Whether a flash flood happens is not decided by how much rain falls over a whole day, but by how much water falls in how little time. The Deutscher Wetterdienst, Germany’s national weather service, defines heavy rain exactly that way: rainfall amounts that arrive in a very short window and overwhelm a city’s drainage or a stream bed’s capacity, regardless of the total spread across the day.
Why time matters more than volume
Forty millimetres (1.6 inches) of rain spread over twelve hours mostly soaks into the ground or drains away in a controlled manner. The same forty millimetres falling within forty minutes overwhelms even well-built drainage immediately. Soil can only absorb water at a certain rate, and during short, intense rain that rate is far lower than the rate at which the water is actually arriving.
The Deutscher Wetterdienst defines heavy rain not by the total amount but by the rate of fall: how much rain arrives within a given short window and therefore overwhelms drainage capacity.
Sealed surfaces in cities, roads, car parks, roofs, make that problem worse. Where natural soil could take water in, asphalt passes it straight on, and the water gathers at the lowest points around, often underpasses, basement stairwells or narrow streets.
How a single storm produces that much water
One slow-moving storm, or one that keeps re-forming, can keep raining over a limited area for hours while barely a drop falls a few kilometres away. That phenomenon, where storm cells build repeatedly over the same spot and rain themselves out there, is called back-building, and it is one of the most common triggers for local heavy-rain events that end in a flash flood.
The reason lies in wind shear, or more precisely in the lack of enough wind shear to move a cell along. If the cell stays over one place instead of moving on, the rainfall total at that exact spot keeps adding up, while the surrounding area stays dry.
Why sealed surfaces make the problem worse
In cities, the steady growth in sealed surfaces, new car parks, additional roads, denser housing, raises the flash-flood risk on top of the weather itself. Every newly sealed surface means less natural ground able to take up rainwater, and more water running straight and unchecked into the drains or across the surface. Older drainage systems, designed decades ago for the building density and rainfall patterns of their time, frequently hit their capacity limits during today’s heavy-rain events, even when the rainfall itself is not historically exceptional.
That means two identical heavy-rain events in different decades at the same location can have quite different consequences, simply because how much of the surroundings is sealed has changed in the meantime.
The difference between a warning and reality
A general severe weather warning for a district says little about whether one particular street or one particular basement will actually be affected. Back-building cells mean heavy rain can be extremely local, often limited to a few blocks, while the rest of the warned area stays dry.
RainViewer shows precipitation rates in real time down to 100 metres (330 feet), which makes it possible to see whether a storm cell is actually stalling over one street or moving on.
Follow the rain rate, not just the general warning
A severe weather warning is the right first signal, but it often covers an entire district while the real flash-flood risk concentrates on individual blocks. Anyone who wants to know whether one specific spot will actually be affected needs the precipitation rate at exactly that spot.
RainViewer
