So in Berlin there was 250mm of rainfall in 24h. Here is a video of that. It will drain into the river Elbe.
It takes about 8 days for a drop of water in the river Elbe to go from the Czech border to Cuxhaven. So just slightly before G20 Hamburg all of this stuff from the video above will be piling in on Hamburg.
See also
https://www.kompf.de/gps/rivermap.html
for detaild “where will this water finally reach the ocean?” information
How long for water in the Elbe to reach the North Sea? https://www.tagesschau.de/inland/elbe140.html
But for Hamburg this does not matter… Water from rainfall (we call it “Oberflächenwasser”) has no real effect on the water level in Hamburg. Remember, a normal tide will make the water rise or fall 4000mm within 6 hours. The few drops from Berlin will not make a difference.
The “Elbe Flooding” from 2013 made the tide in Hamburg about 50cm higher than normal tide. That’s less than 4 Beaufort wind from Northwest will do.
Hamburg gets flooded by Saltwater, not sweet water, if it gets flooded at all.