Per experiences in Iraq - I would estimate that the next time - the explosions will be multiple - strung out in delayed fuses like they were in Boston - but set up along routes to target first responders - and large enough to disrupt local emergency logistics...conducted in multiple cities - and those that survive will be waiting in ambush...so numbers are going to count in the near future.
“The study of insurgencies can be overwhelming and tiresome at times - it’s only when you are directly involved through a life and death struggle as insurgents attack your position does the subject become interesting!” JH White
I would expect something similar to Harrod’s bombing 1983. First a bomb scare to drive people into the street and then bomb goes off in the street where everyone is gathering.