ECMWF is really not meant to accurately predict thunderstorms the next day. It's a medium range forecast model that's best used for 2+ days ahead, and is very useful for initialization of short-range high resolution models that are much better for local predictions like thunderstorms. Currently Windy does not include those models except for France (AROME). The AROME model does include regions outside France like Belgium but those are meant to set the boundary conditions for France and not to accurately predict weather in those regions.
I'm mentioning this because I think if you're using Windy predictions for thunderstorms you should know about this, and because it would be very cool if those short-range predictions would be included in Windy. It is currently the main missing feature as far as weather prediction goes (I'm trying to get coverage in my own country via other posts), and most of them are available for free from their respective meteorological institutes.