We live in a strange new world, one in which the familiar is all mixed up with the radically altered. Such was the case this weekend when a weather pattern that was pretty normal for summer spawned a single thunderstorm that produced a once-in-a-thousand-years flood event in Ellicott City.
Normal Weekend, Typical Weather Pattern, Abnormal Conditions
On Saturday, my wife and I readied to trek out to Shenandoah National Park for a happily-anticipated summer camping trip. As we headed out the door, the weather pattern looked mostly normal for summer, if a little stormy. A high-pressure system out over the ocean was pulling in moisture off its waters and drawing warm air up from the south. A low over western Pennsylvania and a warm frontal boundary over Maryland created instability in a big zone of convection from Northern Virginia on through to Connecticut. Overall, it was a pretty typical pattern that would probably have produced some moderate-to-strong late-afternoon thunderstorms back in the 20th century. Back then, it was far less likely that a similar pattern would have produced a 1,000 year flood event.
(Extremely warm sea surface temperatures on the weekend of July 30-31 helped to fuel the record rainfall event over Elicott City, Maryland. Sea surface temperature anomaly map provided by: Earth Nullschool.)
However, conditions were not normal, not the same as they were back during a time when human fossil-fuel emissions hadn’t forced the world to warm by 1.2 degrees Celsius above 1880s levels. In the new world in 2016, the ocean high-pressure system was circulating over record warm sea surfaces that were 3-5 C hotter than late 20th-century averages. And because of this, the ocean was bleeding off a whole hell of a lot more moisture than it typically would. Any storms that fired in that very wet air mass would, as a result, tend to pump out a lot more rain than is typical.
A Wet Atmosphere Crackling with Unusual Energy
As my wife and I made our way toward the Blue Ridge Mountains and down Interstate 66 and Route 211, large, energetic cumulus clouds sprouted all around us. Wafted in the hot, unstable air, many tops punched up through the troposphere, spreading out into the characteristic anvil shapes of thunderstorms.
Light streamed down between these big, wet beasts. For a while, as we made our way up to the campground, set up our gear, and took a hike along a local rock scramble, we were fortunate — able to enjoy our day despite the loud rumbles and roars of thunder echoing up from the valleys or off the nearby mountainsides in the steamy, moisture-choked air.
(A massive amount of atmospheric moisture fueled powerful thunderstorms on Saturday, July 30 from the Appalachians of northwestern Virginia to the Baltimore City region. One of these storms dumped more than 4.5 inches of rain on Ellicott City, Maryland Saturday in just one hour. Image source: Terp Weather.)
At about 3,000 feet in elevation, these conditions were a bit odd for Shenandoah National Park which typically experiences milder weather. Temperatures were around 80 degrees Fahrenheit (about 5-6 F hotter than average), and the level of atmospheric moisture was amazing. Great, steamy clouds kept rolling up the mountainsides. They made the air heavy and full of shapes dancing with light and shadow, seeming to give it the character of some alive thing sprouting a thousand wet heads and arms.
The big thermals and thunderstorms were supported by hotter temperatures into the 90s (F) down in the valleys. And the storms were taller, bearing more moisture, engorged by a hot atmosphere whose temperatures and water vapor levels are now probably unlike anything seen in at least the past 115,000 years — conditions that would have devastating effect just a couple of hours later and about 100 miles to the east in Ellicott City.
Returning to camp, as we prepared for a hearty dinner of tempeh and veggie pasta, thunder from the southwest grew ever closer and a rolling wave of cloud seemed to spill in through the trees, spreading mist and heavy rain over everything nearby. In just a few minutes, we were scrambling into our vehicle and watching as torrents of rain streamed down, transforming the mountaintop campground into a world of rushing water.
Thunderstorm Dumps More Rain on Ellicott City than Any of the Past Deluges or Hurricanes in its History
At about the same time that my wife and I were scrambling for cover, another massive thunderstorm was bearing down on Ellicott City, Maryland. The storm hit one of the densest pockets of atmospheric moisture in that big bleed off the record-hot Atlantic Ocean and just exploded. Packed with all that unusual and heat-fueled moisture, the storm then began to dump its amazing and unprecedented torrents on this historic town of 65,000 just 20 miles to the west of Baltimore. In only 60 minutes this massive thunderstorm managed to unload 4.5 inches of rainfall. Two-hour rainfall totals approached six inches.
(Restaurant-goers in Ellicott City watch in shock as the street below floods and sends vehicles hurtling past. Video source: Ellicott City Flash Flood.)
Streets were rapidly flooded as the Patapsco River rose to a record 14 feet and leaped over its banks. The main road running through the center of town became a four-to-six-foot-deep torrent that hurtled vehicles along its path or into buildings. First responders scrambled to rescue more than a hundred motorists who were suddenly stranded in the flash flood. Tragically, two people lost their lives.
The force of the flood was so energetic that not only were many of the historic buildings in town damaged, but some had their very foundations torqued off-center, twisted by the great energy of the sudden flood waters churning through town. The Maryland government has declared a state of emergency, but it is uncertain how long it will take to make repairs or how much that is irreplaceable has been lost in the flood.
Two 100+ Year Flood Events For Ellicott City in the Past Five Years
The Washington Post’s Capital Weather Gang appropriately notes that Ellicott City is a pretty vulnerable place for floods:
It’s a highly vulnerable spot, an urbanized strip along the bottom of a deep valley through which the Patapsco River flows. This place, historic Ellicott City, Md., has seen plenty of serious floods: 1868, 1923, 1952. More recently, the remnants of Hurricane Agnes (1972) left an extreme high-water mark, measured in many feet. The Great Mid-Atlantic Flood of June 2006, once again drowned parts of the town.
And another big flood back in 2011 pushed the Patapsco River to 11 feet, prompting a local art gallery to build a 20-inch-high flood barrier. But the unforeseen flood that roared into Ellicott this past Saturday was the worst among all of these. As a result, most protections and flood barriers previously erected were quickly overwhelmed.
(Dramatic microburst over Phoenix, Arizona on July 21, 2016. Each degree Celsius of warming increases the atmosphere’s water vapor content by about seven percent. This increase, along with other factors such as intensified convection and rising cloud tops, escalates the frequency of extreme rainfall events like the July 30, 2016 Ellicott City flood. Image source: GGferg.)
Two-inch-per-hour rainfall amounts are usually enough to completely overwhelm most drainage infrastructure, overtop banks, and turn streets into rivers. Ellicott City saw nearly six inches of rainfall in two hours and 4.56 inches of rainfall in just one. Such events are typically seen as quite rare (100-year events or more) and building codes do not often account for them. Managing so much water is a major engineering challenge and requires a great deal of investment. Spending so much money on flood defense systems for a storm that might happen in 100 years or 1,000 years can sometimes seem like a waste at the time.
However, in a world warmed by climate change, such floods are happening with greater frequency. A 100 to 1,000 year flood may happen every five years or so in some locations (as has been the case with Ellicott for 2011 and 2016). The atmosphere is loaded more and more with both heat and moisture. The troposphere is taller due to the heat. The oceans are warmer and bleed more moisture. All these factors combine to make even pop-up storms more intense and to generate the 100 and 1,000 year events with a higher frequency in the present day. And as the world continues to warm, such severe storms will become ever more common.
Hat tip to Colorado Bob
Hat tip to DT Lange
Hat tip to Greg
Hat tip to Griffin