I personally drove around the northern Serengeti as well as down to south / central Serengeti over the last week.
I noticed several large herds moving south all the way from the Sand River / Lobo / Bologonja / upper Grumeti / Kleins areas.
I then spent a night at Migration Camp and the following day I drove down south only to find that from Lobo right to Mbuzi Mawe all the herds came together, they were coming from all directions and joint up on Togoro plains north of Mbuzi Mawe.
I stayed at Pioneer Camp for one night on the 6th and on my return all the herds started heading east, some south east. There was also a huge herd moving thought the Seronera area.