Potentially serious trouble was in the air; that was evident after the National Weather Service launched an upper-air balloon near Washington Dulles International Airport.
Data from the balloon documented that the air was soaked with invisible water vapor.
It wasn’t quite a record, said Jason Elliott, the senior service hydrologist at the weather service’s Baltimore/Washington D.C. office, but the “precipitable water” content of the air, 2.06 inches, was unmistakably the makings of a downpour.
That had a whole lot to do with what followed: An incredible deluge, a month’s worth of rain in an hour on Monday that turned the Washington D.C. metro area into a life-threatening lake, prompting a rare “flood emergency” alert.
But the atmosphere wasn’t the only factor in the widespread flooding. Human activity was a important co-conspirator, and we have seen that countless times around here.
A number of studies have addressed the issue of how worldwide warming is increasing the frequency of extreme precipitation events.
The hypothesis is that a warmer atmosphere would hold more water vapor. Just how much of an impact that would have remains a subject of research.
However, one indisputable contributor to flooding in urbanized areas is the ever-increasing impervious cover and stress on storm-water systems.
The Washington D.C. metropolitan area has experienced tremendous growth in population, and no doubt that has produced harvests of runoff, a major issue during heavy rains.
“That water has to find its way to drainage structures,” said Elliott, “and if the rain is so heavy that it exceeds the capacity of that drainage system, you’re going to get significant urban flooding.”
Another factor in metropolitan areas is the so-called urban heat island effect.
Buildings and paved surfaces in big cities such as Philadelphia, New York, and Washington D.C. are efficient absorbers of solar energy, which is one reason cities are so much warmer than their surrounding areas.
“Any area with higher heat content has the potential to provide more fuel to produce heavy rainfall,” said Elliott. Some studies have shown that winds can displace the effects downstream.
But this gets complicated. J. Marshall Shepherd, a scientist at the University of Georgia and expert on heat island effects, said it would be too early to determine just how much a factor urban heating was in the Washington D.C.-area deluge.
And Elliott noted Monday’s rains “were extremely intense before, during, and after the time they moved through the immediate D.C. area, so they were just generally extreme throughout.”
“We kind of got lucky here,” said Jonathan O’Brien, a meteorologist at the weather service’s Philadelphia/Mount Holly office, noting the most favorable environment for heavy rain had set up to the south.
Extreme rains can be capricious, especially in summer, and that might become evident again on Thursday as the weather service has most of the Philadelphia and Washington regions under a “marginal risk” for flash flooding.
The Philadelphia region has experienced several extreme rain events in the last two months, both in and around the city and well away from it.
On Saturday, for example, close to 6 inches of rain fell upon Stafford Township, Ocean County, and close to 5 inches in Pipersville.
What was different about Washington D.C. was the venue: the rains swamped the downtown area and flooded the White House basement.
It also triggered a rare “emergency” flood warning, a relatively new alert for the weather service.
Elliott said the government started using it in 2011, and that is “only issued when significant to catastrophic flooding is already occurring.” That sets off the smartphone Wireless Emergency Alerts system.
The Mount Holly office has issued only one, and that was a few years ago for a flooding event in Mercer County, N.J.