Mlukas made this interesting PCA:

Quote Originally Posted by mlukas
Although few Flemish are in the same area as Wallon, most of them are closer South Dutch.

Also except one individual there is no overlap between Flemish and Central Dutch.

Also overlap between Central and North Dutch is not very big.

Average Eurogenes K36 results for each of 5 groups from the PCA:

In case of some admixtures, North-South clines are clearly visible: