Despite its wrath, Hurricane Hone brings much needed rain to Big Island

Anywhere you look on the Big Island right now, whether it’s in the saddle between Mauna Kea and Mauna Loa or outside your front window, bright green and fresh ferns, grass and other flora dominate the landscape.

And it’s all because of some plentiful precipitation produced toward the end of August by Hurricane Hone.

Most of the Big Island was under no kind of drought or even abnormally dry at the beginning of June, according to the U.S. Drought Monitor, which is produced via a partnership between the National Drought Mitigation Center at the University of Nebraska-Lincoln, the U.S. Department of Agriculture and the National Oceanic and Atmospheric Administration.

By Aug. 27, however, 73% of the island was experiencing abnormal dryness or moderate to severe drought. The worst was at the summit area of Mauna Kea and in the far southeast portion of Kaʻū.

Other places in the state, such as southwest Maui, were even seeing extreme drought conditions by late August.

As of Sept. 3, just 16% of the Big Island was abnormally dry, mostly the slopes of Mauna Kea and very far southeastern Kaʻū, or under moderate drought, at the Mauna Kea summit.

The estimated population in drought or abnormally dry areas was just 11,299 people. The island’s total population as of the 2020 Census was 200,629.

“Streamflows show evidence of the improvement of conditions leading to one-category improvements across the entire islands of Maui, Kaho‘olawe and the Big Island,” said the U.S. Drought Monitor in its Sept. 3 summary, adding the National Weather Service Climate Prediction Center also heavily favored an above-normal precipitation outlook for the next 6 to 10 days.

That period ends later this week.

Hurricane Hone’s impacts were felt around the island from Aug. 23-26.

Hone approached as a tropical storm and strengthened to a hurricane before making its closest pass to the island on the early morning of Aug. 25 and peaking as a Category 1 hurricane within 50 miles of South Point.

While it brought strong winds and high surf to many areas, knocking out power to more than 20,000 people throughout the state at one point, it was prolonged heavy rainfall from the storm’s outer rainbands that was the main story from Aug. 24-25.

The post-tropical cyclone report for Hurricane Hone from the National Weather Service forecast office in Honolulu shows that Hakalau received the highest rainfall total from the storm, recording 28.82 inches.

A rain gauge at the Saddle Road Quarry received 25.03 inches of rain. The Kūlani National Weather Radio rain gauge got 24.46 inches of rain and a gauge in Mountain View recorded 21.30 inches.

Rounding out the top 5 rainfall totals from Hone was the 21.04 inches received by a gauge in Waiākea Uka.

Rainfall totals ranged from 19.63 to 12.70 inches for the remaining 5 gauges in the top 10.

The top 10 highest rainfall totals around the island from the hurricane add up to 204.29 inches in just those 2 days. That’s only about 49 inches shy of the 126.72 inches of average annual rainfall in Hilo.

A manually recorded total from a Community Collaborative Rain, Hail and Snow Network rain gauge in Volcano Village near the summit of Kīlauea actually had the highest 2-day total on the island, getting a whopping 33.58 inches of rain from the hurricane.

Flooding along windward and southeast slopes of the island resulted in the closure of some major roads, including one of the most serious on Highway 11 in Wai‘ōhinu.

Other significant road closures included another portion of Highway 11 at Kāwā Flats and Cane Haul Road and Wood Valley Road near Pāhala.

North Kūlani Road near Mountain View was also closed because of a nearby water retention basin overflowing. Several secondary roads in rural areas of the island were shuttered because of flooding as well.

Fortunately, rainfall rates were mostly less than an inch per hour despite the impressive rain totals, helping mitigate even more serious flooding.

Wind shear — or a change in wind speed or direction — typically destroys tropical cyclones from above as they get closer to the islands. Hawai‘i tends to have a lot of wind shear because of prevailing northeast trade winds at the surface and southwest winds aloft.

“There was a brief period where shear relaxed and upper-level outflow allowed it to strengthen,” said National Weather Service forecaster Genki Kino in Honolulu. “Not sure how rare it is, but most of the time in Hawai’i we are protected by strong wind shear.”

It looks like all but 2 of the Big Island’s rain gauges were more than well above their normal monthly rainfall totals for August because of Hone’s passage.

Only the Koloko-Honokōhau and Laupāhoehoe gauges recorded less than 100% of their monthly averages, albeit they were 98% and 99% of normal, respectively.

