Rainfall totals for the month of May on the Garden island were recorded below average, officials reported.
Several leeward locations between Hanapēpē and Kekaha had monthly totals below 50% of average, according to the National Weather Service monthly rainfall summary. The US Geological Survey’s (USGS) rain gage on Mount Waialeale had the highest monthly total of 27.03 inches (88% of average), and the highest daily total of 3.40 inches on May 22.
Līhue Airport’s 0.49 inches (24% of average) marked to lowest May rainfall total since 2009 and the fifth lowest May total on record for this site.
While April and May were generally dry, rainfall totals for 2021 through the end of May were above average across the island due to wet conditions in February and March. Mount Waialeale had the highest year-to-date total of 243.86 inches (157% of average).
May is the first month of the dry season for the Hawaiian islands. According to the summary report, last month an upper tropospheric disturbance near the state during the first week of May created unstable conditions that produced enhanced trade wind shower activity.
On May 6, heavy rainfall shifted to the west half of the state, with 1 to 2 inches occurring along the windward slopes of Kauai and the windward slopes of Oahu’s Koolau Range. As was the case with the events earlier in the week, there were no reports of significant flooding issues.
Conditions stabilized for most of the remainder of the month within moderate to fresh trade winds. A low-pressure system aloft moved over the east 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.
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.