Weather Forecast

Hawaii County Weather Forecast for May 20, 2021

May 20, 2021, 7:16 AM HST
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Hilo

Today: Scattered showers. Partly sunny, with a high near 82. West wind around 5 mph becoming calm in the morning. Chance of precipitation is 50%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Scattered showers. Mostly cloudy, with a low around 65. Light and variable wind becoming west around 6 mph in the evening. Chance of precipitation is 50%. New precipitation amounts of less than a tenth of an inch possible.

Friday: Scattered showers. Partly sunny, with a high near 82. Calm wind becoming east 5 to 9 mph in the afternoon. Chance of precipitation is 40%. New precipitation amounts of less than a tenth of an inch possible.

Kona

Today: Isolated showers after noon. Mostly sunny, with a high near 80. Southwest wind 3 to 8 mph. Chance of precipitation is 20%.

Tonight: Isolated showers. Partly cloudy, with a low around 66. Light and variable wind becoming east around 6 mph in the evening. Chance of precipitation is 20%.

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Friday: Scattered showers after noon. Mostly sunny, with a high near 78. West wind 5 to 8 mph becoming calm in the afternoon. Chance of precipitation is 30%. New precipitation amounts of less than a tenth of an inch possible.

Waimea

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Today: Scattered showers, mainly after noon. Partly sunny, with a high near 75. Breezy, with a northeast wind 16 to 20 mph, with gusts as high as 28 mph. Chance of precipitation is 40%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Scattered showers. Mostly cloudy, with a low around 52. Breezy, with a northeast wind around 17 mph, with gusts as high as 24 mph. Chance of precipitation is 30%. New precipitation amounts between a tenth and quarter of an inch possible.

Friday: Scattered showers. Partly sunny, with a high near 75. Breezy, with a northeast wind 8 to 15 mph, with gusts as high as 21 mph. Chance of precipitation is 40%. New precipitation amounts between a tenth and quarter of an inch possible.

Kohala

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Today: Scattered showers. Partly sunny, with a high near 61. Windy, with an east wind 20 to 25 mph decreasing to 9 to 14 mph in the afternoon. Winds could gust as high as 36 mph. Chance of precipitation is 50%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Scattered showers. Mostly cloudy, with a low around 50. Breezy, with an east wind 13 to 20 mph, with gusts as high as 28 mph. Chance of precipitation is 50%. New precipitation amounts of less than a tenth of an inch possible.

Friday: Showers likely, mainly after noon. Mostly cloudy, with a high near 61. Breezy, with an east wind 9 to 18 mph, with gusts as high as 25 mph. Chance of precipitation is 60%. New precipitation amounts of less than a tenth of an inch possible.

South Big Island

Today: Isolated showers. Mostly sunny, with a high near 79. Windy, with an east northeast wind 21 to 25 mph, with gusts as high as 32 mph. Chance of precipitation is 20%.

Tonight: Scattered showers, mainly before midnight. Partly cloudy, with a low around 72. Windy, with an east northeast wind 22 to 24 mph, with gusts as high as 31 mph. Chance of precipitation is 30%.

Friday: Scattered showers, mainly after noon. Mostly sunny, with a high near 79. Breezy, with an east wind around 22 mph, with gusts as high as 28 mph. Chance of precipitation is 30%.

Puna

Today: Showers likely, mainly before noon. Mostly cloudy, with a high near 79. North northwest wind 6 to 13 mph becoming east northeast in the morning. Chance of precipitation is 60%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Showers likely. Mostly cloudy, with a low around 62. Northeast wind 7 to 9 mph becoming north northwest after midnight. Chance of precipitation is 60%. New precipitation amounts between a tenth and quarter of an inch possible.

Friday: Showers likely, mainly before noon. Partly sunny, with a high near 79. North wind 6 to 13 mph. Chance of precipitation is 60%. New precipitation amounts between a tenth and quarter of an inch possible.

Waikoloa

Today: Sunny, with a high near 83. Breezy, with a northeast wind 14 to 20 mph, with gusts as high as 28 mph.

Tonight: Mostly clear, with a low around 67. Breezy, with an east wind 14 to 17 mph, with gusts as high as 24 mph.

Friday: Isolated showers after noon. Mostly sunny, with a high near 81. East northeast wind 6 to 11 mph becoming light and variable in the afternoon. Chance of precipitation is 20%.

