The herds stay in the Maasai Mara and Serengeti Mara until the beginning of the small rain season; unlike in the season of the heavy rainfalls caused by the monsoon, the small rain season is short and, more importantly, less intense.
The rainfalls usually start in November, but it cannot be taken for granted and their amount and intensity cannot be predicted; wildebeests can perceive, even at a very far distance, the longed-for thunderstorm and rain; it’s time for them to set off and return to their favourite pastures in the Southern Serengeti and the Ngorongoro Conservation Area.
They have to assess the time of leaving very carefully and precisely: if they set off too early and rainfalls were not enough to revive the pastures and fill the rivers and pools they would risk to starve and suffer thirst, thus putting their life at risk.
Miles-long columns of wildebeests and zebras cross the wooded savannah advancing confidently Southwards – an endless march towards their destination; they pass through the Lobo area, the Seronera and finally come to the so longed-for pastures. 
In years of prolonged drought, the migration routes can change somehow, sometimes considerably; in such an event the wildebeests instead of migrating to the South, divert their way by pointing West and stopping in the Western Corridor.
Except in such exceptional circumstances, the migration reaches the Southern pastures in December and stay here in the following months, when puppies come to light, and until the pastures are over exploited and the herds must migrate in search of water and new grassland.

Where is possible to spot the herds of Great Migration during the short rain season 

Lobo: although it is not certain when the migration sets off, Lobo is a good place to watch the migration at this time of the year, when numerous herds can be found despite the migration is moving Southwards; the kopjes and wooded savannah make this area very scenic and visitors can move easily around to spot the herds on their way.
Central Serengeti: when the rainfalls are sufficient and the herds are moving Southwards, the migrating animals can be viewed in this area, but it is quite difficult to predict the best time as it depends solely on rainfall.