Weather Forecast

Hawaii County Weather Forecast for September 12, 2021

September 12, 2021, 7:15 AM HST
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Photo Credit: James Grenz

Hilo

Today: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 77 to 83 near the shore to 63 to 70 at 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Tonight: Cloudy. Numerous showers in the evening, then showers after midnight. Lows 63 to 71 near the shore to 54 to 59 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 90 percent.

Monday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 77 to 84 near the shore to 63 to 70 at 4000 feet. Northeast winds around 10 mph. Chance of rain 70 percent.

Kona

Today: Mostly sunny in the morning, then cloudy with scattered showers in the afternoon. Highs 81 to 86 near the shore to around 67 near 5000 feet. West winds up to 10 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 69 to 75 near the shore to 51 to 57 near 5000 feet. Light winds. Chance of rain 50 percent.

ARTICLE CONTINUES BELOW AD

Monday: Mostly sunny in the morning, then mostly cloudy with scattered showers in the afternoon. Highs 83 to 88 near the shore to around 67 near 5000 feet. Light winds. Chance of rain 50 percent.

Waimea

ARTICLE CONTINUES BELOW AD

Today: Breezy. Partly sunny with scattered showers. Highs 63 to 83. East winds 10 to 25 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 62 to 70 near the shore to 56 to 63 near 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Monday: Partly sunny with scattered showers. Highs 64 to 84. East winds 10 to 15 mph. Chance of rain 50 percent.

Kohala

ARTICLE CONTINUES BELOW AD

Today: Breezy. Partly sunny with scattered showers. Highs 63 to 83. East winds 10 to 25 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 62 to 70 near the shore to 56 to 63 near 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Monday: Partly sunny with scattered showers. Highs 64 to 84. East winds 10 to 15 mph. Chance of rain 50 percent.

South Big Island

Today: Breezy. Sunny in the morning, then cloudy with isolated showers in the afternoon. Highs around 83 near the shore to around 66 near 5000 feet. East winds up to 20 mph. Chance of rain 20 percent.

Tonight: Breezy. Mostly cloudy with isolated showers in the evening, then mostly clear after midnight. Lows around 73 near the shore to around 53 near 5000 feet. Northeast winds up to 20 mph. Chance of rain 20 percent.

Monday: Sunny in the morning, then mostly cloudy with isolated showers in the afternoon. Highs around 83 near the shore to around 65 near 5000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Puna

Today: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 77 to 83 near the shore to 63 to 70 at 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Tonight: Cloudy. Numerous showers in the evening, then showers after midnight. Lows 63 to 71 near the shore to 54 to 59 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 90 percent.

Monday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 77 to 84 near the shore to 63 to 70 at 4000 feet. Northeast winds around 10 mph. Chance of rain 70 percent.

Waikoloa

Today: Sunny in the morning then becoming partly sunny. Breezy. Highs 83 to 88 near the shore to 65 to 72 above 4000 feet. Northeast winds up to 20 mph.

Tonight: Partly cloudy. Lows around 74 near the shore to 51 to 56 above 4000 feet. Southeast winds up to 15 mph.

Monday: Mostly sunny. Highs 83 to 89 near the shore to 66 to 72 above 4000 feet. Northeast winds up to 10 mph in the morning becoming light.

Detailed Forecast

Synopsis

An upper level low northeast of the islands will drift westward into the region producing enhanced showers into Friday. Moderate trade winds will continue through the end of the week, focusing showers over windward and mountain locations with lower shower coverage over leeward areas. This upper low will pass by just north of the island chain on Wednesday, increasing chances for heavy showers and thunderstorm activity over the western half of the state from Wednesday into Thursday. Drier conditions will return with strengthening trade winds from late Friday into Saturday.

Discussion

A band of unstable low clouds is slowly drifting through the Hawaiian Islands this morning with cloud tops in the 8,000 to 12,000 foot range. This cloud band will keep numerous showers in the forecast over windward and mountain slopes with some showers passing over leeward areas through the morning hours. The upper low is roughly 850 miles northeast of the islands, as shown on satellite water vapor imagery. This low is drifting slowly westward bringing periods of enhanced showers to all islands as it approaches the state.
The high pressure ridge far north of the islands will be strong enough to produce moderate trade winds across the region through the week. A slight increase in trade winds into the moderate to locally breezy range remains in the forecast for today. Lighter trade winds are forecast from late Tuesday to Thursday as the upper low passes by just north of the island chain weakening the surface pressure gradient. Lighter large scale winds will allow a sea breeze pattern to set up on the terrain sheltered western slopes of each island from Wednesday into Thursday. Stronger trade winds will return after the low passes by the islands from late Thursday through next weekend.
Weather wise, the upper level low and weakened ridge far north of the state will allow higher trade wind inversion heights than normal. Trade wind inversion height will range between the 7,000 and 10,000 foot range for most areas into the middle of next week producing higher cloud tops. Expect continued periods of enhanced shower activity with higher rainfall amounts focused over windward and mountain areas. Some of the stronger showers will push into typically drier leeward areas, especially in the overnight to early morning time periods.
On Wednesday, forecast guidance shows the upper low passing just north of the state with increasing potential for heavy showers and isolated thunderstorms over the eastern islands from Wednesday into Thursday. Still too early to discuss timing and island by island weather impacts as much will depend on the track and intensity of this upper low relative to the islands. Our confidence in these wet weather details will improve as the forecast time period grows shorter. Stay tuned.

Aviation

High pressure north of the state will keep moderate to locally breezy trade winds in place through tonight. Clouds and showers will favor windward and mauka areas, with a few showers reaching leeward communities from time to time. MVFR cigs/vsbys will be common in showers along windward slopes and coasts through mid morning. Otherwise, VFR conditions will prevail.
AIRMET Sierra remains in effect for mountain obscuration across windward sections of Kauai, Oahu, Molokai, Maui and the Big Island. Conditions should improve later this morning.

Marine

High pressure north of the state will maintain a tight enough pressure gradient across the islands in producing moderate to fresh trade winds the next couple of days. The windier areas surrounding Maui County and south of Big Island will experience periods of stronger winds. Thus, a Small Craft Advisory has been posted for the Pailolo Channel, Maalaea Bay, Alenuihaha Channel and Big Island’s southeastern waters through this afternoon. The passage of a weak easterly trough Tuesday will subtly dampen mid week trades. As this trough moves west of the state, northeast veering southeast winds will pick back up to more moderate to locally fresh magnitudes by Friday and persist into next weekend.
The recent small, relatively longer period south swells that have provided near seasonable south shore surf these past couple of days will decline through Monday. A small, long period Tasman Sea southwest swell is expected to fill in from late Monday into Tuesday with a slow fall during the second half of the week. Kauai and Oahu west coast surf should benefit from some of this southerly wrap and keep waist to near head high sets in play into Thursday. A small, medium period northwest north swell may clip the islands next weekend. Choppy easterly wind wave swell will hold small east facing shore surf through Tuesday with a slight mid week drop under slightly weaker trades.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Southeast Waters.

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Data Courtesy of NOAA.gov

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