"Given the pre-existing saturated ground, some further road flooding is possible, especially rural roads in the West and a few trees may topple at relatively lower gust speeds than would normally be the case".
Police said his death was not being treated as suspicious or being linked to the bad weather.
A tweet from the Ystradgynlais neighbourhood policing team said: "A male has fallen into the river at #Ystradgynlais at approximately 10am this morning and has sadly be (en) found deceased further along the river in the Tebanos (sic) area". We will keep you updated throughout the morning.
Storm Dennis, one of the deepest low-pressure centres to have formed in recent years, has brought severe weather to Iceland and many western and central parts of the UK.
The Met Office, Britain's meteorological service, only issues its highest red warning when it thinks the weather will be so risky there's a "risk to life" and that people must take immediate action to protect themselves. The Met Office also had four other amber warnings in place in England and Wales following the torrential downpours.
There are still 274 flood warnings and 348 flood alerts in effect across England.
At Cray Reservoir, Powys, South Wales, 132.8mm of rain has been recorded between 0700hrs on Saturday 14 and 0800hrs on Sunday 15.
Severe flood warning were issued for the Scottish Borders and for the River Neath in South Wales, while local media reported the River Taff had burst its banks at Pontypool.
Gwent Police said residents in the village of Skenfrith in Monmouthshire, south-east Wales, were advised to evacuate due to severe flooding.
South Wales Police declared a "major incident" after multiple landslides and floods prompted the evacuation of local residents.
The Environment Agency has issued more than 200 flood warnings across the country, which means flooding is expected and action is required.
"Keep an eye on the forecast and visit our website for the latest information on flood warnings".
You can check the latest from the Met Office here.
Hundreds of flights were cancelled due to the high winds while train services were repeatedly disrupted by flooding, affecting tens of thousands of passengers as British families leave for the mid-winter school break.
Flooding saw the M54 westbound closed at Telford, along with the A616 around Stocksbridge in South Yorkshire and the A52 between Stragglethorpe and Gamston in the East Midlands, Highways England said.
Storm Dennis has sparked a record number of flood warnings across England after barreling through the United Kingdom and releasing more than a month's worth of rainfall in just 48 hours.
The Met Office was predicting 140mm of rain to fall in South Wales on Sunday morning before the storm eased in the afternoon.
Various government agencies are scrambling to lessen the damage caused by the storm, which arrived less than a week after Storm Ciara wreaked havoc last weekend.
Continuing, the ESB statement noted: "The vast majority of customers who lost power during Storm Dennis were restored yesterday, Sunday".
"Our volunteers know that when people are caught up in a crisis, one person's needs are different to the next person".
Meanwhile, British Airways and budget carrier EasyJet have confirmed they grounded flights.