Translation:It will be bad weather this afternoon.
I would only ever say "The weather will be bad..." or "There will be bad weather...", personally.
I understand 'uvejr' in the same way as I do High German 'Unwetter' – a storm – aren't these cognates with corresponding meanings?
Yes, uvejr is related to "Unwetter". But it's not just a storm. It's defined as any kind of extreme weather: strong winds, rain, snowfall, hail, or thawing. In common use uvejr is mostly referring to a thunderstorm with rain or hail.