In 4 out of 5 cases, merging cells result in no intensification of the absorbing cell according to the SPC. This was not one of those cases obviously. In addition, the environment which would have supported an EF5 went from unimpressive to off the charts in 5 minutes roughly. There is NOT a meteorologist in this world who could have seen what was coming until the velocity scans came in probably about the time it was over Joplin. The point is, regardless of how advanced we think our radars are, some events just unfold so quickly that no one could have predicted it.