The Met Office issues an amber warning with western and southern areas predicted to be worst hit.
Heavy rain has hit parts of Cumbria with warnings that up to 250mm (9.8 inches) could fall over the next 24 hours.
The Met Office has issued an amber – or medium risk – warning, which will be in place until 23:59 BST on Thursday.
It said the rain may cause localised flooding and disruption.
Western and southern parts of the county look likely to be worst affected, according to the Environment Agency.
Flood warnings – meaning flooding is expected – are in place for parts of Keswick and Grasmere, White Bridge and Stock Lane where river levels have risen.
Rain has already caused disruption to cross-border rail services with Avanti West Coast advising passengers not to travel from stations in the north-west of England and Scotland before Friday.
Cumbria has already seen more than 12 hours of heavy rainfall, according to the Met Office.
Cumbria Fire and Rescue Service said residents should “be alert to the dangers of flood water”.
“Never enter flood water on foot or in a vehicle. Call 999 if life is at risk – we’re here to help,” the service tweeted.
Stewart Mounsey, the Environment Agency’s flood risk manager for Cumbria, advised people to sign up for weather alerts via the organisation’s website.
“We may see up to eight inches of rain over the next 24 hours, which is why the amber warning is in place,” he said.
“Consider a flood plan if you are at risk. What medication might you need? What furniture or valuables might need to be moved upstairs? Where would you park your car to avoid damage?
“It’s always better to be prepared than to get a shock.”
EA teams were monitoring the effect of the rain on rivers, focusing on western and southern parts of the county, “making sure flood defences work”, Mr Mounsey said.
He also advised motorists not to attempt to drive through flood waters and urged people to stay away from rivers as they are likely to be swollen and fast-moving.
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