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Rainfall Summary for April 2021

May 8, 2021, 2:00 PM HST
* Updated May 8, 11:38 AM
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Most of the April rainfall totals across the Big Island were near to below average. According to the National Weather Service’s monthly rainfall report, the only above-average totals were along the Kona slopes and in the Kahuku portion of the Ka‘ū District.

Although the past month was drier than average across most of the Big Island, rainfall totals for 2021 through the end of April were near to above average at most sites.

The notable exceptions were along the slopes of the Kohala Mountains. This region did not receive the drenching that affected other parts of the state in March. The highest year-to-date total was 90.20 inches (142% of average) at Piihonua.

Many of the sites had April totals in the range of 40 to 70% of average. Several sites in the South Kohala District and the leeward side of the North Kohala District had totals below 40% of average.

The Pi‘ihonua gage had the highest monthly total of 14.58 inches (80% of average) and the highest daily total of 5.51 inches on April 5.

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Following the wettest March since 2006, April conditions statewide turned out to be much drier overall. There were no damaging flash flood events and only a handful of heavy rain events that produced minor flooding issues.

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Most of the days included near-surface winds from a northeasterly to easterly direction, with light to moderate intensities. Interestingly, the drenching across most of the state in March 2006 was also followed by a dry April.

From April 3 through April 5, the remnants of a cold front cloud band embedded within fresh to strong trade winds produced the wettest day of the month at several locations. Peak daily totals of 3 to 6 inches were recorded along the windward slopes of the Big Island and Maui, with isolated amounts of 1 to 2 inches from Moloka‘i to Kaua‘i.

Since the rainfall was spread out over time, there were no significant flooding issues.

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The following week, an upper-level trough near the main Hawaiian Islands generated unstable atmospheric conditions. The instability helped produce pockets of heavy rainfall on April 11 and 12 over O‘ahu, Maui, and the Big Island. While there were instantaneous rain rates in excess of 2 inches in a few spots, rain durations were short, which limited event totals and mitigated flooding.

Lastly, a weak, late-season cold front reached Kaua‘i on April 28 before dissipating near O‘ahu later that day. Enhanced rainfall produced minor flooding over the windward slopes of the Ko‘olau Range on O‘ahu, with peak totals of 2 to 4 inches from Maunawili to Kaneohe. This was the last cold front of October 2020 through April 2021 wet season.

Cold fronts reaching the island chain in May are rare.

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