Weather Forecast

Hawaii County Weather Forecast for May 18, 2021

May 18, 2021, 7:16 AM HST
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Hilo

Today: Isolated showers. Partly sunny, with a high near 83. Light and variable wind becoming east northeast 5 to 10 mph. Chance of precipitation is 20%.

Tonight: Showers likely, mainly after midnight. Mostly cloudy, with a low around 66. Calm wind becoming east around 6 mph after midnight. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

Wednesday: Showers likely, mainly before noon. Partly sunny, with a high near 82. East wind 5 to 8 mph. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

Kona

Today: Mostly sunny, with a high near 78. Light and variable wind becoming west southwest 5 to 9 mph.

Tonight: Scattered showers, mainly before midnight. Mostly cloudy, with a low around 65. Light and variable wind becoming north northwest around 6 mph. Chance of precipitation is 40%. New precipitation amounts of less than a tenth of an inch possible.

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Wednesday: Scattered showers after noon. Partly sunny, with a high near 79. Light and variable wind becoming northwest 5 to 7 mph. Chance of precipitation is 30%. New precipitation amounts of less than a tenth of an inch possible.

Waimea

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Today: Isolated showers. Mostly sunny, with a high near 73. Windy, with a northeast wind 21 to 24 mph, with gusts as high as 34 mph. Chance of precipitation is 20%.

Tonight: Scattered showers, mainly after 1am. Mostly cloudy, with a low around 56. East wind around 14 mph, with gusts as high as 18 mph. Chance of precipitation is 40%. New precipitation amounts of less than a tenth of an inch possible.

Wednesday: Scattered showers. Mostly sunny, with a high near 72. East northeast wind 11 to 14 mph, with gusts as high as 20 mph. Chance of precipitation is 40%. New precipitation amounts between a tenth and quarter of an inch possible.

Kohala

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Today: Isolated showers. Partly sunny, with a high near 67. Windy, with an east wind 20 to 25 mph, with gusts as high as 36 mph. Chance of precipitation is 20%.

Tonight: Showers likely, mainly after midnight. Mostly cloudy, with a low around 54. Breezy, with an east wind 16 to 20 mph, with gusts as high as 28 mph. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

Wednesday: Showers likely, mainly before noon. Partly sunny, with a high near 66. Breezy, with an east wind 11 to 16 mph, with gusts as high as 22 mph. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

South Big Island

Today: Isolated showers after noon. Mostly sunny, with a high near 79. Windy, with an east wind 22 to 25 mph, with gusts as high as 32 mph. Chance of precipitation is 20%.

Tonight: Isolated showers before midnight. Partly cloudy, with a low around 72. Windy, with an east northeast wind 21 to 23 mph, with gusts as high as 29 mph. Chance of precipitation is 20%.

Wednesday: Isolated showers after noon. Mostly sunny, with a high near 79. Windy, with an east wind 21 to 23 mph, with gusts as high as 30 mph. Chance of precipitation is 20%.

Puna

Today: Isolated showers. Mostly sunny, with a high near 80. West northwest wind 8 to 14 mph becoming east northeast in the afternoon. Winds could gust as high as 18 mph. Chance of precipitation is 20%.

Tonight: Showers likely, mainly after midnight. Mostly cloudy, with a low around 63. North northeast wind 8 to 11 mph. Chance of precipitation is 70%. New precipitation amounts between a tenth and quarter of an inch possible.

Wednesday: Showers likely, mainly before noon. Partly sunny, with a high near 79. Northeast wind 10 to 13 mph. Chance of precipitation is 70%. New precipitation amounts between a tenth and quarter of an inch possible.

Waikoloa

Today: Mostly sunny, with a high near 81. Breezy, with an east northeast wind 17 to 22 mph decreasing to 5 to 10 mph in the afternoon. Winds could gust as high as 31 mph.

Tonight: Isolated showers before midnight. Partly cloudy, with a low around 66. East wind around 6 mph. Chance of precipitation is 20%.

Wednesday: Isolated showers after noon. Mostly sunny, with a high near 82. North wind around 6 mph. Chance of precipitation is 20%.

Detailed Forecast

Synopsis

Trade winds will continue through the rest of the week, although a slight weakening trend is expected today and again over the weekend. Showers will favor windward and mountain areas with brief rainfall over leeward locations. More humid conditions and an uptick in rainfall is possible this weekend.

