The Bad Weather Coalition, more formally known as the New Hanseatic League, is a group of EU member states Ireland, Netherlands, Denmark, Sweden, Finland, Estonia, Latvia, Lithuania, which are all “fiscally conservative northern European states”. The original Hanseatic league very much included cities like Hamburg, Bremen, Rostock, Stettin (now Szczecin) and Danzig (now Gdańsk). Based on the name and the political aims, I would expect Germany to be part of the club. Why isn't it? Was Germany not invited (perhaps the founders wanted to remain a club of smaller nations), or does Germany not want to be in the Bad Weather Coalition?
Many decision in the EU are pre-arranged by the German-French axis, which in practice means that the smaller countries don't have much influence.
The New Hanseatic League aims to give the smaller countries with similar views more influence. Therefore, it is a counter-balance against the German-French axis.
It would be strange if Germany were on both sides.
An advantage for Germany for not being in this group, is that they can be the negotiator-in-the-middle, while the New Hanseatic League is free to voice concerns shared by Germany.
I think that both the New Hanseatic League and Germany are happy with this arrangement, so there is no reason for them to join.