2021 March Rainfall Summary

March 2021 produced 11 flash flood events. Overall, it was the wettest March. The March 2006 totals set a very high bar so monthly rainfall records were broken at only a couple of locations.

Most of the heavy rainfall and 8 of the flash flood events took place from March 7 through March 13. The weather pattern over the area included a strong low pressure system aloft tapping into an area of enhanced low level moisture embedded within the trade wind flow. Heavy rainfall on March 7 affected the windward slopes of the Big Island and Haleakala on Maui with daily totals of 2 to 4 inches. Rainfall intensified on March 8 over both areas with daily totals of 8 to 14 inches.

Heavy rainfall shifted to Oahu and Kauai, and continued over the Big Island on Tuesday, March 9. The rainfall focused on the lower Puna District on the night of March 9. Highway 132 was briefly closed and there was a report of flooding in Hawaiian Paradise Park.

Although some enhanced rainfall occurred over portions of the state after March 13, atmospheric conditions slowly stabilized to help keep intensities in check and prevent significant flooding. Most of the rest of the month included trade winds over the main Hawaiian Islands. The main break in the trades occurred from March 23 through 25 as a cold front approached the state from the northwest. Low level winds veered in response, coming from a southeasterly through southerly direction. Pre-frontal convergence rain bands set up over Oahu, producing 1 to 2 inches of rainfall on March 24 but no significant flooding problems. The front itself stalled northwest of the main Hawaiian Islands.

Most of the March totals from the Big Island were above average. The exceptions mainly came from the slopes of the Kohala Mountains where several sites had below average totals. Out of the automated real-time sites, the Piihonua gage had the highest monthly total of 38.29 inches (192 percent of average) and the highest daily total of 10.01 inches on March 8. There was a CoCoRaHS manually observed site located on the slopes above Hilo that had a March total of 45.01 inches with a maximum daily total of 12.70 logged on the morning of March 9. Presumably, most of this daily total occurred on March 8, as was the case at Piihonua. The Pahoa gage posted its highest March total on record, beating the previous record of 29.68 inches set in 1991. Kealakekua had its highest March total since 1997, Laupahoehoe’s was the highest since 2004, and Kapapala Ranch, Kona Airport, Pahala, Pali 2, Piihonua, and South Point the highest since 2006.

Rainfall totals for 2021 through the end of March were near to above average at most of the gages on the Big Island. The few below average totals were mostly from the Waimea and Pohakuloa areas, and the South Kohala District. The Piihonua gage had the highest year-to-date total of 75.62 inches (166 percent of average).