Instability Ushered in by Trade Winds Causes Rainy April on Big Island

April definitely put the “rainy” in the 2021-22 rainy season.

According to the monthly rainfall summary prepared by Kevin Kodama, senior service hydrologist at the National Weather Service in Honolulu, last month saw uninterrupted trade winds with above average speeds, with several low pressure systems aloft throughout April that increased instability over the islands. That resulted in periods of enhanced shower activity, especially along windward slopes.

Kodama said in his report that windward Big Island rain gauges posted April totals that were mostly near to above average. April was the last month of the wet season, which began in October.

The most significant rainfall event of the month was produced by a low pressure system that moved over the state April 16. The system generated enough instability to help enhance rainfall within the moderate to fresh trade winds affecting the islands at the time.

“The highest rainfall totals were over the windward slopes of the Big Island, with 2-6 inches observed in the North Hilo, South Hilo and Puna districts,” Kodama’s report said. “East Maui received 2-5 inches of rainfall, and the windward slopes of Kauaʻi and Oʻahu had totals of 1-3 inches.”

There were reports of flooding that weekend in Hilo along Kamehameha Avenue and rocks on the roadway in all three gulches along Highway 19 in Hāmākua. The National Weather Service issued a flood advisory, but later canceled it. There also was a winter storm warning issued for the summits of Maunakea and Mauna Loa.

Fortunately, there were no significant flooding impacts from the heavy rainfall, according to Kodama’s report.

The U.S. Geological Survey rain gauge at Kawainui Stream had the highest monthly rainfall total for April of 33.96 inches, or 218% of average. However, the Papaikou Well gauge had the highest daily total for the month of 6.21 inches on April 16.

The Hilo airport and Honokaʻa rain gauges recorded their highest April totals since 2004. But while it was a wet month in Hilo, the 15.67 inches recorded at the airport, which was 167% of its average, fell well short of the April record of 43.24 inches set in 1986.

The Kawainui Stream gauge had the highest year-to-date total of 55.63 inches, or 101% of average, among automated sites. However, the Wainaku CoCoRaHS gauge had the highest year-to-date total among all sites, with a manually read 56.85 inches.

The continuous trades also meant most Big Island leeward areas had below average rainfall. Despite that, most of the rain gauges in Kona had near to above average rainfall totals for the month. But many of the Kaʻū and South Kohala gauges recorded below average totals.

Big Island rainfall totals for the year through the end of April were still below average at most gauges, according to Kodama’s report. The ongoing dry conditions kept significant drought in place over portions of the Big Island.

For additional rainfall totals from around the Big Island and throughout the state for the month of April, click here.