Comparison of flooding in adjacent mountainous coastal watersheds during a land falling Pacific winter storm

A major flash flood that occurred during a landfalling Pacific winter storm is explored in detail. Two adjacent watersheds (150 and 270 km2), one rural and one heavily populated, experienced major flooding. However, the stream in the heavily populated area experienced a 6- h long decrease in water depth before the most severe rise and ultimately experienced its second- highest flood of record, while the stream in the adjacent rural watershed continued to rise throughout the period and experienced its greatest flood of record. The causes of this important difference are explored using data from coastal boundary layer profilers and a NEXRAD site. It is shown that the lull in the storm over the area with the most societal vulnerability was due to a small directional wind shift that moved the rain shadow associated with an upstream mountain range. Two wind profilers in the area both documented the same wind shift and established that the direction shifted by only 13 degrees. Rain gauge, NEXRAD and wind profiler data are used to establish the orographic nature of the heavy warm-sector precipitation in this event. These watersheds are rather well situated with respect to the local NEXRAD site and QPE from NEXRAD is compared with observed rain fall and stream flow.