Detailed Forecast

Synopsis

A surface high far northeast of the islands is maintaining locally breezy trade winds. Low clouds and passing trade showers will continue to favor windward and mauka sections through Friday. An upper-level trough moving toward the islands is expected to bring lighter winds, higher humidity, and a slight increase in showers this weekend. Expect the trade winds to strengthen again early next week.

Discussion

A nearly stationary 1030 mb surface high centered near 37N 136W, or about 1670 miles northeast of Hilo, is producing a tight pressure gradient across the Aloha State early this morning. This is maintaining locally breezy easterly trade winds in the vicinity of the island chain. Aloft, a mid-tropospheric ridge is keeping stable atmospheric conditions over the region. Loops of satellite imagery and radar reflectivity data show scattered to broken low clouds with embedded scattered showers moving in from the east toward the windward sides of the islands. The greatest coverage of low clouds and showers is currently over eastern Maui and the Big Island.
The forecast guidance shows the surface high will persist through Friday night, which will likely keep locally breezy easterly trade winds in place into early Saturday. The mid-tropospheric ridge will also maintain stable atmospheric conditions across the area, which will keep rainfall totals relatively light during the next couple of days. The low-level trade wind flow will continue to transport low clouds and brief trade showers into the windward sections of the islands. The trade showers will be most active during the nighttime and early morning hours, and may spread leeward over some of the smaller islands. The higher terrain on the Big Island and Maui will block the trade winds, and allow sea breezes to produce clouds and showers over leeward sections of those two islands each afternoon.
The global forecast models appear to be in good agreement that an upper-level trough will dig down toward the islands from the northwest this weekend. This feature will make the atmosphere slightly less stable, and may potentially allow a weak surface trough to develop in the vicinity of the islands. At the same time, a weak surface front is expected to move to a position northwest of the region. These features would likely cause the background flow to weaken across the state from late Saturday into Monday. The forecast models are showing some differences in the amount of deep moisture that may spread up from the southeast toward the eastern islands. For now, the weekend forecast shows an increase in humidity values over most of the state, with showers becoming a bit more active, especially over portions of the Big Island and windward Maui from late Saturday into Monday.
The trough aloft is forecast to move northeast away from the islands starting Monday. Therefore, the trade winds will likely strengthen again early next week. Also, expect more stable and drier atmospheric conditions to become established over the state from Monday night and Tuesday through mid-week.

Aviation

Periods of MVFR VIS/CIG in SHRA are expected over windward slopes and coasts into Friday as breezy trades deliver patches of showery low clouds. Coverage is currently sufficiently widespread to warrant AIRMET SIERRA for mountain obscuration over windward Maui and Big Island. Coverage should diminish later this morning, which should allow the AIRMET to be cancelled. AIRMET Tango will remain in effect below 8000 feet due to low-level turbulence over and S through W of all mountains.

Marine

The pressure gradient back across the islands from high pressure far northeast of the state will maintain these moderate to locally strong trade winds into the weekend. A Small Craft Advisory (SCA) is in effect for the typical windier waters around Maui County and the Big Island through Friday afternoon. The SCA may need to be extended into Saturday as advisory conditions persist over these windy marine zones into the weekend. A front northwest of chain is forecast to be north of the islands by early next week. This boundary will slacken the pressure gradient northeast of the state and subtly weaken trades. Trades are expected to restrengthen again early next week.
The current small northwest swell passing around the islands will fade through today. Another small north northwest swell is scheduled to arrive Sunday and peak surf into Monday morning along north and west facing shores with a gradual fall late Monday into Tuesday. A small northwest swell will fill in through mid week that may peak surf to near head high along many Kauai and Oahu north shores. Small, long period southerly swells will keep surf from completely going flat along southern coasts. The largest southern pulse is timed to arrive next Tuesday with a gradual decline into Thursday. Several days of sustained moderate to fresh trade winds over and upstream of the islands will maintain small period, wind wave choppy surf along most eastern exposures. A slight weakening of trades early next week may result a subtle lowering of recently elevated eastern shore surf.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Friday for Maalaea Bay, the Pailolo and Alenuihaha Channels and the waters south of the Big Island.

Data Courtesy of NOAA.gov

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