Rainfall totals above average on Big Island for month of May

Big Island rainfall totals were near to above average at most of the rain gages throughout the month of May.

Conditions favorable for heavy rainfall centered on the Big Island on May 10, with strong thunderstorms developing over the Hilo area during the mid-afternoon, according to the National Weather Service. Intense rainfall at 3 to 4 inches per hour produced flash flooding in downtown Hilo.

Waiānuenue Avenue and Kamehameha Avenue were briefly closed in both directions, and numerous Bayfront area businesses received flood damage from deep water ponding. By sunset, the thunderstorms weakened and moved offshore to the northeast.

While the weather appeared to settle down, the event was not over for the Big Island. By 9 p.m., heavy rainfall initiated once again, this time along the slopes of the South Kohala District on the northwest side of the island. The heavy rainfall occurred within a roughly northeast-to-southwest oriented line that moved slowly toward the northwest into the North Kohala District, then southward to the Puako and Waikōloa Beach areas.

Rainfall totals of 3 to 6 inches were observed from Waimea to Kawaihae, and flash flooding closed the Queen Kaʻahumanu Highway (Highway 19) at the Kawaihae Road junction for several hours.

By May 11, the most unstable conditions shifted westward resulting in heavy rainfall over Oʻahu and Maui.

The Honokaʻa rain gauge was a notable outlier at just 27% of average, but its May total of 1.78 inches was consistent with the lack of normal trade winds and CoCoRaHS network totals in the area. The USGS’ rain gage at Honoliʻi Stream had the highest monthly total of 32.75 inches (199% of average), and the highest daily total among the automated gages with 4.83 inches on May 12.

The highest overall daily total was 6.19 inches on May 13 from the CoCoRaHS observer near Kurtistown.

The Pāhala rain gauge had its highest May total since 1965. Other notable totals came from Kamuela Upper and Kapāpala Ranch, which had their highest May totals since 1998 and 2002, respectively.

Rainfall totals for 2024 through the end of May were near to above average at most of the gages on the Big Island. Year-to-date totals from the Humuʻula Saddle/Pōhakuloa regions were lagging with most of these sites having totals in the below-average range. The Honoliʻi Stream rain gage had the highest year-to-date total of 100.83 inches (106% of average).

The atmosphere over the island chain stabilized a bit on May 14 and 15 as the low-pressure system aloft weakened. This break in the heavy rainfall was short-lived as a stronger low-pressure system developed northwest of the main Hawaiian Islands and formed a kona low on May 16.

The kona low weakened to the northwest of the Kauaʻi on May 19. However, additional low-pressure systems, including another kona low on May 23, continued to develop far to the west and northwest. While these systems did not significantly impact the state, it kept the winds slightly veering at an easterly to east-southeasterly direction through May 29.
More normal east-northeasterly trade winds did not return until May 30.