The best time to go to Tanzania depends on many things including your interests, exactly where you want to visit and why you’re traveling.
The “long dry season” occurs from June through October. During this time, rainfall is unusual. It’s a great time to visit Tanzania. During November and December the “short rains” occur. These are much lighter than the main rains and less reliable. They also bring new life and water to the Serengeti, causing a burst of grass growth and sparking the migration southwards.
If it has rained during the short rains, then it normally dries up for a few months. January and February, which is Tanzania’s “short dry season”, another great time to visit.
Generally, the main rainy season called the “long rains” occurs in late March, April and May. Afternoon tropical downpours are the norm – which are heavier and more predictable along the coast and on Zanzibar. The humidity is high and daily temperatures reach low-mid 30°C/high 80°F.
THE GREAT MIGRATION
Most travelers visit Tanzania to see the Serengeti’s great wildebeest migration. Linked to the rainfall, this stunning migration of close to 2 million wildebeest along with thousands of zebras follows a fairly predictable pattern, as the wildebeest are constantly seeking fresh grazing and water. The migration can be found during any given month; you just need to know where to look! While we can’t control the weather, generally speaking, the best time to travel to Tanzania for migration viewing is mid December to March (the migration birthing cycle) and June to October.
The short rains begin around early November. A little after this, in late November and December, the herds of the wildebeest migration arrive on the short-grass plains of the Southern Serengeti (south and east of Seronera, around Ndutu) and include the north of the Ngorongoro Conservation Area. Dispersed across these plains, wildebeest and zebras are everywhere – feeding on the fresh, nutritious grasses. They stay here through January, February and March, with most wildebeest calves born in a short window around February. Gradually they spread west across these plains, then around April they start their great migration north.
By May the Serengeti's wildebeest all seem to be moving north, migrating to seek fresh grazing and water. The area around Moru Kopjes and west of Seronera is then hectic with a series of moving columns, often containing hundreds of thousands of animals – joined by many zebras, and a scattering of Thomson's and Grant's gazelles.
Around June the wildebeest migration is often halted on the south side of the Grumeti River, which has some channels which block or slow their migration north. The wildebeest then congregate there, in the Western Corridor, often building up to a high density before crossing the river. The wildebeest migration continues moving northwards during July and August, often spreading out across a broad front: some heading through Grumeti Reserve, others north through the heart of the Serengeti National Park.
September sees the herds spread out across the northern Serengeti, where the Mara River provides the migration with its most serious obstacle. This river gushes through the northern Serengeti from Kenya's adjacent Maasai Mara Game Reserve. Watching the frantic herds of the wildebeest migration crossing the Mara River can be very spectacular; there are often scenes of great panic and confusion. By October the wildebeest herds are migrating again with more accord: all are heading south, through western Loliondo and the Serengeti National Park's Lobo area, returning to the green shoots which follow the rains on the short-grass plains of the southern Serengeti in November.