Los Angeles County saw a sudden and potentially dangerous shift in weather Monday as a colder storm system swept into the region, bringing heavier rain than forecast along with thunderstorms, strong winds and increased flooding concerns,

The Bay Area and Monterey Bay was being pounded by heavy rain and strong winds as an atmospheric river storm passed through the region Thursday.

A powerful low-pressure system is forecast to bring heavy rain, gusty winds and possible thunderstorms to the Los Angeles area beginning Thursday night and continuing through the weekend, with mountain areas potentially receiving up to 4 inches of rainfall,

The storm has brought flooding and downpours, with some areas expecting up to a month's worth of rain in a day.

An upper level disturbance will draw a thick blanket of high clouds over the state while destabilizing the atmosphere. Rainfall will increase late Monday, with the highest chances for heavy rainfall late Tuesday afternoon or early evening into Wednesday, especially for the eastern end of the island chain.