Storm Eva is set to test flood defences deployed as part of a vast operation that saw the Army on the streets of Cumbria.
More than 180 flood alerts and warnings were issued by the Environment Agency (EA) as forecasters said up to 4.7ins (120mm) of rain could fall in some areas already saturated by wave after wave of winter squalls.
The average rainfall for the whole of December in the North West is 5.7ins (145mm) - so not far off a month's rain could lash the region in hours.
The Government's emergency Cobra committee met on Christmas Day while a company from the 2nd Battalion, Duke of Lancaster's Regiment, helped committees across the North West build miles of temporary flood defences.
The EA said 85% of the country's temporary flood barriers had been sent to Cumbria, where rainfall has smashed records, and the Lakeland region braced itself again ahead of the deluge.
Met Office forecaster Greg Dewhurst said the latest weather is "unusual" when the mild conditions enjoyed during December are taken into account.
"And as you can see from the rainfall figures from this weekend alone it's very similar to what we'd expect across the whole month, so it is unusual to see such a prolonged unsettled spell," he said.
Flood warnings extended across northern Britain, including the Calder in West Yorkshire and the Ribble, Wyre and Wenning in North Yorkshire, Wyre & Wenning. Pendle Water, Pimlico Brook, the Burrow and Hellifield Beck were also at risk of bursting their banks, the EA warned.
As Storm Eva approached mainland Britain the Met Office expanded its amber warning of rain for Boxing Day, to encompass areas of Lancashire and western areas of both North and West Yorkshire.
Over high ground between 1.7ins (50mm) and 3.1ins (80mm) of rain is likely to fall on Saturday, while some exposed areas in southern parts of Cumbria could see up to 120mm.
A yellow warning of rain was issued for parts of north Wales, northern England and southern Scotland before the slow-moving band of squally weather.
Forecasters are expected to review the area of warning on Boxing Day morning.
The EA issued dozens of flood warnings to broad swathes of the country throughout Christmas Day, instructing locals to be prepared to take "immediate action".
It issued more than 100 flood alerts from the South West to the North East, warning that some flooding was possible.