Rainfall onset dates (ROD), rainfall cessation dates (RCD), and length of the rainy season (LRS) are crucial for crop production and food security in Eastern Africa, yet they are scantily documented. This paper seeks to investigate the spatial patterns of these parameters.

The data used are Climate Hazards Group InfraRed Precipitation with Station Data (CHIRPS v2.0) and National Oceanic and Atmospheric Administration (NOAA) gridded temperature. A threshold of 1 mm for rainy days, 20 mm over 5 days with at least 3 rain days, and a dry spell not exceeding 7 days in the next 21 days were used to determine RODs.

The Potential Evapotranspiration (PET) and Water Balance (WB) criteria were computed to determine RCDs.

The differences between ROD and RCD were used in calculating LRS. The results showed early rainfall cessation over more than 30 counties in Kenya led to a shortened rainy season by 10–20 days during the MAM season. Similarly, 20–40 days early onset dates are observed in most counties in upper Nile, Unity, and Jonglei states in South Sudan, while 20–40 days delayed rainfall onset was observed in Khartoum and southern parts of Nile state, western Darfur, eastern and Aljazeera states in Sudan, and most parts of Ethiopia and Eritrea districts during JJA season.

The Highlands of the western and Nyanza regions of Kenya and most parts of Uganda observed rainfall onset by March and no sign of cessation before November. Early cessation over northern Uganda districts was behind the shortened LRS; furthermore, the early RODs over western and southwestern Uganda districts increased the LRS.

Prolonged dry conditions over northern Sudan, southeastern parts of South Sudan, northern Kenya, central Somalia, northern Darfour, Kordofan, and northern parts of Sudan were exacerbated by significant delayed onset and early cessation of rainfall.

These findings are important for rain-fed agricultural planning and food security in the IGAD region of Eastern Africa.