The Impact of a Prominent Rain Shadow on Flooding in California's Santa Cruz Mountains: A CALJET Case Study

Data collected during the California Land-falling Jets experiment (CALJET) are used to explore the causes of significant variations in flood severity in adjacent coastal watersheds within the Santa Cruz Mountains during the storm of 2-3 February 1998. While Pescadero Creek (118 km2; rural) experienced its flood of record, the adjacent San Lorenzo Creek (275 km2; heavily populated), attained only its fourth highest flow. This difference resulted from conditions present while the warm sector of the storm, with its associated low-level jet, high water-vapor content, and weak static stability, was overhead. The rainfall in the warm sector was dominated by orographic forcing. While the wind speed strongly modulated rain rates on windward slopes, the wind direction positioned the edge of a rain shadow cast by the Santa Lucia Mountains partially over the San Lorenzo basin, thus protecting the city of Santa Cruz from a more severe flood. Roughly one-third of the flow at flood peak on Pescadero Creek resulted from the warm-sector rainfall. Without this one-third contribution, the peak flow on Pescadero Creek would not have attained record status. These case-study results are complemented by a climatological analysis based on ~50 yr of streamflow records for these watersheds, and two other nearby windward watersheds situated ~20-40 km farther to the east (see follow-on presentation by Andrews et al.). These case study and climatological results illustrate the importance of conditions in the moisture-rich warm sector of landfalling Pacific winter storms. In particular, variations of ñ10 degrees in wind direction can modulate the location of floods. While terrain can increase predictability (e.g., rainfall typically increases with altitude), the predictability is reduced when conditions are close to a threshold separating very different regimes (e.g., in or out of a rain shadow).