Weather

Big Island January Rainfall Summary

February 7, 2020, 8:16 AM HST
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January rainfall records were broken in various East Hawai‘i communities during a 72-hour period last month.

The Big Island experienced above average rainfall for most of the month of January, according to the National Weather Service. From Jan. 11-13, East Hawai‘i was drenched in rain, resulting in high runoff, road closures and and swift rivers and streams.

The USGS’ Saddle Road Quarry rain gage had the highest monthly total of 56.41 inches and the highest daily total 19.40 inches on Jan. 11. This overall total was part of the 32.59 inches of rain recorded during the 72-hour period ending at noon, January 13.

On Jan. 11, an atmospheric disturbance arrived over the island chain from the west. According to NWS, the new system had more jet stream support and greatly enhanced rainfall production over the east half of the Big Island.

Courtesy of National Weather Service

“Instead of bands of rainfall, the rainfall became anchored over the already saturated terrain with fewer breaks for runoff to drain away,” the NWS report states. “There were numerous road closures, especially in the South Hilo, Puna, and Ka‘ū Districts. At one point, access between the east and west sides of the Big Island was cut, with highways closed near Honoka‘a in the north, Kawa Flats in the south, and Saddle Road through the center.”

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One rescue occurred when a vehicle became swamped in a flooded road in the Eden Roc area. Rainfall tapered off as the upper atmospheric disturbance lifted northward and conditions stabilized on Jan. 12.

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“Records for the highest January rainfall were broken at Glenwood, Kahua Ranch, Laupahoehoe, Mountain View, Pi‘ihonua and Waiakea Uka,” the NWS report indicates. “What makes these record totals impressive is that they were set mainly with just half a month of rainfall since the latter half of the month was so dry.”

Hilo Airport and Pahala had their highest January totals since 2002. The Hakalau total of 46.09 inches was its second highest total of any month from a data record going back to 2004. The highest monthly total for all months at this site is 47.83 inches in August 2018, in large part due to rainfall from Hurricane Lane.

For the Ka‘ū District, this was one of the biggest rain events to affect the area since the record-breaking rainfall event on Nov. 1-2, 2000, the NWS report states.

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“Event rainfall totals were 10 to 15 inches from Pahala to Kapapala, which exceeded the totals that occurred from the impact of Tropical Storm Iselle in 2014.”

Rainfall totals for the 72 hours ending at noon, Jan. 13 included several readings of 10 to 20 inches over the South Hilo and Puna Districts of the Big Island, with a peak of over 32 inches at the U.S. Geological Survey’s (USGS) Saddle Road Quarry rain gage.

While the totals were not as high as during Hurricane Lane in August 2018, the runoff it produced was still very impressive, NWS states. The preliminary peak flow value at the USGS’ Wailuku River gage in Hilo was 52,500 cubic feet per second (cfs) at a stage value of 21.61 feet.

“This was the second highest flow value in the last 30 years,” the report states.

Peak flow from Hurricane Lane’s rainfall was 82,300 cfs (24.40 ft stage). At the USGS’ Honoli‘i Stream gage just north of Hilo, peak flow on the night of Jan. 11 had a preliminary value of 14,200 cfs (17.32 ft stage), the fourth highest in the last 30 years. By comparison, peak flow at this site from Hurricane Lane was 19,900 cfs (19.9 ft stage).

Following the wet first half of the month, the large scale weather pattern across the North Pacific changed significantly and resulted in much more stable conditions across the main Hawaiian Islands. During the last 10 days of the month, a surface high pressure ridge became established over or just north of the state, resulting in generally light winds and little rainfall overall.

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