Discussion

Breezy trade winds will ease off slightly today as a broad 1029 mb surface high located far northeast of the islands drifts eastward. Further softening of the trades may occur over the weekend as well. As per GFS, up to Friday, mid level ridging will maintain a stable air mass over the islands with an inversion at around 8k feet. Precipitable water will be hovering from just under an inch to high as 1.20 through Friday. With this said, we are anticipating a typical trade wind pattern with showers favoring the windward and mountain areas, especially the nights and mornings. There will be a day or two where there will be a little more showery, and or a day or two, where it will be not as showery. But there will be plenty of sunshine along the leeward beaches.
After Friday, the GFS and ECMWF models disagree on certain elements. Long story short, the GFS has a mid level low southwest of the Big Island Saturday where the circulation brings in deep tropical moisture, below 18k feet, to across the islands from the east and southeast starting Saturday evening. The ECMWF, on the other hand, has a mid to upper level trough present over the eastern part of the island chain through the weekend. This stationary trough will keep the deep tropical moisture east of the Big Island. Little influence is expected thermodynamically with the pocket of cool air with the mid trough/low where only a minus 9 or minus 10 degree C is noted. But, should the the GFS pan out, a thunderstorm may pop up over the higher terrain of the Big Island Sunday afternoon, along with some spotty heavy trade wind showers. The surface wind field differs between the GFS and EC over the weekend, where the GFS has a surface trough moving across the islands between Saturday night and Sunday night. This trough will lighten the wind field, bringing on a hybrid mix of light easterly winds and a day time sea breeze/night time land breeze pattern, especially islands down wind of Maui and the Big Island. Should the EC solution pans out, the islands will see a continuation and extension of the trade winds, although the trades could be tad lighter, in the moderate range, due to a front northwest of Kauai.
Current satellite and radar shows a scattered field of cumulus clouds and embedded showers. This field is concentrated from Oahu eastward. Windward Kauai will see a few showers this morning, and that is about it. It is mainly dry upwind. Overall, the windward and mountain areas of the larger islands will absorb the bulk of these trade showers. As the day progresses, some of these clouds and showers will dissipate under a strong and stable air mass, resulting in plenty of sunshine, especially the south and west facing beaches. We do expect the lee side of the Big Island to see more clouds this afternoon and evening.

Aviation

High pressure northeast of the area will keep moderate to locally strong trade winds in place over the islands and adjacent waters through tonight. VFR conditions will prevail with brief periods of MVFR conditions over windward slopes possible due to passing low clouds and SHRA.
AIRMET Tango remains in effect for moderate turbulence south through west of mountains and will likely continue through tonight.

Marine

High pressure located northeast of the islands, in relation to lower pressure south of the state, is maintaining moderate to locally strong trade winds. Generally, these moderate to fresh trades will persist through the majority of the week. The potential exists for a slight weakening of winds today and this weekend, with the latter relating to an approaching frontal boundary to the north. A Small Craft Advisory (SCA) is in effect through this afternoon for select coastal waters around the islands of Oahu, Maui and Hawaii primarily due to the acceleration of northeasterlies around these islands. The zones around Oahu are anticipated to drop out by 6 PM HST tonight, leaving a Tuesday night and Wednesday SCA for those typically windier Maui County and Big Island south waters through Wednesday afternoon. An extension of this SCA is likely.
Slightly higher surf is expected along northern and western coasts through Wednesday as a couple of late season small north to west swells pass around the islands. Today’s swell is more northwest and will peak surf at below normal heights. A secondary more west northwest small swell arrives tonight into Wednesday. This swell will hold north facing surf up but may also be just enough north to avoid upstream island shadowing and provide a small bump to west facing Kauai and Oahu shore surf Wednesday. Several days of enhanced trades has created fully developed seas. Thus, choppy conditions along east facing shores will occur throughout the week. South facing shore surf will stay small, or slightly below season norms, the next several days. In the long range, a slightly larger, long period pulse from the southern hemisphere may arrive early next week. This swell may boost late May southern coast surf to near typical summertime heights.

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.
Small Craft Advisory until 6 PM HST this evening for Oahu Leeward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters.

Data Courtesy of NOAA.gov

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