Observed precipitation in Northern California has been greater than normal during most, but not all, recent El Nino events. For example, the most recent El Nino year (1997-1998) was the wettest in 50+ years in San Francisco; by contrast, the previous El Nino year (1991-1992) was drier than normal. Our goal is to understand the cause of these varying regional precipitation responses to El Nino. Using a high-resolution global model of the atmosphere, we performed ensembles of simulations of the 1991-1992 and 1997-1998 winters. All simulations used observed sea-surface temperatures (SSTs) as boundary conditions; different ensemble members used different atmospheric and land surface initial conditions. We find that 9 of 10 simulations of 1997-1998 are wetter than normal in Northern California (as observed), and 1 of 10 is very dry. By contrast, only half the simulations of 1991-1992 are wetter than normal. This suggests that, while there is strong chaotic variability, the varying precipitation responses to El Nino in Northern California are to a significant extent predictable; i.e. result from varying patterns and/or amplitudes of SST anomalies.