In April 2018, a series of thunderstorms produced record-breaking rainfall on the Hawaiian Islands of Kauaʻi and Oahu. An upper-level low moved across the area on April 13, generating a mesoscale convective system that moved over eastern Oahu, producing localized heavy rainfall that reached . The heaviest rainfall occurred on northern Kauaʻi. There, a rain gauge owned by the Waipā Foundation, just west of Hanalei, recorded of rainfall in the 24 hours between 12:45 p.m. on April 14 and 15. This was the greatest 24-hour rainfall total on record in the United States, surpassing the previous record of in Alvin, Texas on July 25–26, 1979, set during Tropical Storm Claudette. Through the event, the same gauge recorded a total of of rain.
The heavy rainfall produced flash flooding and landslides that covered roads and washed away several vacant houses. The floods damaged or destroyed 532 houses. Public damage totaled $19.7 million and overall damage was estimated at over $125 million. Damage on Kauai was the worst from a natural disaster since Hurricane Iniki in 1992. Landslides and flooding covered parts of Kuhio Highway.
Meteorological history
On April 11, 2018, an upper-level low was located west of the Hawaiian island of Kauaʻi. The low moved toward Hawaii and drew deep tropical moisture from the south. The low eventually opened into a trough, which crossed over the Hawaiian islands on April 13. That day, a mesoscale convective system (MCS) developed northwest of Molokai and intensified while moving westward through the trade winds. The MCS moved over eastern Oahu, dissipating after moving ashore. Another upper-level low developed on April 14, fueling additional rainfall through April 16, which caused record rainfall on Kauaʻi. The first of a series of thunderstorms moved over northern Kauaʻi around midnight on April 14. After a five-hour break in rainfall, another set of thunderstorms hit northern Kauaʻi around midnight on April 15 and lasted about eight hours. The third set of thunderstorms began around 10 AM on April 15 and continued until around 7 PM.
Impact
The Honolulu National Weather Service issued various flash flood watches and warnings. The trough began dropping heavy rainfall over Hawaii on April 13. Floods damaged or destroyed 532 houses. The flooding knocked down trees and power lines, eroded many bridges and roads, and washed away crops, beehives, and livestock. Overall damage was estimated at over $125 million. Damage to public buildings was estimated at $19.7 million. Despite the damage, there were no deaths or injuries.
Oahu
Across eastern Oahu, the mesoscale convective system produced rainfall rates of about per hour. Over a 15-minute period, a rain gauge in Niu Valley recorded of precipitation, which has a 4–10% annual return period. The same station recorded of rainfall over a two-hour period, which has a 0.5–1% annual return period. The rainfall on Oahu was largely limited to the eastern one-third of the island. A station at Daniel K. Inouye International Airport recorded of rainfall, just west of Niu Valley, which recorded during the same 24-hour period.
At Waimanalo Stream north of Niu Valley, the rains caused the waters to rise in 30 minutes, and in an hour. During the rain event, Wailupe Gulch reported an rise in floodwaters. Flooding entered cars and hundreds of houses on Oahu. The floods also caused significant runoff, leaving a layer of debris on roads. A portion of Kalanianaole Highway was covered with nearly of water, closing a stretch of the highway from Aina Haina to Makapuʻu. A mudslide covered a portion of the same highway near Kailua. Downed trees closed one block of Maunaloa Avenue in Kaimuki.
