Sorry for cluttering up the discussions. Before watching 'cartoons on Channel 4' for a month or so I hadn't realized that big storms seem about as likely to roll lengthwise (like a very wide tank-tread) as to spin around in a circle. I'm guessing that when really big storms are both rolling and in some way spinning they can transport lots of 'weather' from one region to another.

Anyhow this particular storm had at one point appeared to be narrower at its 'exit' than at its 'entrance' - and the effect had appeared to be rather dramatic. But alas the animation at the page referenced above is no longer showing the effect (or illusion perhaps) in question.