I have always understood "screaming" to mean something along the lines of Janet Leigh in the shower in Psycho or Violet Elizabeth Bott.
I have been on MN for a couple of years now and will admit to being shocked by the number of people (often, but not exclusively, MILs) who resort to screaming when they don't get their own way.
It now occurs to me that posters are more likely to just mean "shouting" which puts things in a whole different light. Is it just me?