As a wet and windy storm blows in off the Pacific, a new San Jose-based radar system is watching it with the greatest precision ever, estimating rainfall in individual communities rather than providing a more general Bay Area forecast.

Discrete patches of incoming moisture are detected by a humming “X-band” radar unit on the rooftop of a treatment plant owned by the Santa Clara Valley Water District, predicting precipitation and flood risk with much greater accuracy than current technology. Four more units will be phased in over the next five years for the Peninsula, East Bay and North Bay.