Synoptic Analysis of the Epic Rainstorm in Kauai on 14–16 April 2018

A case analysis is presented of the extreme rainstorm in Kauai on 14-16 April 2018 in which over 30 inches of rain occurred in less than 48 hours along the northern coastline of the island. An incredible 49.69 inches of rain accumulated in 24 hours at Waipi, Kauai, on 14-15 April, which if verified will set the all-time national daily rainfall record for the United States. The previous national record was 43 inches in Alvin, Texas, on 25-26 July 1979, while the previous state record for Hawaii was 38 inches on 24-25 January 1956. Nearby stations at Wainiha and Hanalei, Hawaii, recorded 32.3 and 28.4 inches of rainfall. Flooding inundated the Wainiha and Hanalei river basins, with the gage height reaching just over 14 feet (6 feet above flood stage) on the Hanalei river prior to failure of the instrument. Damage to public property was in excess of $20 million and approximately 530 homes were damaged or destroyed. The rains triggered landslides that wiped out local highways and trails between Wainiha and Hanalei, with access to highways limited to regularly scheduled convoys even three months after the flood. The Ha'ena State Park and Napali Coast State Wilderness Park remain closed until bridges, highways, and trails can be repaired. The aim of this presentation is to present a synoptic analysis of the atmospheric conditions that led to this extreme event using a synthesis of conventional, satellite, and radar observations, model analyses, and ensemble forecasts. A comparison of the synoptic-scale flow regime for the 14-16 April rainstorm with previous events in Hawaii will also be presented.

Synoptic analysis revealed that anticyclonic wave breaking (AWB) in the north Pacific jet exit region resulted in positioning of an upper-level trough northwest of Kauai by 14 April. Enhanced tropospheric-deep water vapor over Kauai associated with a low-level easterly wave was forced to ascend in a region of isentropic lift and warm air advection in the entrance region of the subtropical jet on the southeast flank of the upper-level trough. A strong surface high pressure system north of Kauai, combined with a surface trough southeast of Kauai, resulted in 10-20 knot north-northeasterly upslope flow on the north and east side of Mount Waialeale. The moist low-level upslope flow, combined with southwesterly flow aloft, helped anchor intense thunderstorms on the northern part of Kauai within a broad synoptic-scale environment favorable for the development of convection. The convection was accompanied by unusually frequent lightning likely because of increased instability associated with anomalously cool temperatures aloft near the upper-level trough.

The surface trough southeast of Kauai appears to be connected to the southern end of a cold front extending southwestward from a midlatitude cyclone just west of the British Columbia coastline. This cyclone produced a rainstorm over the coastal northwest U.S. This result indicates that the Kauai and northwest U.S rainstorms occurred with the same synoptic-scale weather system, thereby suggesting that these events are connected extremes. Preliminary inspection of the previous daily record rainstorm for Hawaii on 24-25 January 1956 reveals a similar upper-level flow pattern with AWB and an upper-level trough northwest of Hawaii and a plume of enhanced water vapor extending from Hawaii to the western U.S. This emerging result suggests that the heaviest rainstorms in Hawaii may occur in similar synoptic-scale flow regimes, and that these regimes also may lead to simultaneous heavy rains in the western U.S.