NWS Reports Above-Average Rainfall for Much of Big Island in September

Much of Hawaiʻi Island experienced near to above-average rainfall totals over the past month, according to the National Weather Service monthly rain summary.

In the month of September, the Hāmākua, North Hilo, South Hilo, Puna, and South Kona Districts were mostly near- to above-average in rainfall while the rest of the districts on the Big Island had mostly below-average totals.

According to the report, the USGS’ rain gage at Kawainui Stream had the highest monthly total of 14.51 inches (196% of average), followed closely by the 14.01 inches (88% of average) recorded at Papaikou Well. The highest daily total was 4.08 inches at the Hōnaunau gage on Sept. 27.

Big Island rainfall totals for 2021 through the end of September were near to above average at most of the gages. The Piʻihonua rain gauge had the highest year-to-date total of 149.62 inches (109% of average). The year-to-date totals from Kawainui Stream (147.85 inches, 137% of average) and Glenwood (146.94 inches, 85% of average) were not far behind.

September provided an above-average number of trade wind days over the main Hawaiian Islands. The report indicates, 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. 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. 14-15 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. 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.

The highest daily rainfall totals for the Hawaiʻi Island, Kauaʻi and Maui Counties all occurred during the last five days of the month.