Storm Dennis really was the "perfect storm" as it delivered a month’s rain in just 48 hours and caused rivers to rise to unprecedented levels.
The Met Office knew it was going to be bad as they warned people about the imminent north Atlantic storm which came less than a week after Storm Ciara.
According to Grahame Madge, at the Met Office, forecasters knew the rainfall was going to be significant and much more damaging than the gale force winds they were also predicting.
Much of the UK was battered by the rain, which triggered nearly 600 flood warnings and alerts in England alone on Sunday, more than any other day on record, with dozens more in place across Wales, Scotland and Northern Ireland.
By Sunday afternoon, South Wales Police declared a “major incident” after multiple landslides and people reporetd trapped in their homes.
Why then was it quite so bad in south Wales?
According to Mr Madge, the amount of rainfall was one of the main factors as well as the way the weather front moved across the country.
Between Saturday and Sunday, Tredegar recorded 116mm of rain, Libanus 108mm and Usk 90mm. To put that in context, in Tredegar, the average for the whole month of February is 145mm.
"The rainfall was particularly heavy in south Wales," explained Mr Madge. "The band of rain that was associated with the weather front was aligned with the flow, which means there was continuous rainfall along the front.
"It wasn't until the front shifted that there was some respite behind on Sunday afternoon.
"We always knew that the impact was more likely this heavy rainfall than the high winds."
While there were yellow weather warnings right across the UK for the weekend, the worst of the weather was centred on south Wales. As Storm Dennis advanced, some regions were upgraded to amber, but it was only Wales which was issued the most severe red "danger to life" rain warning- only the fourth time a red has been issued since the impact based warning system began nine years ago.
"They really are exceptional events," Mr Madge added, although it is still too soon to put a number on just how exceptional.
The ground was already saturated from the downpours during Storm Ciara the previous weekend, meaning the ground couldn't soak up the rain which fell.
Mr Madge said: "The groundwater and aquifers were already saturated so then when we get more rain on top of that you get a much quicker response in the rivers."
The rivers themselves reached "unprecedented levels" as they were unable to contain the volume of water pouring off the steep valley sides.
The River Taff in Pontypridd was at its peak over five metres high, the highest in nearly 40 years and a meter higher than the 1979 floods. At the peak of the flood in Pontypridd, NRW has estimated that 900 tonnes of water per second was flowing down the River Taff.
According to the Cardiff Harbour Authority, on Sunday, the Natural Resources Wales monitoring station at Pontypridd recorded a flow of 803 cubic metres per second (or cumecs) on the River Taff.
That's approximately 4,000 full bathtubs of water flowing past the monitoring station every single second. The River Ely also reached one of its highest flow rates of 70.9 cumecs.
The volumes of water were also considerably higher than those recorded during Storm Ciara the previous week.
A spokesman for the Harbour Authority said: "During Storm Ciara, combined river flows were below 300 cumecs and in the week between generally around 100 cumecs combined."
He added that the flows witnessed during Storm Dennis were exacerbated by such a large volume of rain in such a short period and the fact that it fell on saturated ground.
The Welsh valleys have a specific topography that means the rivers can respond very quickly to intense rainfall. According to flood risk consultancy, Atkins, both the shape opf the valleys, combined with the already-wet ground is likely to have contrinuted to the floods seen on Sunday.
A company spokesman said: "The Welsh Valleys have a specific topography that lends themselves to rapid runoff of rainfall from the land. The steepness of the valley sides combined with a narrow valley floor means that storm water can accumulate quickly. This can cause rivers to swiftly rise and limit the capability of local drainage systems to discharge freely.
"As an example, the River Taff is a flashy river which means that depths can rise quickly and, because of the gradient of the river bed, flow rates can be high.
"This is made worse in winter when the ground is already saturated from heavy rainfall in preceding weeks."
The reason why Pontypridd was hit as bad as it was is due to the fact that it sits at the point where the River Rhondda joins the River Taff. With both rivers full to bursting, there was simply nowhere else fior the water to go.
