Rainy season onset and cessation over the Greater Horn of Africa area: definitions and forecasts.

The demand for more accurate forecasts in rainy season onset, length and cessation has significantly increased over the Greater Horn of Africa area. Recent failed rainy seasons, and an extended drought over much of the region, have highlighted the need for both reliable and timely forecasts so that action can be taken proactively rather than reactively. One of the major challenges in the weather and climate science community is how to appropriately define and characterize onset in such a way that is both robust and useful to the stakeholders.

As part of the EU H2020 project CONFER (Co-production of Climate Services for east Africa), we revisit the rainfall onset and cessation definitions used over the subcontinent, with a particular focus on the large discrepancies, reaching up to 50 days, in the onset and cessation dates that emerge from different definitions. The climate over the Greater Horn of Africa is highly variable with most of the region classified as arid and semi-arid and only a few areas classified as humid. A regionalization of the thresholds used in the definitions that more accurately accounts for user needs and the amount of total rainfall an area receives is suggested. These regional details are then combined with a probabilistic approach developed in CONFER, based on a widely available multi-model seasonal forecast ensemble, to predict rainy season onset over the Greater Horn of Africa area.