Weather Forecast

Hawaii County Weather Forecast for September 20, 2021

September 20, 2021, 7:15 AM HST
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Hilo

Today: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs around 84 near the shore to 69 to 74 at 4000 feet. Northwest winds around 10 mph shifting to the northeast in the afternoon. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then showers after midnight. Lows around 67 near the shore to around 53 at 4000 feet. North winds around 10 mph shifting to the northwest after midnight. Chance of rain 90 percent.

Tuesday: Mostly cloudy. Showers in the morning, then scattered showers in the afternoon. Highs around 84 near the shore to 69 to 74 at 4000 feet. Northeast winds 10 to 15 mph decreasing to up to 15 mph in the afternoon. Chance of rain 90 percent.

Kona

Today: Sunny in the morning, then mostly cloudy with scattered showers in the afternoon. Highs 82 to 87 near the shore to around 69 near 5000 feet. Light winds. Chance of rain 40 percent.

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Isolated showers. Lows 68 to 74 near the shore to around 54 near 5000 feet. Light winds. Chance of rain 20 percent.

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Tuesday: Mostly sunny in the morning, then mostly cloudy with numerous showers in the afternoon. Highs 82 to 87 near the shore to around 69 near 5000 feet. West winds up to 10 mph. Chance of rain 70 percent.

Waimea

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Today: Breezy. Partly sunny with scattered showers. Highs around 82 near the shore to 69 to 79 near 3000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows around 67 near the shore to 55 to 61 near 3000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tuesday: Breezy. Partly sunny with scattered showers. Highs around 82 near the shore to 69 to 79 near 3000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Kohala

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Today: Breezy. Partly sunny with scattered showers. Highs around 82 near the shore to 69 to 79 near 3000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows around 67 near the shore to 55 to 61 near 3000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tuesday: Breezy. Partly sunny with scattered showers. Highs around 82 near the shore to 69 to 79 near 3000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

South Big Island

Today: Breezy. Sunny in the morning, then mostly cloudy with isolated showers in the afternoon. Highs around 86 near the shore to around 73 near 5000 feet. East winds 10 to 20 mph. Chance of rain 20 percent.

Tonight: Partly cloudy. Breezy. Isolated showers in the evening, then scattered showers after midnight. Lows around 69 near the shore to around 52 near 5000 feet. Northeast winds 10 to 20 mph. Chance of rain 30 percent.

Tuesday: Breezy. Partly sunny with scattered showers. Highs around 86 near the shore to around 73 near 5000 feet. East winds 10 to 25 mph. Chance of rain 50 percent.

Puna

Today: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs around 84 near the shore to 69 to 74 at 4000 feet. Northwest winds around 10 mph shifting to the northeast in the afternoon. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then showers after midnight. Lows around 67 near the shore to around 53 at 4000 feet. North winds around 10 mph shifting to the northwest after midnight. Chance of rain 90 percent.

Tuesday: Mostly cloudy. Showers in the morning, then scattered showers in the afternoon. Highs around 84 near the shore to 69 to 74 at 4000 feet. Northeast winds 10 to 15 mph decreasing to up to 15 mph in the afternoon. Chance of rain 90 percent.

Waikoloa

Today: Breezy. Sunny in the morning, then partly sunny with isolated showers in the afternoon. Highs around 86 near the shore to 70 to 76 above 4000 feet. East winds up to 20 mph shifting to the northeast in the afternoon. Chance of rain 20 percent.

Tonight: Partly cloudy. Breezy. Isolated showers in the evening. Lows 69 to 74 near the shore to 50 to 57 above 4000 feet. Southeast winds 10 to 20 mph. Chance of rain 20 percent.

Tuesday: Breezy. Mostly sunny in the morning, then partly sunny with scattered showers in the afternoon. Highs around 86 near the shore to 70 to 76 above 4000 feet. Northeast winds 10 to 20 mph decreasing to up to 20 mph in the afternoon. Gusts up to 40 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A high pressure ridge north of the state will keep moderate to breezy trade winds across the region through Saturday. A typical trade wind weather pattern remains in the forecast with periods of passing showers focused mainly over windward and mountain areas of each island.

Discussion

Water vapor satellite imagery this morning shows an upper level trough just east of the main Hawaiian Islands. This upper trough will tend to produce passing showers over each island through Saturday. Satellite imagery and model guidance continue to show scattered to broken high level cirrus clouds drifting over the islands through Tuesday. These high level ice crystal clouds will enhance sunrise and sunset colors.
A high pressure system far north of the islands will continue to produce moderate to breezy trade winds into Saturday. Slight variations in the strength of the trade winds are expected from day to night, with decreasing wind speed trends for most land areas in the overnight hours. Decreasing trade winds are forecast on Sunday as the ridge north of the state weakens due to a cold front moving into the Central Pacific basin.
A low level easterly wave south of the islands will drift westward and brush through the Big Island from Monday afternoon into Tuesday. Increasing moisture with this disturbance will enhance showers along the east and southeast slopes of the Big Island, also adding a few more showers to the Kona slopes during the afternoon to early evening hours. A weak upper low moving in from the north will approach Kauai and Oahu on Tuesday and Wednesday. This system will move out fairly quickly and just continue to keep passing showers in the forecast.

Aviation

Moderate to breezy trades will carry periodic low clouds and showers over windward and mauka areas through this morning. Brief MVFR will be possible with any heavier showers, but otherwise prevailing VFR can be expected.
AIRMET Tango for lee turbulence remains in effect.

Marine

A ridge of high pressure N of the islands will maintain fresh to locally strong trades this week. Some subtle variations in wind speed can be expected through the period, with the potential for a slight increase Tuesday and Wednesday as the ridge strengthens and a trough passes S of the area. An ongoing Small Craft Advisory will likely remain posted for waters around Maui County and the Big Island for much of this week.
While surf will remain below High Surf Advisory levels along all shores this week, a trio of storm-force N Pacific lows will (or have) generate mid- to long-period NW to N swells that will arrive over the coming days. The first low is now weakening in the Gulf of Alaska, but it generated swell over the weekend that will arrive tomorrow and Wednesday, with peak open ocean swell heights approaching 5 feet. The second low will develop far NW of the area tonight and tomorrow, generating a similarly-sized (or slightly larger) NW swell that should arrive by the end of the week. The third low is expected to develop later this week, and may produce a NW to N swell that arrives in about a week or so.
As is typical in September, S hemisphere storm systems are still sending some swell our way, and a small boost in long-period S swell energy is expected around mid-week. The trade winds will deliver a steady supply of short-period wind waves to E facing shores through the week.

HFO Watches/Warnings/Advisories

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

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Data Courtesy of NOAA.gov

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