Hawai´i Rainfall Summary, October 2021

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.

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.

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 Kaua´i

Windward rain gages recorded mostly near to above average rainfall for the month of October. The few windward sites that recorded below average totals were along the lower elevations, such as Anahola (37 percent of average) and Lihue Airport (45 percent of average). Leeward gages also posted below average monthly totals with most amounts at less than 50 percent of average. The U.S. Geological Survey’s (USGS) rain gage on Mount Waialeale had the highest monthly total of 30.74 inches (91 percent of average) and the highest daily total of 3.96 inches on Oct. 3.

All of the gages on Kaua´i had near to above average rainfall totals for 2021 through the end of October. Mount Waialeale had the highest year-to-date total of 403.23 inches (124 percent of average).