Above Average Rainfall on Kona Slopes Reported in May

The Kona slopes had above-average rainfall last month while other areas on Hawai‘i Island were near to below average, according to the National Weather Service monthly rain summary.

According to the summary report, the Hawaiian Islands entered the first month of the 2021 dry season with nearly continuous trade winds, which created unstable conditions that produced enhanced trade wind shower activity.

Rainfall amounts of 1 to 3 inches along the slopes of the North Kona and South Kona Districts on the Big Island produced minor flooding during the afternoon of May 3.

The Kealakekua rain gage posted its highest May rainfall total on record with its 12.86 inches (240% of average) passing the previous record of 9.76 inches by a wide margin. This site also had the Big Island’s highest daily total of 2.28 inches on May 3.

Windward monthly totals were mostly in the range of 60 to 100% of average. Among the automated rain gage sites, Glenwood had the highest monthly total of 13.74 inches (86% of average). However, a manually read CoCoRaHS volunteer network gage at Holualoa in the North Kona District had a higher May total of 14.99 inches.

Conditions stabilized for most of the remainder of the month within moderate to fresh trade winds. A low-pressure system aloft moved over the eastern half of the state on May 21 and 22, which helped produce afternoon thunderstorms over the slopes of Mauna Loa on both dates. These thunderstorm cells dissipated rapidly and radar rainfall estimates were below 2 inches on each day.

Rainfall totals for 2021 through the end of May were near to above average at most of the Big Island gages. The main exceptions were along the slopes of the Kohala Mountains and in the Pōhakuloa region of the island where several year-to-date totals were 40 to 70% of average. Pi‘ihonua had the highest year-to-date total of 100.98 inches (131% of average).

Near the end of the month, an unseasonably strong low-pressure system far north of the state pushed the subtropical ridge of high pressure over the main Hawaiian Islands. This briefly eliminated the trades, allowing diurnal breezes to dominate local wind conditions. The ridge also brought stable conditions with little rainfall over most areas of the state.