A spokesman for Natural Resources Wales added: "The amount of rainfall combined with already high river levels following Storm Ciara, saturated ground and the steep river catchments in the South Wales valleys, led to record high river levels in Pontypridd, where the Rivers Cynon and Rhondda flow into the Taff, which resulted in unprecedented flooding in Pontypridd and in communities downstream."
During the peak of the storm, there were 61 Flood Alerts, 89 Flood Warnings and two Severe Flood Warnings in force in Wales.
The "flashy" rivers, like the Taff and the Cynon, are very different to the River Wye and Monnow in Monmouth. These rivers drain a much wider and shallower catchment area, meaning water takes significantly longer to find its way into the rivers.
That's why Monmouth braced itself for significant flooding through Monday night and into Tuesday, two days after Storm Dennis had passed. Although the rainfall had long gone, the rivers had yet to reach their peak flows.
On Monday evening, two severe flood warnings – representing a danger to life – were issued for "undefended" parts of the Wye in Monmouth. On Tuesday morning, river levels rose above the highest levels ever recorded since records began 200 years ago. Gauging stations in the rivers Wye and Monnow, which straddle the town of Monmouth, have recorded river levels of 7.13 metres and 6.56 metres, respectively.
The previous highest-recorded levels at the stations were 6.48 metres (Monmouth) and 6.16 metres (Monnow Gate).
As the clean up operation begins in earnest, there are two questions on everyone's lips: Were the flood defences good enough? and, Is this climate change?
Wales' First Minister, Mark Drakeford, visited Pontypridd on Monday to speak to people living there and see the damage firsthand. He said that some extreme weather events, like that which Storm Ciara and Dennis brought, are difficult to predict but acknowledged that they are more likely to happen in the future.
"We have to look to the future and recognise that in an era of global warming, events of the sort we have seen, not just one weekend but two weekends in a row in Wales," he added.
"We have to plan in a different way because this sort of thing may well be a much more common part of our futures."
Mr Drakeford also said that Welsh Government will be spending £350 million on flood defences over the next Assembly term.
Research has shown that the conditions in a previous winter storm, Desmond in 2015, which brought very heavy rain to parts of the UK and caused widespread flooding, were made 40% more likely due to climate change.
In the wake of both Storm Ciara and Dennis, Dr Michael Byrne, lecturer in climate science at the University of St Andrews and research fellow at the University of Oxford, said more water in the atmosphere is "an entirely inevitable consequence of climate change".
"When you warm the planet, the atmosphere holds more water. In many parts of the world, including the UK, rising temperatures go hand in hand with more rain," he told the PA news agency.
He said the jury is still out on whether climate change will strengthen or weaken the high winds in storms such as Ciara and Dennis, but "when the storms come there will be more rain associated with them".
"These storms are nothing new, going back 100 years, but, because we are now more than 1C warmer as a whole versus pre-industrial times, every degree means 7% more water in the atmosphere and more rain in these heavy rain events. When they come, they bring more rain, 100% for certain, because of climate change."
If temperatures rise by 3C, which is what efforts to cut emissions already outlined by countries currently put the world on track for, storms could be bringing around 20% more rain than they would without climate change.
"It would put a huge strain on flood defences if that were to happen," said Dr Byrne.
Hannah Cloke, professor of hydrology at the University of Reading, said: "These types of events are most likely a taster of what is to come and we should be paying very close attention to that." And she warned: "Clearly, we are not ready for them. We've always seen these big floods but we do keep seeing these records being broken, it's very concerning."
She said more people are living in areas at risk, and there is a need to think about how the landscape is managed.
Measures to help reduce flood risk could be looking after soil so it can soak up water and does not run off the land to block watercourses, using uplands to catch water, diverting it on to fields upstream of settlements, and putting in "leaky dams" made of wood in streams to slow the water's flow down to the towns.