Future California precipitation projections possess significant uncertainty, with model projections yielding both wetter and drier trends. This uncertainty is due to both model differences and internal climate variability. The Community Earth System Model Large Ensemble Project (CESM LENS), with 40 realizations, is used to quantify future changes in California precipitation, and the importance of internal climate variability. Seasonal and annual mean 21st century California precipitation trends are examined, including the realization agreement on the sign of the precipitation trend. The important dynamical responses, including changes in storm track activity, are also investigated.

CESM LENS yields significant increases in 21st century annual mean California precipitation. Moreover, 100% of the realizations yield a positive precipitation trend. This high agreement between realizations illustrates internal climate variability is a small portion of the uncertainty in California precipitation projections.

The December-January-February (DJF) season yields the largest increase in California precipitation. Consistent with the importance of east Pacific extratropical cyclones to DJF California precipitation, CESM LENS projects a significant increase in east Pacific extratropical cyclone activity near the California coast. Furthermore, CESM LENS shows a southeastward shift of the upper level winds in the east Pacific, which act to steer more storms toward the California coast. These results suggest California will likely become wetter with continued greenhouse gas emissions.