This is the watch that the SPC issued this afternoon at 1:10 pm for portions of the Upper Midwest. Most watches last between 7-10 hours because of the anticipation of severe storms to develop in that watch region and they issue it about 1-2 hours before storms really get going to alert the public of the severe storm potential.
This watch was only issued for that one lone supercell you see on the radar map above in NE Iowa and only put into effect for 4 hours and 50 minutes. You may be thinking that maybe they issued it because they thought more super cellular storms may form out ahead of that one, but the watch discussion didn't mention that risk at all and, in fact, no new storms formed and that one cell ended up dissipating.
I have only been tracking storms for two years and forecasting for one, but I will admit that this is a first I have ever seen and don't believe a watch has ever been issued before because of one cell.