Hilo

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 57 to 67 near the shore to 48 to 55 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 50 percent.

Friday: Occasional showers in the morning, then numerous showers in the afternoon. Highs 74 to 79 near the shore to 59 to 64 at 4000 feet. Northeast winds up to 15 mph. Chance of rain near 100 percent.

Friday Night: Mostly cloudy with numerous showers. Lows 57 to 67 near the shore to 48 to 54 at 4000 feet. West winds up to 10 mph. Chance of rain 70 percent.

Kona

Tonight: Partly cloudy. Isolated showers in the evening. Lows around 69 near the shore to 44 to 49 near 5000 feet. Light winds. Chance of rain 20 percent.

Friday: Mostly sunny with isolated showers in the morning, then mostly cloudy with scattered showers in the afternoon. Highs 80 to 85 near the shore to around 64 near 5000 feet. Light winds. Chance of rain 50 percent.

Friday Night: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows around 69 near the shore to 45 to 51 near 5000 feet. Light winds. Chance of rain 50 percent.

Waimea

Tonight: Breezy. Partly cloudy with isolated showers. Lows 58 to 68 near the shore to 50 to 58 near 3000 feet. East winds 10 to 20 mph. Chance of rain 20 percent.

Friday: Partly sunny. Breezy. Numerous showers in the morning, then scattered showers in the afternoon. Highs around 75 near the shore to 64 to 73 near 3000 feet. East winds 10 to 25 mph. Chance of rain 70 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 58 to 68 near the shore to 50 to 59 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Kohala

Tonight: Breezy. Partly cloudy with isolated showers. Lows 58 to 68 near the shore to 50 to 58 near 3000 feet. East winds 10 to 20 mph. Chance of rain 20 percent.

Friday: Partly sunny. Breezy. Numerous showers in the morning, then scattered showers in the afternoon. Highs around 75 near the shore to 64 to 73 near 3000 feet. East winds 10 to 25 mph. Chance of rain 70 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 58 to 68 near the shore to 50 to 59 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

South Big Island

Tonight: Mostly clear. Lows around 71 near the shore to around 51 near 5000 feet. Northeast winds up to 15 mph.

Friday: Mostly sunny. Breezy. Scattered showers in the afternoon. Highs around 83 near the shore to around 64 near 5000 feet. East winds up to 25 mph. Chance of rain 50 percent.

Friday Night: Partly cloudy. Breezy. Scattered showers in the evening, then isolated showers after midnight. Lows around 71 near the shore to around 51 near 5000 feet. Northeast winds up to 20 mph. Chance of rain 50 percent.

Puna

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 57 to 67 near the shore to 48 to 55 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 50 percent.

Friday: Occasional showers in the morning, then numerous showers in the afternoon. Highs 74 to 79 near the shore to 59 to 64 at 4000 feet. Northeast winds up to 15 mph. Chance of rain near 100 percent.

Friday Night: Mostly cloudy with numerous showers. Lows 57 to 67 near the shore to 48 to 54 at 4000 feet. West winds up to 10 mph. Chance of rain 70 percent.

Waikoloa

Tonight: Mostly clear. Lows 67 to 72 near the shore to 46 to 52 above 4000 feet. Southeast winds up to 15 mph.

Friday: Sunny in the morning then becoming partly sunny. Breezy. Isolated showers. Highs 78 to 83 near the shore to 62 to 68 above 4000 feet. East winds up to 20 mph shifting to the northeast in the afternoon. Gusts up to 40 mph. Chance of rain 20 percent.

Friday Night: Partly cloudy. Isolated showers in the evening. Lows around 69 near the shore to 47 to 52 above 4000 feet. Southeast winds up to 15 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Trade winds will become breezy tonight and Friday as a band of moisture brings an increase in rainfall to mainly windward areas. Trade winds and shower activity will diminish on Saturday. An area of low pressure will develop northwest of the islands on Sunday then drift southward, causing winds to shift out of the southeast and bringing cloudy conditions with increasing chances of showers and thunderstorms. Strong and gusty easterly winds are expected gradually reduce shower chances on Monday. Drier locally breezy trade winds are possible by Tuesday.

