Harvey, like Sandy & Katrina before it is rewriting our understanding of ‘extreme’ weather events and the language we use to describe these. As one American put it, ‘a 1 in a 1000 year storms can occur today and we are watching play out’. The power and energy of these major storms have each had a major impact; Harvey’s cost is estimated to be $42 billion and rising. Yet Trump rescinds the law that would make developers take account of climate change and these events just days ago. In UK this would be called ‘cutting red tape’. Two articles describe the events that are underway.

Record levels of rainfall: 15 – 30 inches in days

The colours the National Weather Service uses to show rainfall on its weather map couldn’t represent the deluge in southeastern Texas, so the NWS added two more purple shades to its map. The old scale topped out at more than 15 inches; the new limit tops 30 inches.

The move came as forecasters warn of continued flood dangers and continue to update precipitation totals from what the NWS is calling “unprecedented” rainfall from Tropical Storm Harvey, which made landfall as a Category 4 hurricane late Friday.

In a note about the change, the weather agency said on Monday, “Some perspective on the amount of rainfall that Tropical Storm Harvey has put down across Southeastern Texas and Southwestern Louisiana. We’ve had to update the colour charts on our precipitation graphics in order to effectively map it.”

Jeff Masters – Meteorologist explains what has made Harvey such a uniquely devastating storm

Six trillion gallons of water, or 24in of rain, representing half Houston’s annual total, have fallen on the city in the past 48 hours. Even more has fallen on the much larger Houston metropolitan area. Scientific American just posted a fascinating interview with Jeff Masters, co-founder of Weather Underground, who explains Harvey’s extraordinary destructive potency.

On Wednesday it was a tropical storm. By Friday it had been supercharged from a category one hurricane to category four. That’s because Harvey passed over an area of extremely warm ocean water called an eddy. The hot spot was one to two degrees F warmer than the surrounding Gulf of Mexico, which is already one to two degrees F higher than normal at 85 or 86F in some places.

SA also explains that Harvey has become wedged between two areas of high-pressure, one system over the south-eastern US, the other over the south-western US. The dueling systems are trying to push Harvey in opposite directions, effectively wedging it in place. The last time that happened to a comparable storm system was Hurricane Mitch in 1998 that killed an estimated 7,000 people in Honduras. Masters also answers the question why Harvey is still producing so much rain despite being mostly over land. He says Harvey has dropped so much water over such a large area of south-eastern Texas that the storm is pulling that water back up into itself and dumping it again as more rain.

“You only need about 50% of the land to be covered with water for that to happen,” Masters told the magazine. “Obviously we have more than that in Texas.” Finally, he explains why the flooding in Houston is so severe. Masters calls it “compound flooding” when water swollen rivers heading to the sea meet a storm surge coming inland. In Galveston the sea surge was about three feet, but the actual water surge was about nine feet. “The water piles up from both sides,” Masters says.

But the good news in what is otherwise an apocalyptic scenario for the residents of Houston, is that a low-pressure trough system has been setting up north of Harvey and could being to pull it northward by the end of the week.