WRRCTR No.97 Meteorological Aspects of the Oahu Flood of 19 April 1974

Abstract

A detailed study has been made of the severe Oahu rainstorm of 19 April 1974 and its implications for the Hawaii flood forecast problem. Conventional meteorological analyses were performed, using standard surface observations, aircraft reports, and satellite photography. Data from Oahu’s extensive recording rain gage network were analyzed to determine the mesoscale structure of precipitating cloud systems. Conceptual models of the flood-producing convective elements were then constructed. It was found that the synoptic scale situation was one which has historically generated flooding. Fresh trade winds in the lower troposphere were combined with a trough in the upper troposphere to produce thunderstorms over Oahu. Hourly rainfall analysis revealed two centers of high rainfall. One of these centers was produced by a continuous thunderstorm. A conceptual model of a continuous thunderstorm over Oahu was then formulated. Study of the special rain and stream gage network in Moanalua Valley showed that the current monitoring scheme of one telemetered rain gage per valley is inadequate. Finally, a discussion of the current state-of-the-art in Hawaiian flood forecasting and monitoring is offered. It is proposed that the installation of a meteorological radar is the only viable solution to the problem of early detection of flash flood potential.