No. 926 - Greater Horn of Africa Climate Outlook Forum considers September-December rainfall

No. 926 - Greater Horn of Africa Climate Outlook Forum considers September-December rainfall



6 September 2011
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Greater Horn of Africa Climate Outlook Forum considers September-December rainfall

Geneva/Nairobi, 6 September 2011 (WMO) - The Greater Horn of Africa Climate Outlook for the forthcoming season (September through December 2011) indicates a possible return to normal/above-normal rainfall conditions in famine-hit southern Somalia, but a risk of below-normal rainfall remains over northern Somalia and adjoining regions.

September to December constitutes an important rainfall season - after the March-May rains - over southern Somalia and other equatorial parts of the Greater Horn of Africa (Kenya, Uganda, Burundi, Rwanda and northern Tanzania). Rainfall in northern Somalia during this period is typically lower than in southern Somalia.

Over parts of the region, given that September-December rainfall is a relatively small proportion of its annual total (see figures below), there may not yet be much easing of the drought situation in these areas where drought conditions have prevailed for the past several months.

Among the principal factors influencing the evolution of rainfall over the coming season are the currently available scenarios for neutral to weak La Niña conditions over the tropical Pacific, warmer than average sea surface temperatures in the tropical western Indian Ocean, and the associated monsoonal winds.

The Greater Horn of Africa Climate Outlook Forum was convened by the Intergovernmental Authority on Development Climate Prediction and Applications Centre (ICPAC), with the support of partners including the World Meteorological Organization. It has been issuing regular Climate Watch Updates since late last year about the severe drought in Somalia and adjoining regions. Some Western areas of the Greater Horn of Africa, by contrast, have been hit by floods and landslides.

The Forum met 1-3 September 2011 in Entebbe, Uganda, to review the state of the global climate system and its implications for the region. Users from disaster risk management, water resources, agriculture and food security, health and development sectors were among the participants at the meeting.

The outlook is relevant only for seasonal time scales and relatively large areas. Local and month-to-month variations might occur as the season progresses. For example episodic flash floods might be experienced in parts of areas that are predicted to receive near to below normal rainfall and poor rainfall distribution might be experienced in areas expected to receive above to near normal rainfall. Forecast updates will be provided by ICPAC and the respective National Meteorological and Hydrological Services.

Outlook Summary:

The consensus regional climate outlook for the September to December 2011 rainfall season over the Greater Horn of Africa indicates:

  • Increased likelihood of above normal to near normal rainfall over eastern equatorial parts covering southern Somalia; coastal, eastern and central Kenya as well as northeastern Tanzania. (Above-normal rainfall is defined as within the wettest third of long term recorded rainfall amounts).
  • Increased likelihood of near normal to below normal rainfall in Northern Somalia; eastern and southern Ethiopia; northwestern Kenya; northern Uganda; southwestern Rwanda; southern Burundi and western Tanzania. (Below normal is defined as within the driest third of the recorded rainfall amounts).
  • Increased likelihood of near normal to above normal rainfall over parts of Tanzania; northern Burundi; Rwanda; southern and central Uganda; western Kenya; much of South Sudan; western, central and northern Ethiopia; Djibouti; extreme southeastern Eritrea and extreme northwestern Somalia. (Near normal rainfall is defined as the third of the recorded rainfall amounts centred around the climatological median).
  • Generally seasonally dry conditions are indicated over Sudan; much of Eritrea and extreme northern Ethiopia.

WMO Update on El Niño/La Niña

The persistence of the severe drought over parts of Ethiopia, Kenya and Somalia since the last quarter of 2010 has been associated with La Niña conditions (unusually cool ocean surface temperatures in the central and eastern tropic Pacific).

The La Niña episode, which started in July 2010, ended in May 2011 and has been replaced by near-neutral conditions, with the ocean temperatures, tropical rainfall patterns, and atmospheric winds over the equatorial Pacific Ocean near the long-term average.

The continuation of near neutral conditions or the re-emergence of La Niña conditions are the most likely scenarios for the rest of 2011, according to WMO’s latest Update issued 1 September. If a La Niña event does occur, current indications are that it would be considerably weaker than the moderate to strong 2010-2011 episode.

El Niño/La Niña conditions are not the only factors influencing the climate of the Greater Horn of Africa. Other factors include positive sea surface temperatures in the western Indian Ocean and the associated monsoonal winds. Regional and local scale features may also exert additional influences on rainfall.

Much of the equatorial sector of the Greater Horn of Africa (Kenya, Uganda, Burundi, Rwanda, southern Somalia and northern Tanzania) has two major seasonal rainfall peaks during March-May and October-December. The western and coastal parts receive significant rainfall during July-August.

El Niño/La Niña Update

World Meteorological Organization is the United Nations System’s
authoritative voice on Weather, Climate and Water

For more information, please contact:
Clare Nullis, Media Officer, Tel: +41 (0)22 730 8478;
e-mail: cnullis[at]

Dolphine Ndeda. IGAD Climate Prediction and Application Center (ICPAC).
Tel: +254203514426.
ICPAC website:

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