The downpours alert came as the Met Office issued an early warning of “very disturbed weather” on Tuesday, with winds forecast to gust to more than 80mph in what is expected to be named Storm Frank.
Eight flood warnings were in force for rivers in Tayside, while flood-hit Cumbria is braced for another deluge.
The flood warnings cover stretches of the rivers Tay, Tummel, Earn and Lyon.
The Scottish Environment Protection Agency said: “Further heavy and persistent rain is expected overnight and into Saturday which is likely to result in river and loch levels rising once again, with a risk of further flooding.”
The Cluanie Inn, east of Kyle of Lochalsh, had the UK’s highest rainfall on Thursday night, with 19.8mm recorded between 5pm and 5am.
Two yellow “be aware” severe weather warnings have been issued for Scotland by the Met Office for Boxing Day rain.
The first, in force until midnight tomorrow, covers the Borders and eastern Dumfries and Galloway. The Met Office said 50-80mm would fall quite widely over higher ground, with some 120mm possible in the most exposed areas.
The second alert, for Aberdeenshire, Angus and Perthshire, will be in force until 10pm tomorrow night, with persistent rain, and several centimetres of snow on higher roads, such as the A9.
A Met Office spokeswoman said: “A band of rain will arrive from the south overnight Friday night into Saturday and is then expected to last until late Saturday evening, before easing in intensity.
“Please be aware of the potential for some localised disruption to travel, either from flooding or from upland snowfall.”
A more serious amber “be prepared” warning is in force for Cumbria today because of the potentially greater flooding impact from yet more downfalls.
The county has had its wettest December since records began in 1910, with serious flooding hitting three times this month already.
Soldiers have been called in and 700 Environment Agency staff are on stand-by. Flood gates have been closed in Carlisle, Keswick and Cockermouth.