Content area
Full Text
1. Introduction
A series of deep convective cells formed over Oahu’s Ko‘olau Mountain Range (Fig. 1) on 2 April 2006, leading to a quasi-stationary thunderstorm complex that persisted for over 6 h. Ensuing flash floods in many of the northeast-facing watersheds of eastern Oahu caused a road closure and precautionary evacuations. The event exhibited a pattern of precipitation with extreme gradients and rainfall totals of over 225 mm recorded by gauges along the crest of the Ko‘olaus.
The Oahu flash flood occurred at the end of an exceptionally wet period in the Hawaiian Islands that began in mid-February 2006 (Nash et al. 2009). This extended wet period saw a prolonged synoptic blocking pattern over the North Pacific and the formation of numerous low pressure systems to the west of the state. Heavy rainfall occurred throughout the island chain with record-setting amounts measured by gauges on the islands of Kauai and Oahu, and seven deaths resulting from the breaching of Kauai’s Ka Loko Dam on 14 March. Of the many heavy rain events that occurred on Oahu during this period, three cases were accompanied by southeast low-level flow and a pattern of precipitation with extreme gradients and maxima concentrated along the crest of the Ko‘olaus (Figs. 2 and 3). Patterns of precipitation of this type are not unique to the spring 2006 wet period; in fact, they are often found on Oahu in association with southeasterly low-level flow (K. Kodama, NWSFO-HFO, 2008, personal communication).
Heavy rain events present a significant hazard to the Hawaiian Islands and have proven challenging to forecast (Kodama and Businger 1998). Lack of in situ observations combined with mountainous terrain and small watersheds with rapid response times make prediction of flash floods especially difficult (Blumenstock and Price 1967; Schroeder 1977). One of the earliest studies on flooding events in Hawaii was performed by Haraguchi (1977). His study examined 11 heavy rain events on Kauai, Oahu, and Maui with 24-h rainfall totals ranging between 150 and 521 mm. All of the events occurred under one of four general synoptic situations: 1) kona storms, 2) cold fronts–shearlines, 3) upper-level troughs, or 4) tropical systems. The events were found to have 1) maximum rainfall on slopes facing the low-level winds, 2) been associated with thunderstorms,...