There are enhanced probabilities of below-normal rainfall conditions over the period September-December 1998 over much of the eastern part of the eastern Africa sub-region (sometimes referred to as the Greater Horn of Africa). Near-normal to below-normal rainfall conditions are the most likely outcome in the south-western part. Enhanced probabilities of near-normal rainfall are indicated in the northern areas.

THE CLIMATE OUTLOOK FORUM

From 2-4 September 1998, a Climate Outlook Forum was convened to formulate consensus guidance for the September-December 1998 season in the eastern Africa sub-region. The Forum reviewed the state of the global climate system and its implications for this region. Among the principal factors taken into account were the evolving La Niña episode, and cooling sea-surface temperatures in the western Indian Ocean. These sea-surface temperature patterns have been associated with below-normal rainfall conditions over the southern half of the sub-region. These same sea-surface temperature patterns are associated with above-normal rainfall conditions in the northern half of the sub-region. In comparison with the March-May period, rainfall during the September-December season has relatively high predictability over the central and southern parts of the region.

METHODOLOGY

The regional climate assessment began with consensus agreement that the La Niña episode will continue to evolve over the forecast period and that western Indian Ocean sea-surface temperatures will continue to cool. This and other factors affecting the climate of the eastern Africa sub-region were assessed using coupled ocean-atmosphere models, physically-based statistical models and expert interpretation. The current status of seasonal-to-interannual forecasting allows prediction of spatial and temporal averages, and may not fully account for all factors that influence regional and national climate variability. This Outlook is relevant only to seasonal time scales and relatively large areas; local and month-to-month variations may occur. Users are strongly advised to contact the respective National Meteorological and Hydrological Services for interpretation of this Outlook and for updates and additional guidance.

The experts established probability distributions to indicate the likelihood of below-, near- or above normal rainfall for each area (see Map). Above-normal rainfall is defined as within the wettest third of recorded rainfall amounts in each region; below-normal rainfall is defined as within the driest third of rainfall amounts; near-normal is the third centered around the climatological median.

OUTLOOK

September to December constitutes an important rainfall season over the eastern Africa sub-region south of about 10° N, and in coastal areas of the Red Sea. An exception is southern Tanzania, where the rainfall season usually starts late in the forecast period.

Over southern Somalia, the eastern half of Kenya and eastern Tanzania (region I) there is a high probability of below-normal rainfall. This area constitutes the region with highest forecast confidence. Northern Somalia, and south-eastern Ethiopia (region IV) also have increased probabilities of below-normal rainfall. Western Tanzania, Rwanda, Burundi, western Kenya and much of Uganda (region II) show relatively low probabilities of above-normal rainfall, and higher tendencies for near- to below-normal rainfall. Proceeding north, increased probabilities grade from near-normal (north-western Kenya, north-eastern Uganda, southern Ethiopia and southern Sudan (region III), Djibouti, central and northern Ethiopia, and eastern Eritrea (region V)), to above-normal rainfall (central and northern Sudan and the extreme western parts of Ethiopia (region VI)).

It should be noted that in the southern half of the region several of the forecasts indicated increasing probabilities of below-normal rainfall as the season progresses. A second point of note is that the enhanced probabilities of above-normal rainfall in northern and central Sudan are principally the result of rainfall in the early part of the forecast period.

CONTRIBUTORS

Contributors to this consensus forecast included representatives of Meteorological Services from nine countries (Institut Geographique du Burundi; Djibouti Meteorological Department; Meteorological and Hydrological Service of Eritrea; National Meteorological Service Agency, Ethiopia; Kenya Meteorological Department; Rwanda Meteorological Service; Sudan Meteorological Authority; Directorate of Meteorology, Tanzania; Uganda Meteorological Department) and climate scientists and other experts from national, regional and international institutes (ACMAD; CLIPS WMO; Disaster Prevention and Preparedness Commission Ethiopia; Drought Monitoring Centre, Nairobi; North Carolina State University; University of Nairobi; CIMMS, University of Oklahoma; International Research Institute for climate prediction; National Centers for Environmental Prediction, NOAA; Office of Global Programs, NOAA). Additional input was supplied by the United Kingdom Meteorological Office.

MAP CAPTION

The numbers for each region indicate the probabilities of rainfall in each of the three categories, below, near and above-normal. The top number indicates the probability of rainfall occurring in the above-normal category, the middle number is for near-normal and the bottom for below normal. In the case of western Tanzania, for example, there is a 15% probability of rainfall occurring in the above-normal category (i.e., within the range of the wettest third of recorded precipitation totals); a 45% chance in the near-normal category; and a 40% chance in the below-normal category. It is emphasized that boundaries between sub-regions should be considered as transition zones.