West Nile virus [WNV] invaded the SW USA during the summer of 2003, successfully over wintered and then amplified to outbreak levels during 2004, with epicenters in Phoenix, Arizona, and Los Angeles, California. WNV transmission remained at low enzootic level in SE California which lacks corvid populations, but rapidly amplified to epidemic levels after invading the Bakersfield area of the southern San Joaquin Valley during 2004. WNV rapidly dispersed throughout California during 2004, invading biomes never considered at risk for encephalitis virus transmission. Epidemic transmission in northern California followed during 2005 and is compared to the epidemic in the SW during 2004. The current paper identifies the principal avian and mosquito components of the amplification cycle and discusses factors responsible for the success of this invading virus.