This is totally speculation on my part, but since they haven't issued a tornado warning--even if only to be on the safe side--I have to believe they're hearing from spotters that the storms show no indiciation of imminent tornadogenesis. Otherwise you'd warn just to make sure people have some lead time in and around such a huge population center. Again, I don't have any information to support this, just thinking out loud.
EDIT: tornado warning issued moments after I posted that, and a friend confirmed the sirens are blowing in downtown Ft Worth right now at 2112z