Discussion

Increasing trade winds will push a band of enhanced moisture across the islands tonight and Friday. For much of the day, the narrow band has been focused on Kauai, where more than a quarter of an inch of rainfall has fallen over windward slopes, and the feature has been starting to move onto Oahu and Maui County this afternoon. A strong surface high centered 1400 miles north of the state is producing breezy trade winds along and north of the moisture band. The high will drift southeastward, pushing showers over windward areas from Kauai to Maui tonight and eventually to Big Island on Friday, where the band will stall and slowly dissipate. Breezy trade winds along and north of the band will periodically carry some showers over leeward areas, but a ridge aloft will maintain stable conditions that will keep rainfall totals modest.
Model guidance begins to diverge on Saturday, but overall, expect easterly trade winds to ease and rainfall to diminish as high clouds increase. The official forecast leans toward the ECMWF, which is hinting at moisture hanging up on windward Big Island and keeping shower chances higher there.
Uncertainty in the details remains high later in the weekend, but probability of unsettled weather with locally heavy showers is increasing, especially over the western end of the island chain. Guidance continues to show an upper level low developing several hundred miles north of Kauai Saturday night and drifting west of Kauai late Sunday. The current forecast is based on the ECMWF and UKMET solutions of a weak surface low or trough developing under the upper low, instead of the GFS, which appears to deepen the feature excessively and produce a strong surface low. Under the favored ECMWF and UKMET solutions, winds will shift out of the southeast Saturday night and focus showers over windward and southeast slopes as high clouds thicken. The low at the surface and aloft is expected to pass to the west of the state Sunday and Sunday night, and broad convergence on its eastern flank will produce higher shower chances Sunday and Sunday night, especially over Kauai and Oahu. Thunderstorms have been introduced around Kauai and on the Big Island interior for this forecast package, and we will continue to assess the flood threat over the next couple of days.
Longer range guidance leans toward decreased rainfall chances and the development of strong and gusty east to southeast winds as the low drifts away to the west on Monday. Drier locally breezy trade winds are possible by Tuesday.

Aviation

A band of moisture moving in overnight from the north could provide enhanced trade wind showers over windward and mauka areas with some spillover into leeward areas across the state. Primarily MVFR conditions are expected within these showers, and VFR will prevail elsewhere. Trade winds will remain moderate to locally breezy tonight, with potential to slightly strengthen tomorrow after the band of moisture passes.
AIRMET Sierra is in effect for mountain obscurations for Kauai and Oahu due to clouds and showers moving over windward and mauka areas. This will likely be adjusted through the night as the moisture band progresses through the island chain.
AIRMET Tango is not currently in effect, but will likely be needed tonight for upper level moderate turbulence. This is associated with the subtropical jet moving closer to the islands from the south.

Marine

High pressure building north of the state will slightly strengthen the trade winds this evening with moderate to locally strong easterly trades expected to persist through Friday night. The Small Craft Advisory has been extended and may need to be expanded to other coastal waters tonight depending on the ASCAT observations. The moderate to locally strong trade winds should decrease this weekend and veer toward the southeast as a trough of low pressure develops north of Kauai and tracks southwestward. As this trough passes just west of the state, strong to near gale force easterly trade winds are expected to impact the Hawaiian Waters by Sunday evening and likely continuing into the early part of next week. Rough conditions with large seas and heavy rain is expected across many coastal water zones early next week.
Current medium period north-northwest swell will continue to decline tonight and shift to a more northerly direction. Another moderate, medium-period north to north-northeast swell will fill in on Friday especially during the afternoon and peak Friday night into Saturday morning.
With a slight increase of trade winds tonight, a subtle increase of rough and choppy surf is expected along east facing shores. A much stronger fetch of strong to near gale force trade winds will bring a significant increase of easterly wind swell Sunday into early next week, which should exceed advisory thresholds for east facing shores. Surf along south facing shores will remain tiny through the forecast period.
Peak astronomical monthly tides with water levels running roughly 0.5 ft above normal should produce minor coastal flooding through this weekend, and may be enhanced along north facing shores Friday and Saturday due to the incoming north swell. A Coastal Flood Statement has been issued to highlight this flooding potential, which should persist through this weekend with peak high tides expected late tonight and late Friday night.
A building north to north-northeast swell on Friday will also produce moderate surges for north facing harbors, especially for Kahului and Hilo. A Marine Weather Statement remains in effect for maritime interests to watch for these impending harbor surges.

HFO Watches/Warnings/Advisories

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

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Data Courtesy of NOAA.gov