Weather Forecast

Hawaii County Weather Forecast for October 20, 2021

October 20, 2021, 7:16 AM HST
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Photo Credit: James Grenz

Hilo

Today: Mostly sunny with isolated light showers. Highs 76 to 82 near the shore to 61 to 69 at 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows 60 to 69 near the shore to 51 to 56 at 4000 feet. North winds up to 10 mph. Chance of rain 20 percent.

Thursday: Mostly sunny with isolated showers. Highs 78 to 83 near the shore to 64 to 70 at 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Kona

Today: Mostly sunny. Isolated light showers in the afternoon. Highs 81 to 87 near the shore to around 67 near 5000 feet. Light winds. Chance of rain 20 percent.

Tonight: Partly cloudy. Lows around 70 near the shore to 48 to 54 near 5000 feet. Light winds.

ARTICLE CONTINUES BELOW AD

Thursday: Sunny. Isolated showers in the afternoon. Highs 81 to 86 near the shore to around 67 near 5000 feet. Northwest winds around 10 mph. Chance of rain 20 percent.

Waimea

ARTICLE CONTINUES BELOW AD

Today: Mostly sunny with isolated light showers. Highs 63 to 83. East winds around 10 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows 61 to 68 near the shore to 53 to 60 near 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Thursday: Breezy. Mostly sunny with isolated showers. Highs 65 to 84. East winds 10 to 20 mph. Chance of rain 20 percent.

Kohala

ARTICLE CONTINUES BELOW AD

Today: Mostly sunny with isolated light showers. Highs 63 to 83. East winds around 10 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows 61 to 68 near the shore to 53 to 60 near 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Thursday: Breezy. Mostly sunny with isolated showers. Highs 65 to 84. East winds 10 to 20 mph. Chance of rain 20 percent.

South Big Island

Today: Mostly sunny. Isolated light showers in the afternoon. Highs around 82 near the shore to around 64 near 5000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Mostly clear. Isolated showers in the evening. Lows around 70 near the shore to around 51 near 5000 feet. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Thursday: Sunny and breezy. Highs around 83 near the shore to around 65 near 5000 feet. East winds 10 to 20 mph.

Puna

Today: Mostly sunny with isolated light showers. Highs 76 to 82 near the shore to 61 to 69 at 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows 60 to 69 near the shore to 51 to 56 at 4000 feet. North winds up to 10 mph. Chance of rain 20 percent.

Thursday: Mostly sunny with isolated showers. Highs 78 to 83 near the shore to 64 to 70 at 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Waikoloa

Today: Sunny. Highs 81 to 88 near the shore to 66 to 72 above 4000 feet. North winds around 10 mph in the morning becoming light.

Tonight: Mostly clear. Lows around 70 near the shore to 48 to 53 above 4000 feet. Southeast winds up to 10 mph.

Thursday: Sunny. Highs 81 to 87 near the shore to 65 to 72 above 4000 feet. North winds 10 to 15 mph.

Detailed Forecast

Synopsis

A ridge of high pressure approaching the islands will make for gentle trades for one more day today. A dry and stable airmass overhead will produce lots of sunshine, and only a few light showers. The remnants of an old front will reach Kauai later tonight, and may also spread to Oahu Thursday or Friday, bringing an increase in clouds and some showers. Trades will start to increase Thursday and become breezy by the weekend. Drier and more stable air will also return this weekend.

