As people clean up after torrential rains and heavy flooding in cities in the Midwest and along the Atlantic Coast, the events highlight what many climate researchers say is a new "normal" for severe rainfall in the US.
Quite apart from what long-term changes in precipitation say about global warming, these events also provide a reality check on the ability of urban areas to cope with flooding from intense downpours in a warming climate.
They "definitely can tell us a lot about where our vulnerabilities are and what types of things might be on the checklist for fixing," says Joe Casola, staff scientist with the Center for Climate and Energy Solutions in Arlington, Va.
During the past 110 years, average precipitation has increased over much of the continental US. During the past 54 years, the amount of rain or snow falling from the top 1 percent of intense storms has increased in every region, with the Northeast and Midwest recording the largest increases, according to the latest National Climate Assessment from the federal government's US Global Change Research Program, released in May.
Those trends are expected to continue as the climate warms, with the wet areas generally growing wetter and the dry regions growing still more arid.
This week's storms fit the pattern.
The latest set of events began Friday night over Kearney, Neb., where up to 3.89 inches fell overnight, much of it in a two-hour period, according to Steve Hilberg, a climatologist at the Midwest Regional Climate Center at the University of Illinois at Urbana-Champagne. That was nearly an inch more than the city typically sees for the entire month of August.
By Monday, a storm system moving slowly into the Midwest dropped 4 to 6 inches of rain in four hours across a swath of southeastern Michigan, prompting Detroit's police department to send divers to check for motorists who might have been trapped in submerged cars beneath highway underpasses. Detroit airport logged a daily record 4.57 inches of rain.
But that was only a warmup. Tuesday and Wednesday, the storm made its way to and up the East Coast, with Washington receiving from 3 to 5 inches of rain and up to 10 inches falling near Greenhaven, Md.
By the time the storm reached Long Island, its region of most intense rainfall was relatively small, notes Paul Kocin, a meteorologist with the National Weather Service's Weather Prediction Center in College Park, Md. But it dropped 11 inches at Islip in three hours as the elongated region of heaviest rain within the storm kept a swath of western and central Long Island under its path from tip to tail.
The overall total at Islip's McArthur Airport was a whopping 13.57 inches – a total typical of tropical storms or hurricanes, which deposit their water over a longer period of time. Hurricane Irene set the previous record: 11.5 inches.
The large-scale atmospheric pattern that set up the storm "is more typical of fall or spring," Mr. Kocin says – a deep meander in the jet stream, a high-altitude river of air that flows from west to east and helps steer storms. The meander moved eastward slowly and dipped far enough south to ladle plenty of moisture-laden, summertime air from the Gulf of Mexico back north along the meander's leading edge.
By the time the storm made its way up the coast of Maine late Wednesday, it was still dumping up to 6 inches of rain in some spots.
On one level, preparing for such events might seem futile, since any storm dumping 11 inches in three hours is going to overwhelm urban storm-water control measures. And for older cities, such as those in the Midwest and Northeast, digging up old pipes that currently carry storm-water runoff – often the same pipes carrying sewage – and replacing them with large pipes is prohibitively expensive.
Yet the continuing growth of cities, with their paved surfaces and limited open land where rainwater can be absorbed, will increase the risk of flooding, even from storms less extreme that those that occurred during the past week.
"The first step of any 12-step program is to acknowledge you have a problem," says Rob Moore, a senior policy analyst for the Natural Resources Defense Council who focuses on climate's impact on water resources.
Once that occurs, there are ways of increasing a city's resilience to urban flooding that don't require digging up pipes.
One relatively simple approach that can help is through water conservation measures in homes and businesses, he says, especially where storm water and sewage share the same pipes. Less water flowing into the system from toilets, showers or sinks leaves more capacity for runoff.
Another approach involves regular cleaning of underground pipes and catch basins already in place, adds Jeanette Brown, an environmental engineer and senior scientist in the school of engineering at the University of Connecticut at Storrs. Over time, debris and sediment can build up in these, reducing their capacity to carry off storm water.
But, she acknowledges, a lot of communities either don't have the funds or don't really understand the importance of good catch-basin and storm-drain cleaning.
To reduce the effect of paving in preventing the ground from absorbing water, some cities are using porous paving materials – concrete or asphalt – in roads and sidewalks.
Green roofs on buildings and even the widespread use of household rain barrels can help reduce runoff from roofs, another impervious surface.
One way to address the issue of money for improvements to the way cities handle storm water is through the use of so-called state revolving funds the US Environmental Protection Agency makes available for water resource and water-quality projects, Mr. Moore says.
In late July, for example, Gov. Pat Quinn (D) of Illinois made state-revolving-fund money available to communities for green infrastructure projects such as increasing the number of roadside planter boxes that can provide a route from cloud to ground without being intercepted by pavement. Projects also would include those that use porous building paving materials to reduce runoff, as well as other techniques, such as roofs that incorporate vegetation into them to reduce runoff and keep urban temperatures cooler.
Compared with large-scale infrastructure projects, some of these things can be done relatively quickly and sustained through changes in building codes, Ms. Brown says. "They may not have a huge effect, but they will have some effect."