There are enhanced probabilities of normal to below-normal rainfall conditions in the north-eastern part of southern Africa for the period October 1998 - March 1999. Further south, probabilities of normal to above-normal rainfall increase. The highest probabilities for above-normal rainfall are in the south-eastern part of the subcontinent.

THE CLIMATE OUTLOOK FORUM

From 29 September - 2 October 1998, a Climate Outlook Forum was convened to formulate consensus guidance for the October 1998 - March 1999 season in southern Africa. 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. Sea-surface temperature anomalies in the Atlantic Ocean are weak. The sea-surface temperature patterns in the equatorial Pacific Ocean have been associated with above-normal rainfall conditions over much of the southern part of the subcontinent, and below-normal rainfall conditions in the far north-eastern part of the region. However, Indian and Atlantic Ocean sea temperature anomalies can significantly modulate the signal from the equatorial Pacific.

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 southern Africa 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. In cases where climatology is indicated, no skillful forecast information could be provided.

OUTLOOK

October to March constitutes an important rainfall season over southern Africa.An exception is north-eastern Tanzania, where the rainfall season usually is divided in two (OND and MAM), and the extreme south-western part of South Africa, where winter rainfall predominates. No forecast information is provided for countries that were not represented at the forum.The outlook presented here are devided into two main seasons, namely October to December 1998 and January to March 1999.

Over eastern Tanzania (region I) there is a high probability of below-normal rainfall. West and central Tanzania, northern Zambia, northern half of Malawi (region II), and Mauritius (region VII) have enhanced probabilities of normal to below-normal rainfall. Normal to below-normal rainfall is expected over the northern part of Mozambique, southern half of Malawi and central Zambia. Southern Zambia, southern half of Malawi, central Mozambique, and Zimbabwe, excluding the south-western portion (region IV), have enhanced probabilities of near-normal rainfall. The south-western portion of Zimbabwe, Botswana, the southern part of Mozambique, Swaziland, central and eastern South Africa, Lesotho, and northern Namibia (regions V and VI) have enhanced probabilities of normal to above-normal rainfall.

Eastern Tanzania (region I) usually experiences a dry season during January-March and so climatology is predicted for this region. West and central Tanzania, (region II) have increased probabilities of normal to below-normal rainfall. For the rest of the subcontinent (regions III, IV and V) and for Mauritius (region VI) there are enhanced probabilities of normal to above-normal rainfall. Climatology is also predicted for the far south-western part of South Africa and for the western coastal regions of Namibia.

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 October-December 1998, 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. In cases where climatology is indicated, no skillful forecast information could be provided. It is emphasized that boundaries between sub-regions should be considered as transition zones. No information is provided for countries that were not represented at the forum.

CONTRIBUTORS

Forecast contributors included representatives from Meteorological Services of ten SADC countries (Botswana, Malawi, Mauritius, Mozambique, Namibia, South Africa, Swaziland, Tanzania, Zambia and Zimbabwe) and climate scientists and other experts from national, regional and international institutes (Drought Monitoring Centre - Harare, CIMMS - University of Oklahoma, International Research Institute for climate prediction, Climate Prediction Centre, United Kingdom Meteorological Office, Universities of Pretoria and Zululand).