The Met Office has issued a 'danger to life' weather warning as more rain is set to hit the UK.
Parts of the country could face flooding with homes and businesses submerged in water.
The weather alert was raised in parts of Wales between Bangor and Swansea for today and tomorrow.
Two amber weather warnings have been issued by the forecaster, with the relentless downpours likely to bring flooding to homes and businesses.
Parts of Wales could be washed out by up to four inches of rain.
The downpour is also expected to cause some damage to some buildings, while fast flowing or deep floodwater is likely and could claim lives.
The heavy rainfall could also result in power cuts affecting swathes of homes and businesses.
Meteorologist Greg Dewhurst said: "Through Monday it will be a fairly bright start but we will see rain spread in from the west by the afternoon.
"Then we will see heavy rain right through the rest of Monday, overnight and into Tuesday morning."
In Wales, the average rainfall for March is 117mm, meaning the worst affected areas could see three-weeks worth of rain in just over 24 hours.
"The ground is already saturated and with all the rainfall we have had over recent weeks it is likely to lead to further flooding in places."
UK five-day weather forecast
Today:
A few showers may turn into heavy rains with strong winds, arriving in Northern Ireland first and moving eastwards across most parts by evening
Tonight:
Windy with further rain or drizzle affecting many parts, heavy on western hills. Temperatures will be mild across the country especially in the south.
Tuesday:
Rain and drizzle will pour southwards across England and Wales. Temperatures will be very mild. It will also be brighter and colder in the north with windy scattered showers.
Wednesday to Friday:
Cold showers with possible longer spells of rain, falling as snow on hills, especially in the north of the country.
Parts of the country will experience gales.
The extreme rainfall is likely to grind public transport to a halt as train and bus services face disruption.
While motorist are being warned that spray and flooding could lead to driving conditions and some road closures.
A good chance some communities cut off by flooded roads.
Power cuts and loss of other services to some homes and businesses likely.
