Weather Forecast

Hawaii County Weather Forecast for May 21, 2021

May 21, 2021, 7:16 AM HST
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Hilo

Today: Occasional showers, with thunderstorms also possible after 1pm. High near 79. West wind 5 to 7 mph becoming light and variable. Chance of precipitation is 80%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Tonight: Scattered showers and thunderstorms, then showers likely after 7pm. Mostly cloudy, with a low around 65. Calm wind becoming west around 6 mph in the evening. Chance of precipitation is 60%. New precipitation amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Saturday: Scattered showers, with thunderstorms also possible after 1pm. Mostly cloudy, with a high near 82. Light and variable wind becoming east northeast 5 to 9 mph in the afternoon. Chance of precipitation is 50%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Kona

Today: Scattered showers and thunderstorms after noon. Partly sunny, with a high near 79. Calm wind becoming west 5 to 8 mph in the afternoon. Chance of precipitation is 40%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Tonight: Scattered showers, mainly before midnight. Mostly cloudy, with a low around 63. Southwest wind around 5 mph becoming light and variable after midnight. Chance of precipitation is 30%. New precipitation amounts of less than a tenth of an inch possible.

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Saturday: Scattered showers and thunderstorms after noon. Partly sunny, with a high near 79. Light and variable wind becoming west southwest 5 to 9 mph in the morning. Chance of precipitation is 40%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Waimea

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Today: Scattered showers, with thunderstorms also possible after noon. Mostly cloudy, with a high near 75. Breezy, with a northeast wind 11 to 17 mph, with gusts as high as 24 mph. Chance of precipitation is 50%. New rainfall amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.

Tonight: Scattered showers. Mostly cloudy, with a low around 52. Breezy, with a northeast wind 13 to 17 mph, with gusts as high as 24 mph. Chance of precipitation is 40%. New precipitation amounts between a tenth and quarter of an inch possible.

Saturday: Isolated showers, then scattered showers and thunderstorms after noon. Partly sunny, with a high near 75. Breezy, with a northeast wind 7 to 15 mph, with gusts as high as 21 mph. Chance of precipitation is 50%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Kohala

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Today: Occasional showers, with thunderstorms also possible after noon. High near 61. Breezy, with an east wind 13 to 16 mph, with gusts as high as 22 mph. Chance of precipitation is 80%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Tonight: Showers likely, mainly before midnight. Mostly cloudy, with a low around 50. Breezy, with an east wind 11 to 16 mph becoming light after midnight. Winds could gust as high as 22 mph. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

Saturday: Scattered showers, with thunderstorms also possible after noon. Partly sunny, with a high near 61. East southeast wind 3 to 6 mph. Chance of precipitation is 50%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

South Big Island

Today: Isolated showers, then scattered showers and thunderstorms after noon. Partly sunny, with a high near 81. Windy, with an east northeast wind 22 to 24 mph, with gusts as high as 31 mph. Chance of precipitation is 30%.

Tonight: Isolated showers. Partly cloudy, with a low around 71. Windy, with an east northeast wind 22 to 24 mph, with gusts as high as 31 mph. Chance of precipitation is 20%.

Saturday: Scattered showers and thunderstorms after noon. Mostly sunny, with a high near 81. Windy, with an east wind 18 to 23 mph, with gusts as high as 30 mph. Chance of precipitation is 30%.

Puna

Today: Occasional showers, with thunderstorms also possible after noon. High near 76. West northwest wind 6 to 10 mph becoming east northeast in the afternoon. Chance of precipitation is 80%. New rainfall amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.

Tonight: Showers likely, mainly before midnight. Mostly cloudy, with a low around 62. East northeast wind 6 to 8 mph becoming northwest after midnight. Chance of precipitation is 60%. New precipitation amounts between a tenth and quarter of an inch possible.

Saturday: Scattered showers, with thunderstorms also possible after noon. Partly sunny, with a high near 79. Northwest wind 7 to 11 mph becoming east northeast in the afternoon. Chance of precipitation is 50%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Waikoloa

Today: Scattered showers and thunderstorms after noon. Partly sunny, with a high near 82. Breezy, with a light and variable wind becoming east northeast 12 to 17 mph in the afternoon. Winds could gust as high as 24 mph. Chance of precipitation is 30%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Tonight: Isolated showers before midnight. Partly cloudy, with a low around 64. Breezy, with an east northeast wind 14 to 17 mph, with gusts as high as 24 mph. Chance of precipitation is 20%.

