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Hawai´i Rainfall Summary, October 2021

November 4, 2021, 1:24 PM HST
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The start of the October 2021-April 2022 wet season brought persistent trade winds across the main Hawaiian Islands through Oct. 18.

Trade wind strength was in the moderate-to-fresh range from Oct. 1 through Oct. 6, then intensified to the fresh-to-strong range from Oct. 7 through Oct. 14. It was during this period of stronger trade winds that the most significant rainfall of the month occurred.

On Oct. 11, an area of deep tropical moisture moved onto the east-facing slopes of the Big Island within the strong low level trade wind flow. This air mass was destabilized by the subtropical jet stream and a low pressure system aloft. Bands of moderate to heavy rainfall developed and moved over the windward Big Island slopes on the evening of Oct. 11 through the early morning hours of Oct. 12.

Rain gage and radar data showed a large area of 10- to 15-inch totals from the Upper Waiakea Forest Reserve northward to the Hakalau Forest National Wildlife Refuge. Some of the heavy rainfall went far enough upslope to produce multiple areas of runoff debris on the Mauna Kea Access Road, and even the brief closure of the road above the Hale Pohaku camp. The lower elevations in the South Hilo District, including the city of Hilo, had mostly minor flooding and a few partial road closures.

The windward slopes of Haleakala on the island of Maui was brushed by the area of rainfall, but the totals were considerably lower, within the range of 1 to 3 inches with no significant impacts.

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The second half of the month produced much drier conditions as strong low pressure systems far to the north and northwest of the state pushed the low level subtropical ridge close to, and even directly over, the main Hawaiian Islands. This resulted in lighter winds, more stable atmospheric conditions and low rainfall totals.

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The overall dry weather in most leeward areas kept drought conditions intact. Maui County, in particular, has been hit hard by drought over the past several months. For the latest drought information across the state, refer to the Drought Information Statement at https://www.weather.gov/hfo/DGT.

Island of Hawai´i

October rainfall totals from the Big Island’s windward slopes were near to above average, mostly due to wet conditions in the first half of the month. The rest of the island had mostly below average monthly totals with many at less than 30 percent of average.

The Papaikou Well gage had the highest monthly total of 20.66 inches (131 percent of average), followed closely by the USGS’ Saddle Road Quarry gage at 20.47 inches (191 percent of average). The Saddle Quarry gage also had the highest 24-hour total of 11.39 inches from noon on Oct. 11 to noon the following day. The heavy rain event on Oct. 11 and 12 also helped push the Hakalau monthly total to its highest October value since records started at this site in 2004. In contrast, the Honaunau and Kealakekua gages had their lowest October totals since 1995.

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Rainfall totals for 2021 through the end of October were near to above average at most of the gages on the Big Island. The USGS’ rain gage at Kawainui Stream now has the highest year-to-date total of 167.73 inches (147 percent of average), followed closely by Piihonua’s 167.44 inches (111 percent of average) and Glenwood’s 165.68 inches (87 percent of average).

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