All weather forecasts can't be accurate, just like this sentence.

This topic belongs to the subordinate relationship between outspoken propositions. It can be deduced that "all weather forecasts can't be accurate".

"Some weather forecasts can't all be accurate" means "Some weather forecasts are not necessarily inaccurate"

It doesn't necessarily mean it's possible, but it adds up to "some weather forecasts may be inaccurate"