Favorable Conditions for the Development of a Heavy Rainfall Event over Oahu during the 2006 Wet Period

Abstract

uring the 2006 wet period, as eastward-moving transient disturbances passed through a semipermanent low pressure system west of Hawaii, southerly winds east of the low strengthened bringing in higher than usual amounts of moisture from the deep tropics to Hawaii. All five heavy rainfall episodes during the wet period occurred during a southerly wind regime. Favorable conditions for the development of the Kahala Mall flood case on 31 March 2006 are examined. A high low-level ?e axis across Hawaii indicated the existence of convective instability over Hawaii. A transient midlatitude trough extending southward merged with the semipermanent subtropical trough. The tropopause folding associated with the deepening subtropical trough contributed to the spinup of the Kona low. The advection of high-PV air in the upper troposphere enhanced upward motion downstream over Hawaii. The Weather Research and Forecasting Model (WRF) simulation shows that latent heat release contributed to an eastward shift of the moisture tongue and enhanced moisture convergence at low levels. The horizontal distributions of instability indices, especially the K index, from WRF modeling can provide useful forecast guidance for the development of heavy rainfall. On 31 March, heavy rainfall occurred on the lee side of the Ko?olau Mountain Range with maximum rainfall at the summit as a convective line followed by an intense storm moved inland along the south shore and continued to advance northward through the range. As the convective cells moved across the mountain range, radar echoes intensified with deeper echo tops and higher vertically integrated liquid water content.