The African Monsoon Recent Evolution and Current Status

1. The African Monsoon Recent Evolution and Current Status Update prepared by Climate Prediction Center / NCEP 14 September 2009 For more information, visit: http://www.cpc.ncep.noaa.gov/products/Global_Monsoons/African_Monsoons/precip_monitoring.shtml

2. Outline • Highlights • Recent Evolution and Current Conditions • NCEP GEFS Forecasts • Summary

3. Highlights:Last 7 Days • The western end of the Sahel and Guinea continued to register heavy rains sustaining flood conditions in Senegal, The Gambia, and Guinea. • Portions of southern Sudan benefited from needed moderate to heavy rains.

4. Rainfall Patterns: Last 7 Days During the past seven days heavy rains sustained flooding in parts of western Sahel, including Senegal. Rainfall was significantly above average in western Senegal, The Gambia, parts of southwestern Mali, and western Burkina Faso. The exception in western Sahel was the western end of Mali, which received slightly below average rainfall. In the Gulf of Guinea region, rainfall was below average over central Nigeria, and along the coast from Ghana to Liberia. However, rainfall was above average in Guinea. Rainfall was also slightly above average in most areas over central Africa. Moderate rains brought relief to parts of southern Sudan and sustained moisture surpluses over western Ethiopia. Light rains sustained moisture deficits in northeastern Ethiopia.

5. Rainfall Patterns: Last 30 Days During the past 30 days rainfall was above average over most areas in the Sahel as heavy downpours continued to fall. The areas including Senegal and Gambia, western Mali, and Burkina Faso continued to register rainfall in excess of 150 mm. Rainfall continued to be extremely heavy in Guinea and parts of Sierra Leone. In contrast, rainfall was below average along coastal Guinea from Liberia to Nigeria. However, many areas in central Nigeria registered above average rainfall. Rainfall was above average over most areas in central Africa and parts of northern Ethiopia. Rainfall was below average over much of Sudan.

6. Recent Rainfall Evolution Daily evolution of precipitation during the last 90 days at selected stations shows that season-to-date totals are above average over the western sector of the Sahel (lower panel – left). Rainfall tampered off over Burkina Faso and brought relief to the floods (lower panel – right). Beneficial rains in southeastern Ethiopia over the past helped reduce moisture deficits in local areas (upper panel – right).

7. Atmospheric Circulation:Last 7 Days The 850 hPa wind anomaly (left panel) reflected westerly wind anomalies from the tropical central Atlantic into West Africa. An anomalous cyclonic couplet dominated northern Africa. The Mascarene and St Helena High in the southern hemisphere were stronger than normal. The 200 hPa wind anomaly featured an amplification of the subtropical ridges across both hemispheres.

8. NCEP GEFS Model ForecastsNon-Bias Corrected Probability of precipitation exceedance Week-1: Valid 15 - 21 September, 2009Week-2: Valid 22– 28 September, 2009 For week-1 and week-2, the global ensemble forecast system (GEFS) suggests over a 90% chance for precipitation to exceed 25 mm over most areas in the Gulf of Guinea region, parts of central Africa, and western Ethiopia.

9. Experimental Week-1 Precipitation Outlooks Week-1 Outlook 15-21 September, 2009 An increased chance for above average rainfall over the western ends of the Sahel and the Gulf of Guinea region: The strong meridional SST gradient between the Gulf of Guinea and the tropical eastern Atlantic and the GFS projected enhanced low level westerly wind anomalies are expected to increase precipitation in the region. Confidence: Moderate

10. Experimental Week-2 Precipitation Outlooks Week-2 Outlook 22-28 September, 2009 No Forecasts.

11. Summary • During the past seven days heavy rains sustained flooding in parts of western Sahel, including Senegal. Rainfall was significantly above average in western Senegal, The Gambia, parts of southwestern Mali, and western Burkina Faso. The exception in western Sahel was the western end of Mali, which received slightly below average rainfall. In the Gulf of Guinea region, rainfall was below average over central Nigeria, and along the coast from Ghana to Liberia. However, rainfall was above average in Guinea. Rainfall was also slightly above average in most areas over central Africa. Moderate rains brought relief to parts of southern Sudan and sustained moisture surpluses over western Ethiopia. Light rains sustained moisture deficits in northeastern Ethiopia. • During the past 30 days, rainfall was above average over most areas in the Sahel as heavy downpours continued to fall. The areas including Senegal and Gambia, western Mali, and Burkina Faso continued to register rainfall in excess of 150 mm. Rainfall continued to be extremely heavy in Guinea and parts of Sierra Leone. In contrast, rainfall was below average along coastal Guinea from Liberia to Nigeria. However, many areas in central Nigeria registered above average rainfall. Rainfall was above average over most areas in central Africa and parts of northern Ethiopia. Rainfall was below average over much of Sudan. • For week-1, there is an increased chance for above average rainfall over the western ends of the Sahel and the Gulf of Guinea region. It is a no forecast for week-2.