Discussion

The full moon VIIRS image from 12z shows a nice, quiet night out there tonight with just scattered clouds upwind of mainly Oahu to the Big Island, and relatively sparse small cumulus near Kauai. Indeed, radars are mainly rain-free this morning except for a few small showers to the ENE of Oahu. Only one rain gauge has recorded measurable rain in the last six hours, although interestingly the Honolulu airport did have a brief random sprinkle early this morning.
Looks like a very nice day today. Both soundings have strong and relatively low inversions near 5000-5500 feet, with PW below normal for October. Sea level dewpoints also relatively dry, with most readings in the upper 50s and lower 60s. A weakening surface ridge about 100 mi NE of the islands will fade away today, as a weakening frontal boundary about 250 mi N of Kauai continues pushing our way. Gentle trade winds are expected today with areas protected from the trades switching to daytime sea breezes by mid-morning.
Clouds and some showers associated with the old front are expected to reach Kauai tonight, mainly after midnight. This moisture will continue to become more diffuse as it sags southward, perhaps spreading a few additional showers to Oahu later Thu into Thu night. Not surprisingly, some mixed signals in the models as to whether the band will actually hold together near Oahu, or whether we’ll just see more of an increase in trade wind showers for all islands starting Thu night instead. Either way, the trend will be a bit wetter. This trend should continue into Friday for Kauai and possibly Oahu as weak upper troughing and the moisture from frontal remnants linger.
A surface high to the N of the old front will start to increase the pressure gradient over the islands on Thursday, and winds will gradually ramp up to locally breezy levels by the end of the day, becoming breezy to locally windy on Friday.
Over the weekend, mid-level ridging builds in and subsidence takes back over, leading to more sunshine and fewer windward and mauka showers on Sat. Although the airmass will remain quite stable into Sunday and Sunday night, a very minor increase in trade showers is expected associated with a patch of moisture coming in on the trades. Depending on the timing of this area with respect to the diurnal cloud cycle, it might not be very noticeable. Another high will move in from the W far to the NW of the islands, keeping breezy to locally windy trades blowing through the weekend.
Fewer than normal trade wind showers are expected overall through Tuesday. The high to the N should weaken and lead to a downward trend in trade wind speeds by Tuesday.

Aviation

Expect a VFR day today, while a front, currently 170 nm north of Kauai, falls apart as it approaches Kauai and Oahu today. With the prevailing light northeast winds, a couple of light isolated showers may reach the windward shores. Some of the lee areas of the smaller islands, may go sea breeze this morning, but a stable and dry air mass, will squelch any development of convective showers in the interior and lee areas.
The remnants of the dissipating front will reach Kauai early this evening, providing that island with some scattered trade wind showers. AIRMET Sierra is a possibility for mountain obscuration along the island’s windward slopes. These clouds and showers will be spreading east to the other smaller islands the rest of tonight and Thursday. Surface high pressure settling north of the islands Thursday, will lead to strengthening trades Thursday through Friday, warranting the issuance for AIRMET Tango for low level turbulence.

Marine

The tail end of a weak cold front moving closer to the state will push a ridge of high pressure over the north offshore waters. This will support gentle to moderate trade winds, especially over the windward waters as the ridge axis stalls in the vicinity of these waters. There may be an areal increase of near coastal, western island showers Thursday and Friday as moisture pools near this weakened, diffuse front. Starting Thursday, high pressure will re- establish itself north of the state. This will restrengthen local trades into the fresh to locally strong range by Saturday.
A moderate, long period northwest to north swell (340 degrees) is arriving this morning. The forerunners of this new swell reached the northwest offshore buoys last night and are now just arriving along the northwest Kauai coastline during the pre-dawn hours. This swell will build through the day and peak tonight into Thursday morning with a Thursday afternoon decline. A reinforcing north swell (010 degrees) is expected to fill in just behind this swell Thursday night and peak Friday. The later of these two swells could lift surf to near High Surf Advisory thresholds during this Friday or Friday night peak. Friday’s 6 to 7 foot swell will come from a near due north direction and this will increase the harbor surge threat within Kahului and Hilo’s harbors. Background South Pacific energy will hold south shore surf. South surf should experience a small late week bump as new medium period, low southwest to southeast swells fill in Friday and Saturday. Surf along east facing shores should experience some wrap from the pair of north swells the next few days. East wind wave swell will be on the rise into the weekend in response to strengthened trade winds.

HFO Watches/Warnings/Advisories

None.

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Data Courtesy of NOAA.gov

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