Trade Winds Help Produce Enhanced Shower Activity in April

April put more of the “rainy” back in the 2021-22 rainy season.

According to the monthly rainfall summary prepared by Kevin Kodama, the 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.

April rainfall totals were near to above average over the east half of Kauaʻi, and mostly below average over the west half, according to Kodama’s report.

The U.S. Geological Survey’s rain gauge on Mount Waiʻaleʻale had the highest monthly total of 52.28 inches, or 139% of average, and the highest daily total of 7.31 inches on April 10. The gauge at Mākaha Ridge had its lowest April total since 2005.

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.”

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

Continuous trades also meant most leeward areas of the state had below average rainfall. The ongoing dry conditions kept significant drought in place over portions of Oʻahu, Maui County and the Big Island.

Nearly all rain gauge locations on Kauaʻi continued to report below average rainfall totals for 2022 through the end of April. Līhuʻe Airport is the only site that reported a near average total. The USGS rain gauge on Mount Waiʻaleʻale had the highest year-to-date total of 87.66 inches, or 70% of average.

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