Residents have been warned that where isolated thunderstorms hit on Sunday, there is a risk of flooding.
There is also a risk of travel disruption where rain is heaviest, the Met Office said, with the whole of Lancashire affected by the weather warning.
A warning in place on Saturday predicts heavy rain in some parts of the North West, while others may avoid the downpours. A separate warning on Sunday also predicts storms.
A spokesman added: "A band of rain will affect the area through Sunday, becoming heavy at times, with the added risk of isolated thunderstorms developing.
"There remains some uncertainty in the location of the rain-band, so some places may see very little rain, but others could catch several batches of heavy rain.
"There is a small chance of 40 mm falling within an hour from any thunderstorms that develop in the afternoon and evening over northern England."
The warning is in place from 9am on Saturday and all day on Sunday.