NWS Monthly Summary: May Rainfall on Big Island Was Near to Above Average

Most Big Island rain gauges saw near to above average rainfall amounts last month, according to a report from the National Weather Service in Honolulu.

“The May through September dry/warm season started with a flash flood event,” NWS senior service hydrologist Kevin Kodama said in his monthly precipitation summary for May. “With fresh to strong trade winds blowing across the state, a low pressure system aloft produced unstable conditions which generated periods of heavy rainfall over the windward slopes of Maui and the Big Island from May 1-4.”

Thunderstorms with intense rainfall on the night of May 1 over east Maui caused significant washouts along Piʻilani Highway between Kīpahulu and Kaupō. Radar rainfall estimates of about 5 inches in a three-hour period occurred over the Kālepa Gulch area.

On the Big Island, four-day rainfall totals of 10-15 inches were recorded in portions of the South Hilo and Puna districts, with 5-10 inches in the North Hilo and Hāmākua districts. There were no reports of significant flood damage from the Big Island.

The U.S. Geological Survey rain gauge at Honoliʻi Stream had the highest monthly rainfall total of 26.69 inches, or 163% of average, and the highest daily total of 7.90 inches on May 3.

“This relatively new site is the closest automated gauge to the Big Island’s climatological rainfall maximum, so it will often have the highest monthly total,” Kodama said in the summary.

The Mauna Loa Observatory had its highest May rainfall total since 2002, and the Pōhakuloa Training Area Kipuka Alala, Waiākea Uka and Hilo Airport rain gauges had their highest May totals since 2006.

Not all of the island fared as well. Below average rainfall totals were mainly recorded in Kaʻū, where the Kapāpala Ranch had its lowest May total since 2012.

“Significant areas of drought continue to affect portions of the state, with the worst conditions in Maui County and the Big Island,” Kodama said in his report.

For more details, refer to the Drought Information Statement.

Most Big Island gauges had observed below average rainfall totals so far for the year through the end of May.

Among those that saw near to above average yearly totals, most were along the slopes of the South Kona District and in the South Hilo District. The rain gauge at Honoliʻi Stream had the highest year-to-date total of 81.16 inches, or 85% of average.

From May 16-20, a late-season cold front disrupted the trade winds and moved into the offshore waters northwest of Kauaʻi.

“In conjunction with this large-scale surface pattern, a disturbance aloft destabilized the rain bands embedded in the low level southerly flow,” Kodama’s report said. “These bands produced heavy showers over Kauaʻi and, to a lesser extent, over Oʻahu on May 19-20.”

However, despite models and radar estimates showing a threat of heavy rain, rainfall totals observed over Kauaʻi and Oʻahu were at most in the range of 1-2 inches. No flooding issues occurred during this event.

“On May 21, a more stable air mass settled in over the state, with light to moderate trade winds and generally benign weather persisting through the remainder of the month,” Kodama reported in the summary.