All parts of the UK will get wet weather, with western and hilly areas in line to receive the biggest drenching as Storm Aiden moves in from the west, said the Met Office.
Most of Lancashire has a yellow warning for rain, while parts of Fylde have also been issued with a warning to expect strong winds.
Met Office deputy chief meteorologist Martin Young said: “As the heavy rain and strong winds from Storm Aiden sweeps eastwards on Saturday, another system quickly follows on Sunday that contains the remnants of ex-Hurricane Zeta, bringing further heavy rain and strong winds.
“We’re urging people to keep a close eye on the forecast, flood alerts and warnings in their areas.”
The warning for Saturday also covers parts of Wales, north-west England and Yorkshire and Humber.
Predicted rainfall throughout Saturday is expected to be 20-40mm in lower-lying areas, while hilly areas could see 40-60mm, the Met Office said.
Another yellow warning for rain covering Wales and north-west England is in force from 6pm on Sunday to 6am on Monday, and warns some communities could be cut off by fast flowing and deep floodwater.
Very strong winds could affect north-west Scotland between noon and 9pm on Sunday, including the possibilities of power cuts and delays for high-sided vehicles and public transport.
Gary White, the duty tactical manager for Natural Resources Wales, said flooding is likely up and down the country over the next few days.
He said: “Our emergency response workers will be at key sites checking defences are in good working order and making sure any drainage grids and screens are clear to reduce the risk to people and their homes.”