Hilo

Today: Mostly cloudy with scattered showers. Highs around 78 near the shore to 61 to 66 at 4000 feet. South winds up to 15 mph shifting to the southeast in the afternoon. Gusts up to 40 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 59 to 66 near the shore to around 52 at 4000 feet. Southeast winds around 10 mph. Chance of rain 70 percent.

Friday: Numerous showers in the morning, then occasional showers in the afternoon. Highs 75 to 80 near the shore to around 63 at 4000 feet. Southeast winds up to 10 mph. Chance of rain near 100 percent.

Kona

Today: Mostly sunny. Isolated showers in the afternoon. Highs around 83 near the shore to around 66 near 5000 feet. Northwest winds up to 10 mph. Chance of rain 20 percent.

Tonight: Mostly clear. Isolated showers in the evening. Lows around 67 near the shore to 40 to 48 near 5000 feet. Northwest winds up to 10 mph shifting to the east after midnight. Chance of rain 20 percent.

Friday: Mostly sunny in the morning, then partly sunny with isolated showers in the afternoon. Highs around 83 near the shore to around 66 near 5000 feet. North winds up to 10 mph shifting to the northwest in the afternoon. Chance of rain 20 percent.

Waimea

Today: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 76 near the shore to 66 to 74 near 3000 feet. Southwest winds up to 15 mph shifting to the east in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 59 to 68 near the shore to 50 to 57 near 3000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Friday: Mostly sunny in the morning then becoming partly sunny. Scattered showers. Highs around 77 near the shore to 66 to 73 near 3000 feet. East winds around 10 mph becoming up to 15 mph in the afternoon. Chance of rain 50 percent.

Kohala

Today: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 76 near the shore to 66 to 74 near 3000 feet. Southwest winds up to 15 mph shifting to the east in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 59 to 68 near the shore to 50 to 57 near 3000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Friday: Mostly sunny in the morning then becoming partly sunny. Scattered showers. Highs around 77 near the shore to 66 to 73 near 3000 feet. East winds around 10 mph becoming up to 15 mph in the afternoon. Chance of rain 50 percent.

South Big Island

Today: Windy. Mostly sunny with isolated showers. Highs around 81 near the shore to around 65 near 5000 feet. East winds 10 to 30 mph decreasing to up to 30 mph in the afternoon. Chance of rain 20 percent.

Tonight: Mostly clear. Breezy. Isolated showers in the evening. Lows around 69 near the shore to around 46 near 5000 feet. East winds 10 to 20 mph. Chance of rain 20 percent.

Friday: Breezy. Sunny in the morning, then partly sunny with isolated showers in the afternoon. Highs around 81 near the shore to around 65 near 5000 feet. East winds 10 to 20 mph. Chance of rain 20 percent.

Puna

Today: Mostly cloudy with scattered showers. Highs around 78 near the shore to 61 to 66 at 4000 feet. South winds up to 15 mph shifting to the southeast in the afternoon. Gusts up to 40 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 59 to 66 near the shore to around 52 at 4000 feet. Southeast winds around 10 mph. Chance of rain 70 percent.

Friday: Numerous showers in the morning, then occasional showers in the afternoon. Highs 75 to 80 near the shore to around 63 at 4000 feet. Southeast winds up to 10 mph. Chance of rain near 100 percent.

Waikoloa

Today: Sunny. Highs around 82 near the shore to 62 to 68 above 4000 feet. Light winds.

Tonight: Mostly clear. Lows 66 to 71 near the shore to 44 to 49 above 4000 feet. Light winds becoming southeast around 10 mph after midnight.

Friday: Mostly sunny in the morning, then partly sunny with isolated showers in the afternoon. Highs around 82 near the shore to 62 to 68 above 4000 feet. Light winds becoming northwest up to 10 mph in the afternoon. Chance of rain 20 percent.

Detailed Forecast

Synopsis

A mostly dry and stable airmass will linger over the islands today, but give way to a wetter pattern by tomorrow. Moderate to locally breezy east to southeast winds will prevail over the eastern islands, while lighter east- southeast winds are expected over the western end of the state. Shower coverage and intensity will gradually increase tomorrow and Saturday, with additional showers expanding area wide Sunday night into early next week as a cold front moves through the islands chain.

