Horseshoe Vortices?

I have seen horseshoe vortices a handful of times on both chase days and fair weather days. Most recently, I saw one a couple of weeks ago on an otherwise beautiful day; the vortex was somewhat isolated from a patch of small Cu. So my questions are...

How do horseshoe vortices form?
Does seeing one on a day with severe weather potential indicate anything useful about the atmosphere that day?