Saturday: Scattered showers and thunderstorms after noon. Mostly sunny, with a high near 82. East wind 9 to 14 mph becoming light and variable in the afternoon. Winds could gust as high as 18 mph. Chance of precipitation is 30%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Detailed Forecast

Synopsis

High pressure northeast of the state will keep moderate to locally breezy trades in place through Saturday, with clouds and showers favoring windward and mauka areas. The trades will be disrupted Sunday through Tuesday as a front approaches from the northwest. This bring more humid conditions and a land and sea breeze pattern to the islands, with showers favoring coastal areas at night and interior and mauka areas during the afternoon and early evening hours. Trade winds appear to return by the middle of next week, bringing a return of more typical trade wind weather to the island chain.

Discussion

Currently at the surface, a 1032 mb high is centered around 1600 miles northeast of Honolulu, and is driving moderate to breezy trades across the island chain this evening. Infrared satellite imagery shows partly to mostly cloudy skies in windward areas, with clear to partly cloudy conditions in most leeward locales. Radar imagery shows scattered windward showers with mainly dry conditions in leeward areas. Main short term concerns revolve around trade wind trends across the state as well as thunder potential over the Big Island during the next few days.
High pressure northeast of the state will keep moderate to locally breezy trade winds in place through Saturday. The trades will ease Saturday night, then shift east-southeast Sunday through Tuesday as a front approaches and stalls out northwest of the state. This will bring a land and sea breeze pattern to the smaller islands, with moderate east-southeasterly winds prevailing over the Big Island. Models appear in good agreement showing the front fizzling out west of the state and a new strong high building northeast of the islands by the middle of next week. This should bring a resurgence in the trades Tuesday night and Wednesday and prevailing through late next week.
As for the remaining weather details, typical trade wind weather should prevail through Saturday night, with mainly windward and mauka showers and the occasional leeward spillover. We should see an increase in trade wind showers over windward Big Island and Maui tonight and Friday however, as a slug of low clouds seen on satellite imagery just east of the state moves through. Another increase in showers will be possible Saturday night as some deeper moisture begins to work into the islands from the southeast. Additionally, the airmass appears unstable enough due to a disturbance aloft over the islands, that a thunderstorm or two could develop over the Big Island each afternoon.
Conditions will begin to feel more summerlike by Sunday as we lose the trades and humidity levels climb. The loss of the trades will also allow for a convective weather pattern to develop over the islands which appears to hold in place through Tuesday. Showers will favor locations near the coast in an east-southeasterly flow regime at night, and interior/mauka areas during the afternoon and early evening hours. Additionally, a thunderstorm can’t be ruled out each afternoon over the Big Island. Trade wind weather appears to return Tuesday night or Wednesday, bringing a decrease in humidity levels and more typical windward and mauka showers.

Aviation

A high pressure ridge north of the region will keep moderate to breezy trade winds in the forecast through Friday. An upper level low will produce brief periods of MVFR conditions in showers over windward and mountain areas, mainly in the overnight to early morning hours. A band of clouds moving into the Big Island will produce periods of showers for windward slopes starting around midnight with tempo mountain obscurations affecting the eastern Big Island and Maui through the morning hours.
AIRMET Tango remains in effect for moderate turbulence south through west of island mountains due to stronger trade winds.

Marine

The pressure gradient back across the islands from eastern Pacific high pressure positioned far northeast of the state will maintain these moderate to locally strong trade winds into Saturday. A Small Craft Advisory (SCA) is currently in effect for the typical windier waters around Maui County and the Big Island through Friday afternoon. This SCA may need to be extended into Saturday as advisory conditions persist one more day over these windy marine zones. A front approaching and stalling out northwest of the islands will slacken the pressure gradient and ultimately weaken trades Sunday through early next week. Reestablishing high pressure northeast of the islands mid next week will allow trades to restrengthen, possibly reaching SCA levels over the typically windier areas.
The current small northwest swell passing through the islands will fade through the night. The next 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 Thursday. Small, long period southerly swells will keep surf from completely going flat along southern coasts. A couple of larger, long period southern pulses Tuesday and Thursday will pick south facing surf up to near seasonal heights. Several days of sustained moderate to fresh trades over and upstream of the islands will maintain small period, wind wave choppy surf along most eastern exposures. A weakening of trades early next week will result in a subtle lowering of recently elevated eastern shore surf. A boost in eastern surf will likely occur as trades pick back up during the middle of next week.

HFO Watches/Warnings/Advisories

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

Data Courtesy of NOAA.gov

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