The rest of the gauges were from 102% above average, which was at Hilo Airport with 11.56 inches compared with its usual 11.30 inches for the month, all the way up to 706% more than typical at the Silversword gauge on the upper northeast slopes of Mauna Kea, which received 21.10 inches of rain compared with its normal 2.99 August total.

Records were broken at Kahuā Ranch, which received 9.91 inches of rain last month compared with its normal 4.65 inches for August, 213% above its average, and in Honokaʻa, which had its highest August total since 1995 with 13.32 inches compared with its typical 6.42 inches for the month, more than 200% above average.

The monthly precipitation summary for August from the National Weather Service said the low number of records broken is because of many sites having their highest rainfall totals set in 2018 when Hurricane Lane drowned the island.

The highest monthly total for an automated rain gauge was the U.S. Geological Survey’s gauge at Kawainui Stream, which recorded 35.94 inches for the month, or 402% of its average 8.93 inches.

The highest overall August total went to the manually observed Volcano Village CoCoRaHS gauge with 36.14 inches.

The U.S. Geological Survey gauge at Honoliʻi Stream had the highest year-to-date total with 156.98 inches, or 103% more than its normal 152.53 inches by the end of August. It was followed closely by the Kawainui Stream gauge with 156.57 inches, which is 156% above its typical 100.26 inches by the end of the eighth month of the year.

Below-average year-to-date totals were recorded mainly in South Kohala and North Kona, as well as the Pōhakuloa region of Hāmākua.

Before the hurricane’s passage, persistent trade winds at mostly moderate to fresh levels prevailed.

A weak surface low-pressure trough Aug. 15, the remnants of tropical cyclone Emilia that dissipated far east of the state, moved across the islands and produced a brief uptick in rain; however, totals were mainly less than an inch.

Remnants of Tropical Storm Gilma passed just north of the state Aug. 30, producing enhanced rainfall along the east slopes of the Big Island, Maui, Molokaʻi and Oʻahu.

The windward slopes of Haleakalā on Maui saw the most rain from Gilma’s last breath, with an estimated 2 to 4 inches over the Keʻanae area.

Otherwise, the additional tropical moisture left over from Gilma and former Tropical Storm Hector created super muggy conditions on the Big Island to end the month.

As of 8 p.m. Monday, there were no tropical cyclone threats expected during the next 7 days in the Central Pacific.

There was an area of low pressure expected to form sometime during the middle of this week just south of the coast of southwestern Mexico that could gradually develop into a tropical depression as it moves north-northwest near the coast, but it doesn’t look like it will have any impact on Hawai’i any time soon, if at all if it moves away from the continent.

The National Weather Service forecast calls for daytime sea breezes and nighttime land breezes to bring clouds and showers over the island’s interiors during the day and clearing at night through midweek, with trade winds returning Thursday and through the weekend.

Forecast models show clouds and showers increasing with their return, and there’s the possibility of a typical summertime tropical trough setting up west of the state providing a bit of enhancement to those showers.

Showers would focus over windward and mauka areas.

The forecast from Harry Durgin, citizen meteorologist as well as administrator and founder of the Puna Weather group on Facebook, concurs.

“Today through Thursday continued mostly dry with comfortable humidity, especially considering it’s still summer,” posted Durgin late Monday morning. “Not much rain and only light to variable easterly trade winds. A turn to more humidity and also some scattered showers on Friday. This weekend we’re likely to be back to mostly dry with comfortable humidity.”

He said longer-range forecast models show more significant tropical moisture building in about a week.

Durgin also reported Friday morning that it doesn’t look like there are any tropical threats on the Big Island’s horizon despite wind shear forecast to be light and water temperatures quite warm to the southeast.

“Mid-level dry and stable air covers the region and is not a good environment for tropical storms, so we shouldn’t expect any risks in the short term,” he posted. “In the longer term [in a week or more] the [Global Forecast System] shows very high shear over the entire region which is also not good for tropical storms. This is all good news.”

While there’s no immediate threat, it’s not time to put planning for tropical cyclones on the back burner.

Hurricane season runs from June 1 through Nov. 30 in the Central Pacific, and the most active months are August and September.

Check out the hurricane preparedness information on the National Oceanic and Atmospheric Administration website and also refer to the resources available on the Hawai‘i County Civil Defense website, including the emergency and power outage preparedness information.