The orange alert applies mostly to western Belgium, i.e. the provinces of East and West Flanders and Hainaut in Wallonia, but was extended to the whole of Belgium later this afternoon.
The thunderstorms can come with heavy gusts of wind, hail stones and a lot of rain in a short time, which may cause local floods. Between 10 and 20 litres of rain per square can be expected in some places, or in the worst case even 30 to 40.
The heavy thunderstorms are triggered by a big temperature contrast: while we hardly have 20 degrees Celsius in the west today, eastern parts of the country have up to 28. Sunday should be drier, but more thunderstorms are expected on Sunday evening before the weather will get cooler in the whole country, next Monday and Tuesday.