My guess is that "bad weather" doesn't always indicate a storm (though it probably does most the time). In Colorado, "bad weather" in the summer might just mean overcast, while "bad weather" in the winter might even mean sunshine (for us snow lovers :) )
it sounds alot like the dutch word: onweer, it would litterly translate to bad weather in english too, but it is used for thunderstorms. wondering if it is the same in danish? or does it really just point to bad weather?
It has the same connotation in Danish (and German 'Unwetter', I might add). Mainly thunderstorms but also other kinds of awful weather like heavy rain or hail, or blizzards.
Literal translation is more like "un-weather".
Noah, yes, it's standard to place a comma between two clauses (as long as they're not connected by og or eller). Here you're talking about two actions, "I have heard" and "it will be bad weather", so a comma between them is appropriate.