Synoptic analysis of the epic rainstorm in Kauai on 14-16 April 2018

Madelyn B. Powell, Thomas Galarneau, and Eric Betterton

Hydrology and Atmospheric Sciences
The University of Arizona

The extreme rainstorm that occurred in Kauai on 14-16 April 2018 is presented using a synoptic and mesoscale ingredients-based approach for heavy rainfall. A staggering 46.69 inches of rain fell in 24 hours on 14-15 April at Waipa Gardens, Kauai, setting the newly certified national 24 hour rainfall record, surpassing the previous record of 43 inches at Alvin, Texas, on 25-26 July 1979. This record breaking rainstorm flooded both the Hanalei and Wainiha river basins, producing flows that caused multiple stream gauge failures. Damage to public property is in excess of $20 million and nearly 530 homes sustained damage or were destroyed. Synoptic analysis revealed that an upper-level trough in the north Pacific jet exit region was positioned northwest of Kauai by 14 April. The upper-level cold trough was positioned above and just west of a low-altitude warm moist tropical air mass, resulting in anomalously low bulk column stability. A strong surface anticyclone positioned north of Kauai helped drive moist upslope northeasterly flow on the north and east side of Kauai’s Mount Waialeale. The focused upslope flow provided a lifting mechanism for the continuous redevelopment of sustained heavy-rain-producing thunderstorms in a moist, unstable environment on the north side of Mount Waialeale.

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