Hilo

Tonight: Mostly cloudy with numerous showers. Lows 68 to 73 near the shore to around 54 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 70 percent.

Sunday: Partly sunny with scattered showers. Highs around 84 near the shore to 68 to 73 at 4000 feet. East winds up to 10 mph. Chance of rain 50 percent.

Sunday Night: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 68 to 73 near the shore to around 54 at 4000 feet. West winds up to 10 mph. Chance of rain 70 percent.

Kona

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 70 to 76 near the shore to around 55 near 5000 feet. Light winds. Chance of rain 50 percent.

Sunday: Mostly sunny with isolated showers in the morning, then mostly cloudy with scattered showers in the afternoon. Highs 82 to 87 near the shore to around 68 near 5000 feet. Southwest winds up to 10 mph. Chance of rain 50 percent.

Sunday Night: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 70 to 76 near the shore to around 55 near 5000 feet. Light winds. Chance of rain 50 percent.

Waimea

Tonight: Breezy. Mostly cloudy with scattered showers. Lows around 70 near the shore to 57 to 62 near 3000 feet. East winds 10 to 20 mph decreasing to up to 20 mph after midnight. Chance of rain 50 percent.

Sunday: Breezy. Partly sunny with scattered showers. Highs around 82 near the shore to 69 to 79 near 3000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Sunday Night: Mostly cloudy with scattered showers. Lows around 70 near the shore to 57 to 62 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Kohala

Tonight: Breezy. Mostly cloudy with scattered showers. Lows around 70 near the shore to 57 to 62 near 3000 feet. East winds 10 to 20 mph decreasing to up to 20 mph after midnight. Chance of rain 50 percent.

Sunday: Breezy. Partly sunny with scattered showers. Highs around 82 near the shore to 69 to 79 near 3000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Sunday Night: Mostly cloudy with scattered showers. Lows around 70 near the shore to 57 to 62 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

South Big Island

Tonight: Partly cloudy. Isolated showers in the evening. Lows around 71 near the shore to around 53 near 5000 feet. Northeast winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 20 percent.

Sunday: Sunny and breezy. Scattered showers in the afternoon. Highs around 85 near the shore to around 72 near 5000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Sunday Night: Partly cloudy. Breezy. Isolated showers in the evening. Lows around 71 near the shore to around 53 near 5000 feet. Northeast winds up to 20 mph. Chance of rain 20 percent.

Puna

Tonight: Mostly cloudy with numerous showers. Lows 68 to 73 near the shore to around 54 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 70 percent.

Sunday: Partly sunny with scattered showers. Highs around 84 near the shore to 68 to 73 at 4000 feet. East winds up to 10 mph. Chance of rain 50 percent.

Sunday Night: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 68 to 73 near the shore to around 54 at 4000 feet. West winds up to 10 mph. Chance of rain 70 percent.

Waikoloa

Tonight: Partly cloudy with isolated showers. Lows 71 to 76 near the shore to 51 to 58 above 4000 feet. Southeast winds up to 15 mph. Chance of rain 20 percent.

Sunday: Sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 86 near the shore to 69 to 75 above 4000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Sunday Night: Partly cloudy with isolated showers. Lows 71 to 76 near the shore to 51 to 58 above 4000 feet. Southeast winds up to 10 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Moderate to locally breezy trade winds will ease for Sunday and Monday as the surface ridge to the north briefly weakens. Rainfall will favor windward and mauka locations, but a few showers, some heavy, may develop across leeward areas as a disturbance aloft moves overhead. Breezy trades and more stable conditions are expected to return on Tuesday.

Discussion

A moderate to locally breezy trade wind flow will begin to ease and destabilize tonight. A surface ridge sitting nearly 1,000 miles to the north is driving the trades, which have stubbornly remained breezy today. The winds should begin to gradually fall off this evening as the ridge starts to weaken as drift southward. An upper-level low centered a couple hundred miles north of Kauai has so far had minimal effect on local weather aside from some thin high clouds, but as the low aloft digs southward tonight, the inversion will lift and weaken. As a result, mainly windward showers should become a bit heavier than the past few days, and a narrow and diffuse band of moisture in the trades will likely bring higher shower coverage from Maui to Oahu overnight.
Trade winds will drop a notch, and showers will likely be heavier Sunday and Monday. The surface ridge to the north will erode, causing trades to decline to moderate strength. Meanwhile, the upper-level low will stall over the western end of the island chain, causing 500 mb temps to drop to around -9C, steepening the lapse rate aloft, and significantly weakening the low-level inversion. The GFS and ECMWF show a lack of organized low-level moisture in the trade wind flow, which suggests that a general increase in shower activity is unlikely. However, showers that do develop could be locally heavy.
A stronger and more stable trade wind flow is expected by midweek. The locally breezy trades will be driven by a tighter pressure gradient between the strengthening low-level ridge to the north and a pair of tropical disturbances passing several hundred miles south of the islands. The upper-level low fill and drift away to the west, and a ridge aloft building in from the east will bring stability. The GFS and ECMWF differ on timing but show a pocket of moisture in the building trades passing over the islands late Monday and Tuesday. This will likely be too late for enhanced rainfall and is expected to produce typical windward showers. Through the rest of the work week, we will monitor areas of tropical moisture that could clip the Big Island, but expect typical windward rainfall for most islands.

Aviation

Moderate to locally breezy trades will weaken slightly heading into tomorrow and isolated to scattered showers will favor windward and mountain areas. With temperature inversion heights expected to remain elevated and weaken heading into Sunday as an upper level low approaches from the north, expect brief moderate to heavy downpours in showers with some spillover into leeward areas. MVFR conditions can be expected in heavier showers with isolated IFR conditions possible. VFR should prevail outside of shower activity.
AIRMET Tango is in effect for tempo moderate turbulence between 12,000 and 40,000 feet this afternoon as the upper level low approaches from the north. This AIRMET will likely be needed through Sunday morning.
AIRMET Sierra may be needed for mountain obscuration for windward locations during the overnight hours through this weekend, depending on if the shower coverage becomes extensive enough to warrant it.

Marine

Fresh trade winds will ease slightly tonight and Sunday as a ridge to the north of the islands weakens. Winds have already dropped below Small Craft Advisory (SCA) thresholds and are expected to remain below these levels through Monday. The trades should strengthen back to moderate and locally strong levels Monday night and hold steady in this range through the remainder of the upcoming work week.
Surf along north and west facing shores will remain tiny through Monday. A moderate, long-period northwest swell will arrive on Tuesday and push surf along north and west facing shores well above seasonal average, though below High Surf Advisory levels, Tuesday night and Wednesday. This swell will decline as it shifts out of the north-northwest on Thursday, then fade on Friday.
Surf along south facing shores will remain just below seasonal average over the next couple of days as an inconsistent southwest swell moves through. A reinforcing southwest swell is expected late Monday into Wednesday. With trade winds on a decline, east shore surf will be well below seasonal average during the next few days. An increase in trade winds will produce larger east shore surf near seasonal average by Tuesday night or Wednesday.

HFO Watches/Warnings/Advisories

None.

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Data Courtesy of NOAA.gov