Below, I have a posting that talks about “wall clouds” and their association to tornadoes. Here is an excellent video from North Dakota yesterday (in the same general area where a tornado warning is in effect now) that shows the speed/scale of rotation in a wall cloud in relation to a tornado. Viewing this video will be helpful when assessing whether rotation threatens you in the absence of a tornado warning. Video by Roger Hill via YouTube.
Hat tip: Tim Marshall and Will Wilkens