(Bloomberg) — Floods that killed more than 150 people in eastern Spain this week were caused by a storm that dumped a year's worth of rainfall in less than 24 hours.

The tragedy has raised questions about whether earlier warnings could have prevented so many deaths, while the widespread damage shows how many cities aren't adapted to withstand rainfall amplified by climate change.

An extremely hot Mediterranean and warmer air temperatures exacerbated a type of storm system that is becoming more common in the Iberian peninsula due to global warming. Experts say that a building boom in the region in recent decades may have put people in harm's way and reduced stormwater drainage.

"It may happen that the paths we have left for water aren't large enough, and in recent decades that's happened too many times," said Ernest Blade, professor of river and hydrologic engineering at the Universitat Politècnica de Catalunya.

The storm which hit southeastern Spain earlier this week was a particularly intense example of a "dana" storm, a common phenomenon in the area at this time of year. Danas (short for "depresión aislada en niveles altos") develop when cold autumn air moving south meets warm, moist air typically flowing from the Mediterranean, creating high-altitude storm clouds. These can dump very large amounts of rainfall in a short space of time. This year, the Mediterranean reached a mean temperature of 28.5C (83.3F) in mid-August. It's remained anomalously warm, making this type of intense storm more likely to develop. Climate change is driving up air temperatures, which allows storms to hold more water — a 7% increase for every 1C of temperature rise.

The system moved inland and dumped rainfall over higher ground, inundating settlements downhill and overwhelming urban areas with flash floods. As the storm stalled, it dropped more than 48 centimeters (19 inches) of rainfall in just eight hours in the mountainous area west of the city of Valencia, inundating areas as far inland as Utiel, more than 80 kilometers (50 miles) from the coast. The town's position in a natural basin and close to several small rivers made it vulnerable to flooding. Rain continued to fall on Thursday, also flooding Castellón, north of Valencia.

"It is a storm that, in the location where it formed, in the south of the peninsula, with very strong easterly winds, has created the perfect breeding ground for a significant episode of rain," said Jose Miguel Viñas, a meteorologist at weather forecaster Meteored. "The magnitude has exceeded expectations."

A rapid analysis by the World Weather Attribution initiative found the recent extreme rainfall in Spain was around 12% heavier and twice as likely as it would have been in a pre-industrial climate. One-day bursts of rain in the region are increasing as fossil fuel emissions warm up the planet, the group said.

The flooding was also more likely due to the hot, dry year the region had. That's left soil hard-baked, reducing how much water could soak into the ground.

The government of Valencia said that 155 people had been reported dead as of mid-afternoon on Thursday. Several more died in other regions. The shockingly high toll has turned attention toward Spain's flood warning systems. While weather services said on Monday that big storms were on the way, the main emergency alert was not issued until Tuesday evening, when the rain was already falling in earnest.

"Ultimately all you can do when you're getting that amount of rain is alert people in good time, give warnings and make sure that people are aware of how they can keep themselves safe," said Jess Neumann, associate professor of hydrology at the University of Reading. "It sounds like that was not done adequately."

Adapting for heavier rainfall will be essential for lowering the risk of future disasters. "Reducing the volume and speed of run-off can be done by replanting upland areas and improving soil health, as well as increasing the capacity of urban areas to absorb, hold or disperse water," said Kevin Collins, a senior lecturer in environment and systems at the Open University in the UK. He suggested strengthening dams and bridges, which have proven to be vulnerable, too.

Yet cities can only do so much, particularly those not designed for the level of rainfall seen this week in Spain. With so much pavement, water has nowhere to go.

"It wouldn't matter whether it was Spain or anywhere else, if there was that much rainfall falling on impermeable ground, you are going to get flooding," Neumann said. "In the long term, this isn't something that's going to go away."

Hayley Fowler, professor of climate change impacts at Newcastle University, also warned about limitations of adapting to these types of storms, which "appear to be becoming more frequent in Europe and are projected to increase further with additional warming."

"The question is not whether we need to adapt for more of these types of storms," she said, "but can we?" See also:|

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