Forecasters have predicted heavy rain and strong winds from Storm Desmond over the weekend, with people warned to prepare for flooding in some places.
The Met Office has issued an amber "be prepared" warning for rain on Saturday and Sunday across the Strathclyde, Central, Tayside and Fife, Lothian and Borders and south west of Scotland areas.
Parts of the north west and north east of England are also covered by the warning, which cautions that up to 200mm of rain could fall on high ground.
The warnings are valid from 2am on Saturday until 6am on Sunday.
The Met Office said: "Outbreaks of rain will develop later on Friday, becoming persistent and heavy overnight and lasting through Saturday, before clearing southwards early on Sunday.
"Be prepared for the likelihood of flooding affecting properties and parts of communities. Watercourses may become dangerous, deep and fast-flowing, while some transport disruption seems likely."
Its chief forecaster warned that the spell of heavy rain and strong winds would be "prolonged".
Forecasters said rainfall of 60mm to 100mm is likely quite widely in the amber zones, with some mountainous areas seeing in excess of 150mm over a 30-hour period, perhaps even reaching 200mm.
Meanwhile, yellow "be aware" warnings for wind and rain are in place on Friday across much of Scotland, Northern Ireland and the north of England.
Motorists have been warned to be aware of difficult driving conditions, and the risk of travel disruption on roads and to ferries.
The Scottish Environment Protection Agency has 15 flood alerts in place around the country and three flood warnings in Tayside.
The weather conditions led to Glasgow's George Square festive attractions, including the city centre ice rink, closing for the day.
It comes after heavy snow and rain caused severe disruption and flooding to areas in the south of Scotland on Thursday.
Routes in Midlothian, the Scottish Borders and parts of Edinburgh were among the worst affected by what police described as ''extreme weather''.
The sudden heavy fall of snow on Thursday evening resulted in some road closures and had a major impact on key routes through the Borders.
The fire service said some vehicles became stuck in the snow in places and fire engines had difficulty reaching emergency calls.
A fire engine crashed into a number of cars in Gorebridge, Midlothian, due to the conditions, but no-one was injured.
Marc Becker, hydrology duty manager for the Scottish Environment Protection Agency (SEPA), said: "Flooding is expected to affect many central and southern parts of Scotland which could affect communities and cause travel disruption across many areas of Scotland over the coming days.
"During Friday persistent and heavy rainfall will move south east across Scotland with gale force south-westerly winds. Rainfall will be heaviest over central and southern areas of Scotland during Friday evening and throughout Saturday which will cause river levels to rise. River levels in the Tay, Clyde and Tweed catchments are already high and are expected to rise considerably in the next 24 hours.
"Over the coming days we will continue to monitor levels and update the public on our latest forecasts via social media and Floodline. We would encourage members of the public to remain vigilant and be mindful of the conditions in their locality and when travelling."
RAC spokesman Simon Williams said anyone driving in Scotland on Sunday may be faced with "extremely challenging conditions" with possible flooding, wind damage debris and more high winds.
He said: "Motorists encountering flood water on the road should think twice before trying to drive through. Water getting sucked into engines can cause catastrophic damage which could lead to a frightening repair bill at the most expensive time of year or even an insurance write-off.
"If you are at all unsure how deep a flood is the best advice is to err on the side of caution and take another route."
Scottish Environment Minister Aileen McLeod said the Scottish Government and Transport Scotland are closely monitoring the situation, supporting the efforts of local authorities and the emergency services.