The southern plains of the Serengeti are very fertile but they need rains to ripen the grass for a massive population of grazers.
The "short" and light rains fall in November and December (sometimes as early as October). This draws the migration rapidly south from Kenya's Masai Mara down the eastern side of Tanzania's Serengeti into these sweet short-grass plains.
The wildebeest and other ungulates settle in the southern plains between about January and April - usually the "easiest" time to see the migration in Tanzania. In April and May the "long" or heavy rains set in. The depleted southern plains are less attractive than the long grass plains up in the western corridor and the migration has started moving north (westerly) again.
Large river crossings on the Grumeti and Mara Rivers occur as the migration heads back into Kenya's Maasai Mara - the season dries out and fresh grazing and water can be found in the far north. The Mara is usually at its best in August, September and October. The small maps below give a general idea on the movement of the migration, see a more detailed map of the Serengeti ecosystem plus explanations on how the migration really works!