Hawaii gets a temporary reprieve

The rains have stopped for the past day on Hawaii's Kauai Island, where over 100 inches of rain has fallen in the mountains in the past three weeks. The heavy rains are due to a persistent upper-level low pressure system, a common occurence in the Central Pacific during a La Nina event like the region is now experiencing. The rains moved away from the islands yesterday, which have been under a continuous series of flash flood warnings and watches for weeks. The improved weather will aid in the search for victims of the failure of an earthen dam that burst on Kauai Tuesday. When the dam broke, a 20-foot flood of water moving at 15-20 mph smashed through a populated area, killing two and leaving six people missing and presumed dead. If the missing people are declared dead, this week's disaster would be the deadliest weather disaster in Hawaii's history.

Figure 1. Radar estimated precipitation on Kauai from March 13 - March 17. The radiating rays to the north are caused by mountains that block the radar beam, and the actual precipitation on the northern part of the island is greater than shown here.

Hawaii is probably the safest state as far as deaths from weather-related causes go. High surf is the leading cause of weather-related deaths in Hawaii, with 92 deaths between 1960-2004. Flash floods have killed 46 people in Hawaii in the past 45 years, high wind, six people, and hurricanes, six. Prior to this week's flood disaster, the previous most deadly flood disaster on Hawaii occurred on December 14, 1991, when a rainstorm dropped over 20 inches of rain in 12 hours on Kauai, causing five deaths, intense flooding, bank failures, erosion, and slides, with more than $5 million in property damages. Hawaii's only billion-dollar weather disaster occurred in 1992, when Hurricane Iniki passed directly over Kauai as a Category 4 hurricane, killing 6 and causing over $3 billion in damage. Hurricanes are fairly rare in Hawaii--only three hurricanes have brought hurricane-force winds over the islands since 1957. The National Climatic Data Center lists Hawaii as having the second fewest billion dollar weather disasters of any state since 1980 (one). Only Alaska has had no billion-dollar weather disasters.

The 100+ inches of rain that fell on Kauai's Mt. Waialeale the past three weeks has not set any records yet--Mount Waialeale averages 424 inches of rain per year, and is second only to the the monsoon-drenched Himalaya Mountains of India as the wettest spot on Earth. Mt. Waialeale recorded 683 inches of rain in 1982. The U.S. record of 704 inches in a year was set the same year at nearby Maui Island's Puu Kukui, at 5,788 feet elevation. The Hawaiian state 24-hour rainfall record is 38 inches at Kilauea Plantation on the island of Kauai, on Jan 24-25, 1956. Mt. Waialeale has three factors that help it catch more rain than other locations in the Hawaiian islands (thanks, Wikipedia):

1) Its northern position relative to the main Hawaiian Islands provides more exposure to frontal systems that bring rain during the winter.
2) It has a relatively round and regular conical shape, exposing all sides of its peak to winds and the moisture that they carry.
3) Its peak lies just below the so-called trade wind inversion layer of 6,000 feet (1,800 m), above which trade-wide-produced clouds cannot rise.

The long range outlook for Kauai and the rest of the Hawaiian islands is not good--another upper-level low pressure system is expected to move over the islands Saturday, bringing an increased chance of flash flooding. The rains are part of a typical La Nina weather pattern for the Hawaiian islands which is expected to continue for several months, according to the latest seasonal outlook issued yesterday by the Climate Prediction Center.

Jeff Masters