Northern Monterey Bay and Southern Monterey Bay and Big Sur Coast as well as San Francisco County are included in a coastal flood advisory issued by the National Weather Service on Wednesday at 8:07 p.m. A flash flood warning is issued when a flash flood is either imminent or already occurring. Flood warning: Take action! A flood warning is issued when flooding is imminent or occurring. Flood advisory: Be aware:A flood advisory is released when flooding is not expected to reach a severity level necessitating a warning.