SAN FRANCISCO (KRON) -- A powerful atmospheric river storm driven by a bomb cyclone moved into the San Francisco Bay Area overnight Wednesday. The storm brought heavy rain to the North Bay Wednesday, and a flood watch continues for the area through 4 a.m. Saturday. Starting Friday, the atmospheric river is expected to shift south, bringing the heaviest of rain “directly to the core of the Bay Area,” according to KRON4 Meteorologist John Shrable. What is a bomb cyclone? The National Weather Service urged residents to be prepared by cleaning leaves and debris from storm drains, checking to make sure cars are in good shape for weather and preparing for longer commutes. Officials remind motorists to never drive through flooded roadways. By the end of the week, parts of the Bay Area could see a month's worth of rain in just a matter of days, according to forecasters. Bay City News contributed to this report. Follow our live blog for the latest updates