southwest swell will hold this afternoon maintaining small surf along favored exposures. Forerunners from a moderate, long period south southwest swell originating from an extratropical cyclone east of New Zealand will gradually fill in tonight into Friday. This swell will peak over the weekend near or above the High Surf Advisory threshold before slowly fading early next week.
Choppy surf along east facing shores will hold into the weekend even though locally induced tradewind swell will be on the decline. This is thanks to a low that developed near the California coast earlier this week which is sending a moderate, medium period northeast swell later today and should peak Friday before fading Saturday. Surf along north facing shores will remain small however could see some minor northeast wrap tonight into Friday before returning to near flat Friday night through the weekend.

HFO Watches/Warnings/Advisories

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

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