Hilo

Today: Mostly cloudy. Showers in the morning, then scattered showers in the afternoon. Highs 74 to 80 near the shore to around 63 at 4000 feet. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 80 percent.

Tonight: Mostly cloudy. Scattered showers early in the evening, then showers likely in the late evening and overnight. Lows 61 to 69 near the shore to around 54 at 4000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Saturday: Cloudy. Showers likely in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 75 to 81 near the shore to 62 to 67 at 4000 feet. Northeast winds up to 10 mph shifting to the east in the afternoon. Chance of rain 70 percent.

Kona

Today: Sunny in the morning, then cloudy with scattered showers in the afternoon. Highs 81 to 86 near the shore to around 65 near 5000 feet. West winds up to 10 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Scattered showers early in the evening. Lows 67 to 73 near the shore to 47 to 54 near 5000 feet. Light winds. Chance of rain 50 percent.

Saturday: Partly sunny. Scattered showers and isolated thunderstorms in the afternoon. Highs around 83 near the shore to around 65 near 5000 feet. Light winds becoming west up to 10 mph in the afternoon. Chance of rain 40 percent.

Waimea

Today: Partly sunny. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 62 to 82. East winds up to 20 mph. Chance of rain 60 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows 60 to 65 near the shore to 52 to 60 near 3000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Saturday: Mostly cloudy. Breezy. Scattered showers in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 63 to 83. East winds up to 20 mph. Chance of rain 50 percent.

Kohala

Today: Partly sunny. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 62 to 82. East winds up to 20 mph. Chance of rain 60 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows 60 to 65 near the shore to 52 to 60 near 3000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Saturday: Mostly cloudy. Breezy. Scattered showers in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 63 to 83. East winds up to 20 mph. Chance of rain 50 percent.

South Big Island

Today: Breezy. Sunny in the morning, then cloudy with scattered showers in the afternoon. Highs around 80 near the shore to around 65 near 5000 feet. East winds up to 25 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Breezy. Scattered showers early in the evening, then isolated showers in the late evening and overnight. Lows around 69 near the shore to around 50 near 5000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Saturday: Partly sunny. Breezy. Isolated showers in the morning, then isolated showers and thunderstorms in the afternoon. Highs around 81 near the shore to around 66 near 5000 feet. East winds up to 20 mph. Chance of rain 20 percent.

Puna

Today: Mostly cloudy. Showers in the morning, then scattered showers in the afternoon. Highs 74 to 80 near the shore to around 63 at 4000 feet. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 80 percent.

Tonight: Mostly cloudy. Scattered showers early in the evening, then showers likely in the late evening and overnight. Lows 61 to 69 near the shore to around 54 at 4000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Saturday: Cloudy. Showers likely in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 75 to 81 near the shore to 62 to 67 at 4000 feet. Northeast winds up to 10 mph shifting to the east in the afternoon. Chance of rain 70 percent.

Waikoloa

Today: Sunny with scattered showers in the morning, then partly sunny with isolated showers in the afternoon. Highs 78 to 86 near the shore to 63 to 68 above 4000 feet. North winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Isolated showers early in the evening. Lows 65 to 73 near the shore to 48 to 53 above 4000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Saturday: Partly sunny. Isolated showers and thunderstorms in the afternoon. Highs 79 to 85 near the shore to 63 to 69 above 4000 feet. East winds up to 10 mph shifting to the north in the afternoon. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Locallly strong trades will weaken slightly today, becoming locally breezy by tonight. Showers will tend to focus over windward and mauka zones through the weekend, especially at night, with a low potential for inland showers on Saturday afternoon.

Discussion

Locally strong trades are beginning to weaken and veer to ESEly as high pressure establishes over the northeast Pacific Basin. Winds will further weaken into the locally breezy category overnight with additional slight veering possible into Saturday. Trades will tend to focus showers over windward and mauka zones during this time, particularly overnight and during the early morning. Trades will acquire an increasingly unstable, or at least weakly stable, character during the next 72 hours as an upper jet impinging on the forecast area from the west forces a period of mid-level troughing to develop directly over the islands.
Water vapor imagery indicates cyclonic flow has already begun to take shape over the islands, its effects already manifesting in the form of thunderstorm development directly beneath the upper wave/southwest of the islands, as well as in the modest deepening of trade wind showers affecting windward slopes this morning. A few locally heavy showers are evident on radar and cannot be ruled over windward areas, but strong trades will ensure any heavier showers move quickly. Likewise, diminishing stability and strong trades will allow for effective leeward penetration of showers in favored areas. However, it is already evident that showers over the smaller islands are being steered northwestward, parallel to the island chain, by stout southeasterly flow centered at 850mb. This is producing some degree of shadowing west of Maui, a trend that is likely to continue through the weekend.
As the low deepens overhead Saturday and Sunday, 500mb temperatures falling to a decent-but-not-impressive -10C or so will support 700-500mb lapse rates falling to around -7C/km, mainly over the eastern end of the state. With the addition of daytime heating, interior portions of the Big Island should have no issue generating some afternoon thunderstorms Saturday and Sunday. The primary change to the morning forecast package was therefore to add thunder to the Big Island for both Sat and Sun afternoon. For the smaller islands, expect the next 3 days to be characterized by unstable trades supportive of briefly heavy showers mainly over windward and mauka zones. Shadowing concerns alluded to above only increase during this time as low- level winds also veer to ESE. Absent island interaction/leeward plume development, suspect this period of unstable ESE trades may underachieve for many locations west of the Big Island.
Height rises in the wake of the departing upper trough bring a return to more typical stable, locally breezy trades by early next week.

Aviation

AIRMET Tango for low-level turbulence will remain in place as locally strong and gusty easterly trade winds continue into Saturday. VFR conditions will prevail, but the trade winds will deliver low clouds and showers that will primarily affect windward slopes and coasts with brief periods of MVFR VIS/CIG in SHRA.

Marine

A high pressure system located far north-northeast of the islands will keep fresh to strong east-southeasterly trade winds in the forecast across the region today. A Small Craft Advisory (SCA) was extended in time in effect for most central and eastern coastal waters through early Saturday morning due to strong winds and rough seas. Wind speeds in the windier waters and channels near Maui and the Big Island will remain near gale strength today. Winds will begin to ease this weekend, and SCA conditions will decrease in coverage back to the more typical windier waters and channels surrounding Maui and the Big Island. Combined seas will approach 10 feet over most windward waters today then ease a bit through the weekend.
The current northwest swell will decline today with surf heights rapidly decreasing along north and west facing through through Sunday. Nearly flat surf conditions expected along north and west facing shores by Monday and Tuesday. A period of fresh to strong trades upstream of the islands will keep short period, higher wind waves along many northeastern and eastern facing shores into the weekend.
A small background south swell will continue today, as a slightly larger and longer period south swell will begin to build into the region later this afternoon. This next south swell energy will peak on Saturday. This next south swell will raise surf heights along south facing shores by a couple of feet into the fun size range through Sunday and then decline into early next week.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Saturday for Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Maalaea Bay, 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