April storm on Garden Isle brought record-breaking rainfall totals

Kaua‘i saw record-breaking rainfall totals after a rain event in April.

According to the monthly rainfall summary from the National Weather Service in Honolulu, the mid-April heavy rain event resulted in above-average totals over the entire island of Kauaʻi. Rain gauges near the town of Waimea recorded monthly totals greater than 10 times the April average.

Records for the highest April rainfall were broken at the Anahola, Hanapēpē, Kalāheo, Līhuʻe Airport, Līhuʻe Variety Station, ʻŌmaʻo and Wailua UH Experiment Station gages.

Rain was brought on due to the development of a Kona low northwest of the main Hawaiian Islands starting April 10. The location of the low-pressure system resulted in a veering of the surface winds from the usual east-northeasterly trades to southeasterlies and southerlies.

Conditions aloft became favorable for heavy rainfall over Kauaʻi on the evening of April 11 with strong thunderstorm cells moving over the island from the southwest.

The 15-minute data from rain gages along the southern and southeastern sides of the island indicated rates exceeding 4 to 5 inches per hour. Periods of intense rainfall continued through the night and into the morning of April 12.

By the end of the event, several gages recorded 10 to nearly 13 inches of rain in 12 hours. These totals have a probability of occurrence of less than 1% annually over south Kauaʻi. Radar data indicated stronger cells over the coastal waters just south of the island, with 12-hour radar rainfall estimates of 14 to 15 inches.

At the Kalāheo gage, the 12.68 inches of rainfall over the 12-hour period ending at 5 a.m. on April 12 exceeded the monthly rainfall record for this site. The previous April rainfall record at this location was 8.41 inches in 2001.

A higher 12-hour rainfall total of 12.80 inches for the period ending at 6 a.m., April 12 was recorded at the Līhuʻe Variety Station gage. This amount of rain over a 12-hour period has an annual probability of occurrence of less than 0.2%.

The Līhuʻe Variety Station gage also had Kauaʻi’s highest 24-hour total of the month with 13.10 inches of rain for the period ending at 11:45 a.m. on April 12. The USGS’ Mount Waiʻaleʻale rain gauge had the highest monthly total of 58.23 inches (154% of average).

The intense rainfall produced flash flooding that caused significant damage within drainage basins from Wailua to Kōloa. Numerous homes received varying levels of damage, and several residents were evacuated. Many roads were also closed due to flooding and landslides.

Heavy rainfall also spilled over the mountains into the drainages on the north side of the island. As is often the case in these types of events, the Hanalei River overflowed its banks and inundated Kūhiō Highway for several hours. Fortunately, there were no injuries reported from the flooding event.

Another round of heavy rain occurred over Kauaʻi on April 14 as the dissipating kona low filled and exited the area to the northeast. Rainfall was less intense during this episode since the system was weakening and moving relatively quickly across the area. Radar and rain gauge data indicated widespread amounts of 2 to 4 inches over Kauaʻi, and 1 to 2 inches over Oʻahu.

Despite the lower amounts, the fully saturated ground over south Kauaʻi resulted in flash flooding once again, though the impacts were mainly limited to brief road closures and small landslides. Impacts on Oʻahu were limited to minor flooding over the west half of the island.

At the end of March, all of the Kauaʻi gages had near- to below-average rainfall for 2024. The recent rainy conditions have pushed all totals for 2024 through the end of April into the near-to-above-average range. The Mount Waiʻaleʻale rain gauge had the highest year-to-date total of 128.49 inches (103% of average).