A major flash flood occurred with a land-falling Pacific winter storm in California's coastal Santa Cruz Mountains on 2-3 February 1998 during the California Land-falling Jets (CALJET) experiment. Two adjacent watersheds, one rural and one heavily populated, experienced major flooding. However, the stream in the heavily populated watershed experienced a 6-h-long decrease in depth before ultimately rising to its second highest mark on record, while the stream in the adjacent rural watershed continued to rise throughout the period and it experienced the greatest flood of record. A suite of wind profilers, an operational NEXRAD radar, and a network of rain gauges are used to establish the orographic nature of the precipitation. It is concluded that the lull in the storm over the area with the most societal vulnerability was due to a rain shadow associated with an upstream mountain range to the south.

Using 50 years of stream-flow data from California's coastal mountains, together with the compositing of 925 mb geopotential heights from the NCEP/NCAR global reanalysis database, it is demonstrated that the varying impact of the rain shadow observed across the Santa Cruz Mountains during the single case study was also apparent when examining flooding events whose exceedance probabilities were less than 0.2, i.e., all flooding events whose discharges exceeded the magnitude of a 5-year flood. Specifically, flooding events on the eastern side of the Santa Cruz Mountains required low-level flow that was more westerly than during flooding events on the western portion of these mountains. The phase of the El Niņo-Southern Oscillation (ENSO) also impacted the probability of flooding on the eastern and western sides of this mountain range. The low-level flow was more meridional during the warm phase of ENSO (i.e., El Niņo) and more zonal during the cold phase (i.e., La Niņa). Hence, the eastern Santa Cruz Mountains were anomalously dry due to rain shadowing during El Niņo years and anomalously wet during La Niņa years, an ENSO relationship that is opposite relative to the other coastal sites in central and southern California.