Any info about how the weather conditions for this happened?
You need warm and very wet air in order to form massive clouds in the first place....
The warm wet air came from the North Sea, while a upper level low caused the needed rapid cooling of the air to produce the rain.
Lets start with the situation before:
Here is the situation when the first warnings arrived, a upper level low (HTK) over france (cold air sinks from the upper atmosphere, producing a lower pressure above), and a low pressure system (T) to the east over Austria. So far, nothing bad, but the low pressure system pumps lots of heated air from central europe over the north sea, triggering a good amount of evaporation there. The high pressure systems (H) in the north constrained the warm air even more, directing it.
For people much wiser than me, this must have been like looking into a loaded cannon. The pieces are placed on the board and will move according to their physical rules.
Here you can see the beginning of the real trouble one day later:
The upper level low has moved east, while the low pressure system moved counter clockwise around it to the north. Now it is pumping warm moist air from the north sea mostly into Belgium and France. Note how stable the HTKs are - the jetstream is barely moving them around.
And now, 6 hours later, lets go to the ugly part:
The low pressure system has moved further north with the upper level low and is now pumping much more moisture into the stream of cold air from above. Pay attention how little the low pressure system at ground level moved in six hours, steadily moving evaporation from the north sea into western Europe.
In the evening, the heavy rainfall shifted further into France again.
The low pressure system kept on dancing counterclockwise around the upper level low, which barely moved at all.
The maps had been produced by the ZAMG, a research agency of the Austrian government.