Hurricane Harvey has produced rains that submerged parts of Houston, the fourth most-populous US city, killed at least 11 people, displaced tens of thousands and wreaked havoc along the Gulf Coast.
US meteorological experts explain what fuelled the "perfect storm".
* Why has the National Weather Service called the storm unprecedented?
It is mainly because of the amount of rain Harvey has dropped in one place - 132cm of rain in one part of Texas and rising. That is more than any previous storm in the history of the continental United States, according to the National Weather Service.
The 209km/h winds at landfall and a storm surge that caused damage are not unprecedented. But the storm's path after stalling, keeping it close enough to the coast, allowed it to continue to draw moisture that it then dumped inland, said Marshall Shepherd, director of the atmospheric sciences program at the University of Georgia.
"It's a very rare" for such a storm to stall so long over land, said Suzana Camargo, executive director of the Initiative on Extreme Weather and Climate at Columbia University.
* What allowed Harvey to stall?
Most tropical storms are "steered" by winds that push them along, including over land where they eventually lose power. In this case, two duelling high-pressure systems created a "dead spot" that allowed the storm to stay put, and even wobble its way briefly back out to the Gulf of Mexico, like a spinning top.
Such circumstances are uncommon, making Texas "rather unlucky," according to Jeffrey Chagnon, an assistant professor of meteorology at Florida State University.
* What fueled its rapid increase from Category 1 to a Category 4 hurricane before making landfall?
Warm Gulf of Mexico waters helped give the storm its power, and there was an absence of wind shears, meaning changes in wind speed and direction, whose presence in the atmosphere can disrupt the structure of a cyclonic storm and remove heat and moisture. Both were key factors that helped Harvey build to Category 4 as it closed in on the coast.
"It wasn't particularly strong until it approached the coast and moved into water temperatures that were very warm," Chagnon said.
* Did climate change intensify Harvey?
Weather experts said they could not directly link Harvey to climate change. The winds that helped the storm stall were a random circumstance.
But the larger volumes of rainfall are probably linked to climate change associated with global warming that increases the amount of moisture in the atmosphere, the World Meteorological Organization said.
Climate change may not have caused Harvey, but it could eventually be found to have exacerbated it, said Hal Needham, founder of the private company Marine Weather and Climate in Galveston, Texas.
* What factors beyond the storm made the flooding so severe?
Houston's large sprawl of urbanised areas covered with impervious surfaces such as concrete, along with its location on a flood plain, exacerbated the impacts of the rain, Shepherd said.
"You had a very vulnerable city, the track of a storm that just stalled out and a huge amount of rainfall," Camargo said.
"Each separately wouldn't necessarily be a problem. When you put everything together - talk about a perfect storm."
