Weather Forecast

Hawaii County Weather Forecast for September 21, 2021

September 21, 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. East winds up to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with numerous showers. Lows around 69 near the shore to around 55 at 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Wednesday: Partly sunny with scattered showers. Highs around 84 near the shore to 69 to 74 at 4000 feet. Northeast winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

Kona

Today: Mostly 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 30 percent.

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 70 to 75 near the shore to around 56 near 5000 feet. Light winds. Chance of rain 50 percent.

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Wednesday: Mostly sunny in the morning, then partly sunny with isolated showers in the afternoon. Highs 82 to 87 near the shore to around 69 near 5000 feet. Southwest winds up to 10 mph shifting to the west in the afternoon. Chance of rain 20 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 25 mph. Chance of rain 50 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows around 69 near the shore to 57 to 63 near 3000 feet. East winds 10 to 25 mph. Chance of rain 50 percent.

Wednesday: Breezy. Mostly sunny with scattered showers. Highs around 82 near the shore to 69 to 79 near 3000 feet. East winds up 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 25 mph. Chance of rain 50 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows around 69 near the shore to 57 to 63 near 3000 feet. East winds 10 to 25 mph. Chance of rain 50 percent.

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

South Big Island

Today: Mostly sunny in the morning then becoming mostly cloudy. Breezy. Isolated showers. Highs around 86 near the shore to around 73 near 5000 feet. East winds up to 25 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Breezy. Scattered showers. Lows around 71 near the shore to around 54 near 5000 feet. East winds 10 to 25 mph shifting to the northeast up to 25 mph after midnight. Chance of rain 50 percent.

Wednesday: Mostly sunny in the morning then becoming partly sunny. Breezy. Isolated showers. Highs around 86 near the shore to around 73 near 5000 feet. Northeast winds up to 25 mph shifting to the east 10 to 20 mph in the afternoon. Chance of rain 20 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. East winds up to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with numerous showers. Lows around 69 near the shore to around 55 at 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Wednesday: Partly sunny with scattered showers. Highs around 84 near the shore to 69 to 74 at 4000 feet. Northeast winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

Waikoloa

Today: Breezy. 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 15 mph shifting to the north up to 20 mph in the afternoon. Chance of rain 30 percent.

Tonight: Partly cloudy. Breezy. Isolated showers in the evening. Lows around 73 near the shore to 52 to 59 above 4000 feet. East winds up to 20 mph. Chance of rain 20 percent.

Wednesday: Mostly sunny. Breezy. Isolated showers in the afternoon. Highs around 86 near the shore to 70 to 76 above 4000 feet. North winds up to 20 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

A high pressure ridge north of the state will maintain moderate to breezy trade winds across the region through Saturday. Wind speeds will decrease on Sunday and Monday as the ridge north of the region weakens. A few disturbances moving through the region over the next several days will keep periods of passing clouds and showers in the forecast, favoring windward and mountain areas, and trending higher in the overnight to early morning hours.

Discussion

The satellite picture this morning continues to show an upper level trough just east of the Hawaiian Islands with a weak upper level low approaching Kauai from the north. A weak low level trough currently moving through the Big Island will drift westward through the area. Satellite imagery and model guidance continues to show scattered to broken high level cirrus clouds drifting over the islands today. These high level ice crystal clouds will enhance sunrise and sunset colors.
The high pressure ridge north of the islands will remain strong enough to produce moderate to breezy trade winds each day through 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 to early morning hours. Decreasing trade wind speeds are forecast across the region on Sunday and Monday as the ridge north of the state weakens due to a cold front moving into the Central Pacific basin.
A combination of upper level and low level disturbances across the region will weaken trade wind temperature inversion heights and produce periods of passing showers for all islands into the weekend. Expect day to day changes in shower and cloud coverage as these disturbances move through the state. A low level disturbance will move into the Big Island this morning enhancing showers along the east and southeast facing slopes Maui and the Big Island. Higher moisture levels may add 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 through Wednesday. This system will move out fairly quickly and continue to keep passing showers in the forecast. Another unstable low level trough will move from east to west across the island chain on Saturday and Sunday.

Aviation

High pressure north of the state and a surface trough passing by to the south, will keep breezy trade winds in place during the next 24 hours. Fairly typical trade wind weather is expected for the smaller islands, with showers favoring windward and mauka areas, particularly during the overnight and early morning hours. Brief MVFR cigs/vsbys will be possible in passing showers across these windward areas. Over the Big Island, more unsettled weather is expected. An area of enhanced moisture on the northern fringe of the surface trough will bring widespread MVFR cigs/vsbys to windward sections through much of the day. Conditions should improve over windward Big Island this evening.
AIRMET Sierra is in effect for mountain obscuration across windward sections of the Big Island, Maui, Molokai, Oahu and Kauai. This AIRMET will likely remain in place for the Big Island through much of the day, while the smaller islands will likely see improving conditions by mid morning.
AIRMET Tango remains in effect for moderate low level turbulence over and downwind of the terrain of all islands. This AIRMET will likely remain in place through at least Wednesday.

Marine

The northern Pacific has begun to transition out its summer mode featuring a series of gale- to storm-force lows driving overlapping swells toward the islands this week. Surf will respond along exposed north and west facing shores and trend up beginning today. This first north-northwest pulse will peak tonight, hold Wednesday, then ease Thursday. A larger north-northwest pulse should begin to fill in by Thursday night, peak just under the advisory level Friday, then slowly fade into the weekend.
Surf along south facing shores will trend up by midweek as a new, long-period south swell arrives and moves through. This event should peak around Thursday before fading into the weekend.
Surf along east facing shores will remain small and choppy due to the fresh/strong trades locally and just upstream of the state.
Fresh to strong trade winds will persist each day over the local waters through the week. The Small Craft Advisory winds are expected over the windier areas and channels from the Kaiwi Channel (will be added to the advisory later tonight) to the Big Island. Seas will near the advisory-level of 10 ft Friday due to a combination of the long-period north-northwest swell and trade wind seas.

Fire weather

Leeward areas will exceed critical fire weather thresholds for a few hours this afternoon due to a combination of breezy winds and low humidity levels. A Red Flag Warning remains in effect through the afternoon hours. Fire weather conditions on Wednesday are expected to approach but not exceed these critical thresholds as wind speeds will decrease just below critical levels.

HFO Watches/Warnings/Advisories

Red Flag Warning until 6 PM HST this evening for Kauai Leeward, Oahu South Shore, Waianae Coast, Oahu North Shore, Olomana, Central Oahu, Waianae Mountains, Molokai Leeward, Lanai Makai, Lanai Mauka, Maui Leeward West, Maui Central Valley, Leeward Haleakala, Kona, South Big Island, Kohala.
Small Craft Advisory until 6 PM HST Wednesday for Kaiwi Channel, Maui County Windward Waters, 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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