Discussion

Have made changes to the forecast beginning with tomorrow. The forecast has been nudged heavily to the latest NBM solution, which as expected aligns well with the global models depiction of the weather over the islands. The forecasted PoPs for Thursday and Friday have been increased for leeward areas over the western end of the island, and windward Big Island PoPs were lowered some Saturday, with a drying trend Saturday night. The arrival of the frontal moisture has been delayed a bit Sunday, with moisture spreading across most areas Sunday night through Monday night. Tuesday is expected to see a west to east drying pattern over the islands.
High pressure remains far to the northeast of the islands, with a ridge extending from the high to the west-southwest. The ridge remains north of Kauai early this morning. This pattern will maintain moderate to locally breezy east to southeast winds over the eastern half of the state, and light to gentle east to southeast winds over the western half. The lighter winds will allow for local land and sea breezes to form.
An upper level trough will move down the island chain Friday and Saturday increase instability over the islands. This will result in an uptick in showers, particularly for exposed windward areas, but also some leeward areas particularly on Kauai and Oahu. The ECMWF, GFS and as a result the NBM, have lowered the small chance for thunderstorms with this upper trough passage. The upper level trough will also bring colder temperatures aloft, and if those cold temperatures coincide with moisture reaching the Big Island summits, snow is a possibility. At this time the greatest chance for that would be Friday night and early Saturday morning when the coldest temperatures reach the Big Island. Again have not added this to the forecast quite yet, but will continue to monitor.
Unsettled weather is expected to continue into early next week as a new cold front approaches from the northwest. The latest run of the ECMWF shows the front slower than previous runs, and more in line with the GFS solution. This would have the front slowing as it approaches Kauai Sunday night, and then slowing its progress down the island chain, lingering over the western islands. Models agree with southerly winds over the islands ahead of the front, along with moisture being drawn northward over the islands as a result. The forecast remains heavily weighted towards the NBM resolution as medium ground. The models show a weak mid-level trough moving through the islands along with the front. These models are showing an increase in thunderstorm potential with the mid-level trough swinging through. Confidence remains low this far out, so no thunderstorms have been added at this time. The summit level temperatures will be several degrees warmer than with the upper level trough moving through Friday/Saturday, lowering the chance for snow for the Big Island summits, but it remains a non- zero chance.

Aviation

Breezy east to southeast winds will continue to impact the eastern half of the state today with high pressure to the northeast and a low pressure system to the northwest. Tempo moderate lee turbulence will remain a concern over Maui and the Big Island today so AIRMET Tango remains posted for areas downwind of the terrain. Low level wind shear will also impact portions of the eastern islands this afternoon as diurnal winds increase. In addition, as an upper level trough develops west of the state later today expect areas of moderate to occasionally severe turbulence between FL220 and FL350.
A relatively dry airmass will continue to limit shower activity, with only isolated to scattered light showers favoring windward and southeast slopes. Expect mainly VFR conditions, with brief periods of MVFR under passing showers.

Marine

Moderate to strong east to east-southeast winds will prevail over the eastern waters, while light to moderate east-southeast winds hold over the western waters through Saturday. Winds will weaken and shift southeasterly on Sunday, then pick up as they swing around to the south and southwest at moderate to locally strong speeds early next week as a new cold front approaches from the northwest. A Small Craft Advisory (SCA) is now in effect for most of the eastern waters through 6 PM Friday. This will likely need to be extended through Saturday. Also, a large northwest swell in combination with trade wind swell is expected to build seas to the 10 foot SCA threshold over most of the western waters beginning Thursday morning, and as a result a SCA is now in effect for these waters from 6 AM Thursday through 6 PM Friday.
A moderate to large sized long-period northwest swell will build through the morning, then peak at advisory levels along most north and west facing shores of the smaller islands tonight through Friday. Offshore NDBC Buoy 51101 has continued to climb overnight, so a steady rise through the day is expected. As a result, a High Surf Advisory is now in effect from 6 AM today through 6 PM Friday for north and west facing shores of Niihau, Kauai, Oahu, Molokai and north facing shores of Maui. This swell will gradually decline Friday night through the weekend. A new long period northwest swell similar in size as the current swell will build Monday, peak Monday night and Tuesday, then gradually decline through the middle of next week.
Surf along east facing shores will steadily climb through the week due to a combination of the southeasterly winds locally and the upstream trade wind belt expanding with fresh to strong breezes. By early next week, east facing shores look to gradually decline as the upstream trade winds diminish. Surf along south facing shores will remain small through the weekend, with only background southerly swells. South shore surf could get rough and choppy Monday through the middle of next week as south to southwest winds pick up as a cold front approaches and moves through the islands.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST Friday for Niihau, Kauai Leeward, Waianae Coast, Oahu North Shore, Maui Windward West, Kauai North, Molokai Windward, Molokai North, Molokai West, Maui Central Valley North, Windward Haleakala.
Small Craft Advisory until 6 PM HST Friday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel.
Small Craft Advisory until 6 PM HST Friday for Maui County Windward Waters, Maui County Leeward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

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Data Courtesy of NOAA.gov