The death toll, following severe flooding in China's Henan province and its capital Zhengzhou, has risen to at least 33, authorities said Thursday.

An unprecedented downpour dumped a year's rain in just three days on the city of Zhengzhou, weather officials said, instantly overwhelming drains and sending torrents of muddy water through streets, road tunnels and the subway system.

Hundreds of thousands of people in the area surrounding the city were also affected by the floods, with farmland inundated and road and rail links severed.

In worst-hit Zhengzhou, grim images of horror inside the subway system were relayed in real-time over social media, showing water rising during Tuesday's rush hour from the ankles of passengers to their necks.

At least a dozen people died before rescuers were able to cut survivors free from carriages.

Questions were starting to swirl Thursday over how prepared authorities were for the disaster. Angry users took to Weibo to question why the metro was not closed earlier, with one thread racking up more than 92 million views Thursday.

"Why was it that water levels on the street were almost waist-high, but the subway was still allowing commuters in?" asked one.

In a sign of mounting pressure, the transport ministry put out a statement ordering rail operators nationally to "absorb the lessons of recent incidents and ... improve their emergency plans," warning them to close stations quickly when faced with severe weather.

As the water retreated – with piles of cars a monument to its deadly power – residents prepared for another day of bad weather Thursday, moving vehicles to higher ground and trying to plot journeys out from the stricken city, where communications and power were still patchy.

Trucks pumped muddy water from underground tunnels as business owners counted the cost of the torrent, and meteorologists issued "red" rain alerts, warning of the threat of fresh landslides and flooding in surrounding areas.

Mass communication blackouts added to the challenges, with state TV reporting telecommunications equipment and cables were damaged in the floods.

"I am waiting for the power to be restored, but it may take several more days, I think," Chen, the owner of a local food and pork sandwich restaurant, told AFP.

"My losses? They are OK, compared to what happened in the tunnel there," he said, gesturing toward the tunnel where floods trapped many cars on Tuesday – potentially with motorists still inside.

The death toll looked set to rise as rescuers scoured through debris.

The state-run Global Times newspaper shared a video on its social media feed of rescuers pulling a three-month-old baby from a collapsed building in Zhengzhou.

The newspaper said the baby's mother was still missing.

Topography, typhoon, climate

Questions turned to how China's bulging cities could be better prepared for freak weather events like Tuesday's storm, which experts say is happening with increased frequency and intensity due to climate change.

Anyang city, a short journey north of Zhengzhou, issued a red alert on Thursday for heavy downpours after some areas received over 100 millimeters of rain, ordering schools to close and most workers to stay at home.

Weather experts dissected the reasons behind Tuesday's record downpours.

Chen Tao, the chief forecaster of the National Meteorological Center (NMC), said a mix of Henan's topography and Typhoon In-Fa favored the rains.

Although the typhoon has not landed in China, under the influence of winds, "a large amount of water vapor from above the sea gathered toward Henan," providing a source of water for the downpours, Chen said.

The changing climate also makes these kinds of extreme weather events more common as the world continues to heat up, with catastrophes seen across the world.

Like much of China, Henan province is striated by rivers, dams and reservoirs. Many were constructed decades ago to manage the flow of floodwater and irrigate the agricultural region.

But endless city sprawl is putting pressure on drainage.

State media rebuked suggestions that dams may have had a part to play in subverting the normal flow of water, with the Global Times citing experts as saying construction had "no direct connection to flooding."