The flood disaster resulted from an unusual combination of weather factors that conspired to produce exceptional rainfall.
In short, a storm system or zone of low pressure over Central Europe, named “Bernd,” became trapped between flanking areas of high pressure to the west and east. The low-pressure zone, which tapped tropical moisture from the Mediterranean, unloaded bursts of torrential rain over two to three days. And human-caused climate change may have played a role by increasing the intensity and duration of the event.
In the hardest-hit parts of Germany, two months’ worth of rain fell in 24 hours, according to the Deutscher Wetterdienst, Germany’s meteorological agency. A map tweeted by the agency revealed that it was a 1-in-100-year deluge over a large swath of western Germany, or one that has just a 1 percent chance of occurring in any given year.
Numerous locations in western Germany received 5 to 7 inches of rain, with locally higher amounts, between Tuesday and Thursday. Some of the most extreme downpours occurred Wednesday night into Thursday, when more than a half-foot of rain fell in less than 12 hours. Here are some select totals from western Germany:
- Reifferscheid received 8.1 inches in just 9 hours, according to CNN.
- Cologne received 6 inches in the 24 hours ending Thursday morning, according to CNN.
- Lüdenscheid received 4.5 inches of rain in 24 hours, according to Deutscher Wetterdienst.
- Düsseldorf received 1.89 inches in 12 hours Tuesday night into Wednesday, according to AccuWeather. Its three-day (between early Tuesday morning and early Thursday morning) total was 5.6 inches.
Heavy rain not only affected Germany but also Belgium, the Netherlands, Luxembourg and France.
How it happened
The deluge unfolded as a plume of deep moisture, sourced from the Mediterranean, surged into Central Europe on Tuesday and Wednesday. A high-pressure zone over the eastern Atlantic, off France and the United Kingdom, acted as a pump, steering the moisture plume through France into Belgium, the Netherlands and Germany.
The high-pressure system over the Atlantic was flanked by an unusually intense zone of low pressure to the east over Central Europe that helped generate the downpours. The low was sandwiched between another area of high pressure over Eastern Europe, creating a weather system traffic jam that allowed heavy rain to persist.
Computer models showed that the amount of moisture in the atmosphere, or a quantity known as total precipitable water, was comparable to those seen along the U.S. Gulf Coast during hurricane landfalls.
Before the torrents, Deutscher Wetterdienst had warned that weather models were predicting extreme amounts of precipitation in some areas.
The intensity of rain overwhelmed local rivers that flow into the Rhine River basin, and the volume of water overflowed several dams.
Much of the rain over Central Europe has ended as the high-pressure zone over the eastern Atlantic has extended further over the continent, pushing the low-pressure zone southward.
The climate change connection
The exceptional intensity of the rainfall is consistent with what scientists anticipate with rising temperatures caused by human-induced climate change. Higher temperatures speed up evaporation, placing more water in the atmosphere for the kind of downpours that have occurred.
A recent study in the Journal of Climate anticipates “large magnitude increases” in extreme precipitation in much of Europe in the coming decades.
Another study found that intense, slow-moving storms across Europe may become as much as 14 times more frequent by the end of the century. The study authors explained that the storms may slow, especially in the fall, because of weaker upper-level steering winds.
Climate change is expected to decrease the strength of steering currents as the high latitudes warm more quickly than the mid-latitudes, reducing the north-to-south temperature differences that drive the wind. This may lead to an increase in what are known as “blocking” patterns, in which storms get stuck.
In other words, the weather pattern seen in recent days over Central Europe, characterized by weak steering flow and a stalled, havoc-wreaking storm system, may indeed be a sign of things to come.