All the SPC outlooks are based on probability. They determine the percentages for the chances that a wind, hail, or tornado report will occur within 25 miles of a point. They used the hatched areas to denote when there's a 10% or greater chance of more significant severe weather within 25 miles of a point. All those probabilities are converted to the risks you see. A high risk means that the odds of having severe weather are much higher than any other random day. It's only implied that an event would be expected to occur based on the conditions, but it does not mean that it's 100% certain. Sure, SPC wishes every high risk could be certain to occur, but it's just not possible. Please view this link from SPC for the conversions of the outlook risks:
http://www.spc.noaa.gov/misc/SPC_probotlk_info.htmlEveryone in society can't handle probabilistic forecasts. They want to know if it's going to happen or not, and the science has just not evolved yet to the point where that question can be handled with complete accuracy. Thus, you run into this issue of perception when the reality is that no such theory for mesoscale meteorology exists, and there's no magic formula to tell you exactly when and where these outbreaks will happen. The models were doing a terrible job trying to pinpoint this event, so SPC and everyone else was limited on how well they could predict it. Think about it for the future when you're trying to forecast this kind of event, and you'll see just how difficult it really is to predict severe weather.
I literally just got back from work where 25 severe thunderstorm warnings were issued in the Blacksburg county warning area today, and I actually issued 7 of those. All of them are verified with reports of winds gusts up 70 mph, power outages, numerous trees down, and quarter-sized hail. We even know of two deaths that occurred today. I'm exhausted to say the least.
