Met Office yellow rain warnings are in place for much of the UK
The dry start to autumn is due to come to an abrupt end within hours, as predicted gale force winds and heavy rainfall lash parts of the UK.
Winds of up to 60mph will hit the north of England and south of Scotland, while other areas could see 80mm of rain before Monday evening, forecasters say.
The Environment Agency has issued about two dozen flood alerts, mainly in south-west England.
Northern Ireland was also expected to be severely affected.
Met Office amber warnings - advising people to be prepared for severe weather - were in place for Devon, Cornwall, Dorset, Hampshire and Northern Ireland.
Met Office yellow warnings - indicating that people should be aware - were in place for much of the rest of the UK.
Helen Chivers, Met Office forecaster, said temperatures were set to drop to between 12C (54F) and 14C (57F) - below the average for this time of year.
"The centre of the low pressure is expected to move north towards the south-west of England during Sunday night leading to increasingly strong winds," she added.
"Most of the UK will see between 20mm and 40mm of rain with some areas getting as much as 80mm of rain.
"Winds of up to 60mph are expected across northern England, southern England, particularly coastal areas, and southern Scotland."Debris and flooding
The public are being warned to take care travelling on Monday, as there will be "a lot of debris around due to severe gale force winds as well as surface water on the roads."
The Met Office said there was going to be a big change in the weather compared to what the first part of September.
Parts of Britain have seen temperatures in the mid-20s during September, a welcome break from the rain and wind which dogged much of July and August.
Ms Chivers added: "This week will be dominated by this band of low pressure that will stay with us until Thursday when it will start to calm down."