Hurricane Helene’s death toll has surpassed 150 as searchers use helicopters to get past washed-out bridges and hike through wilderness to reach isolated homes. Crews were still trudging through knee-deep muck and debris in the wake of the deadly category 4 storm that dumped more than 40tn gallons of rain on the southern US after it crashed ashore in Florida on Thursday. The amount of rainfall is enough to fill Lake Tahoe – with its depth of 1,645ft and surface area of 191 sq miles – or 60m Olympic-sized swimming pools. It could also fill the Dallas Cowboys’ 80,000-seat stadium 51,000 times over. “That’s an astronomical amount of precipitation,” said Ed Clark, head of the National Oceanic and Atmospheric Administration (NOAA)’s water center in Tuscaloosa, Alabama, according to the Associated Press. “I have not seen something in my 25 years of working at the weather service that is this geographically large of an extent and the sheer volume of water that fell from the sky.” Clark said the estimate of 40tn gallons was, if anything, conservative. Also, if that amount of water had fallen in the parched western states, it would have been enough to fill Lake Powell and Lake Mead twice over, he said. The 40tn gallon calculation was made by meteorologist Ryan Maue, a former NOAA chief scientist, using precipitation measurements made in 2.5-mile-by-2.5 mile grids as measured by satellites and ground observations. “It was not just a perfect storm, but it was a combination of multiple storms that that led to the enormous amount of rain,” Maue told the AP. “That collected at high elevation – we’re talking 3,000 to 6,000ft. And when you drop trillions of gallons on a mountain, that has to go down.” The measurement comes as more than 130 people have been reported dead – and entire towns washed away – by the deluge in western North Carolina, Georgia, Tennessee, the Carolinas and Florida. “These storms are wetter and these storms are warmer,” North Carolina state climatologist Kathie Dello said. “There would have been a time when a tropical storm would have been heading toward North Carolina and would have caused some rain and some damage, but not apocalyptic destruction.” Deanne Criswell, the Federal Emergency Management Agency (Fema) administrator, told CBS News on Sunday that Helene had taken time to develop. But once it had, Criswell said, it intensified quickly because of high Gulf of Mexico water temperatures driven by the climate crisis, whose primary factor is humans’ burning of fossil fuels. Criswell said the effect was to create “more storms that are reaching this major category level than we’ve seen in the past”. She also said there were were greater amounts of destructive storm surge in the coastal areas and “greater amounts of rainfall as it moves up north”. “In the past, when we would look at damage from hurricanes, it was primarily wind damage, with some water damage, but now we’re seeing so much more water damage, and I think that is a result of the warm waters, which is a result of climate change,” Criswell added. The hurricane struck after previous storm systems had passed over the region. A low pressure system caused the remnants of Helene to stall as well. Helene held so much moisture because it was young and moved fast before it hit the Appalachians, University of Albany hurricane expert Kristen Corbosiero said to the AP. Understanding the storm’s damage may take weeks. “These are historic flooding levels in an area where the terrain is not conducive to being able to withstand those levels of precipitation,” Janey Camp, a civil engineer at the University of Memphis, told Scientific American. The region’s mountainous topography forced the water to cascade down quickly and accumulate in lower-elevation areas, making floods more dangerous. “Unfortunately, it’s a perfect storm for one of the worst-case situations you could have,” Camp told the outlet. The area in North Carolina around Asheville, where the French Broad River and Swannanoa River meet, received more than 20in (51cm) over three days. For context, 8.4in over that time span would have been considered a once-in-1,000-year event by a metric created prior to climate change. Camp said local infrastructure wasn’t designed to be resilient even under once-in-100-year or once-in-500-year circumstances. Had they been, “those design guidelines and standards kind of got thrown out the window”. “They wouldn’t have really helped,” Camp said. Authorities in North Carolina resumed search and rescue efforts Tuesday with hopes of delivering aid and making contact with hundreds of people left isolated after Helene washed out roads and damaged cellphone towers, Reuters reported. More than 1.7m million homes and businesses in six states from Florida to West Virginia remained without power Tuesday.