Cellars were under water and police were forced to close many underpasses after water collected on the streets. Weather service Meteomedia had issued their highest storm warning for the entire state prior to the flooding.
"We estimate that some 1,000 cellars in the city could be flooded," a police spokesperson said on Monday, adding that emergency services were "in control" of the situation.
The fire department in Mönchengladbach was called out repeatedly. "A lot of water came down," a spokesperson said.
While Cologne, Wuppertal, western Lower Saxony, and the Münster region got off lightly, gutters in the the Ruhr Valley filled up quickly, with water levels reaching kerb-level, authorities reported.