September rainfall totals were near to above average at most of the gauges across Kauaʻi, the monthly rain summary reports.
According to the National Weather Service in Honolulu, below-average totals were mainly along the lower leeward slopes from Hanapēpē to Waimea. The US Geological Survey’s (USGS) rain gauge on Mount Waialeale had the highest monthly total of 29.87 inches (99% of average). This gauge also recorded the highest daily total of 3.52 inches on Sept. 26 as an upper-level low moved into the area.
All of the gauges on Kauaʻi had near to above-average rainfall for 2021 through the end of September. Mount Waialeale had the highest year-to-date total of 372.49 inches (127% of average).
September provided an above-average number of trade wind days over the main Hawaiian Islands. According to the report, there were two brief breaks in the trade winds, both caused by weak low-pressure troughs moving westward to the north of the island chain.
“The resultant disruption to the trades allowed land and sea breezes to dominate local wind conditions,” NWS reported.
The first trough, which had a minimal impact on rainfall, moved into the area in late August and had its final day of influence on local weather on Sept. 1. The second trough disrupted the trades on Sept. 14 and 15, and produced brief heavy showers that resulted in minor flooding along the lower leeward slopes of Haleakala on Maui, and over the windward slopes of the Big Island.
The rest of the month included persistent trade winds blowing at moderate to fresh intensities statewide. Upper tropospheric low-pressure systems moving over and near the island chain helped increase the instability of the local air mass which enhanced rainfall activity. In fact, the highest daily rainfall totals for the Big Island, Kauai County, and Maui County all occurred during the last five days of the month.