A weather warning for rain has been upgraded to amber .
The heavy and persistent rain is set to fall in areas that were recently hit by Storm Dennis.
Around 100mm could fall as three weeks worth of rain is forecast to fall in just two days.
The Met Office weather amber warning states: "Rain will become persistent and heavy later on Monday, lasting until later on Tuesday morning.
"Between 50 to 70 mm rain is likely to fall widely, with around 100 mm possible over higher ground. This is likely to lead to flooding and travel disruption."
The amber warning is in place between 6pm on Monday and 10am on Tuesday.
There is also a yellow weather warning in place for others parts of Wales.
It is in place from 3pm on Monday until midday on Tuesday.
The yellow warning states: "Wet and windy weather will spread eastwards across Wales on Monday afternoon and persist until Tuesday afternoon. Some prolonged and heavy rain is expected, especially over higher ground. 20 to 40 mm of rain is likely to fall widely with many upland areas seeing 50 to 70 mm.
"Some hills and mountains may see around 100 mm in few places, the highest rainfall expected over high ground in mid-Wales."
It is more bad news for communities hit by flooding when Storm Dennis battered Wales on February 16.
Meteorologist Greg Dewhurst told the Press Association news agency: "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."
