The African Monsoon Recent Evolution and Current Status Update prepared by Climate Prediction Center / NCEP 13 September 2010 For more information, visit:

Outline Highlights Recent Evolution and Current Conditions NCEP GEFS Forecasts Summary

Highlights: Last 7 Days During the past seven days, heavy rains sustained moisture surpluses and flood conditions in parts of western Sahel including Senegal. Dry conditions sustained moisture deficits in southeastern Nigeria.

Rainfall Patterns: Last 7 Days During the past seven days, much above average precipitation sustained moisture surpluses across western Sahel from Burkina Faso to Mauritania. The areas that received the heaviest rains include western Senegal with amounts averaging about 150 mm (100 mm above the mean) according to the satellite rainfall estimates. The persistent rains resulted in flooding in many localities. Rainfall was also above average over portions of the Gulf of Guinea region. However, light rains sustained moisture deficits over central and southeastern Nigeria. Rainfall was above average locally over parts of central Africa, but below average over northern DRC and eastern CAR. Rainfall was above average over western and central Ethiopia.

Rainfall Patterns: Last 30 Days During the past 30 days, rainfall was above average over most parts of West Africa except for local areas in western Niger, Sierra Leone, and most areas in Nigeria. In central Africa, rainfall was above average over southern Chad, northern CAR, but below average over southern Cameroon, and parts of Gabon, Congo, and DRC. Rainfall was also below average over western Uganda, but above average over western Kenya, southern Sudan and north-central Ethiopia.

Rainfall Patterns: Last 90 Days During the past 90 days, rainfall was above average over most parts of West Africa except for Sierra Leone, western Liberia, and northern Nigeria. Rainfall was above average over the southern half of Chad, CAR, southern Ethiopia and the northern edge of Congo. Rainfall slightly below average over eastern DRC and parts of Sudan and Uganda. Wet conditions resulted in above average rainfall over northern Ethiopia and local areas in southwestern Kenya.

Rainfall Patterns: Last 180 Days During the past 180 days, rainfall was above average over most areas in West Africa, except for portions of Sierra Leone and northern Nigeria. In central Africa, rainfall was above average over southern Chad, CAR, southern Cameroon, Gabon, and the northern areas of Congo and DRC. Rainfall was below average over southeastern DRC,. In East Africa, rainfall was above average over northern Ethiopia, and locally over southern Sudan, western and eastern Kenya, and southern Somalia.

Recent Rainfall Evolution Daily evolution of precipitation during the last 90 days at selected stations indicates persistent rains over the past week sustained moisture surpluses over western Senegal (lower panel – left). Lighter than average rains resulted in moisture deficits in local areas in northern Nigeria (lower panel – right. Rainfall has been steady over northern Uganda, but amounts remained below average (upper panel – right ).

Atmospheric Circulation: Last 7 Days The 850 hPa wind anomaly (left panel) reflected westerly wind anomalies over West Africa with a cyclonic vortex off the coast of Mauritania and a trough over west Africa. The 200 hPa wind anomaly (right panel) featured a strong upper level divergence across northwestern Africa. This pattern combined with the low level conditions as featured in the 850 hPa wind anomaly acted to support convection and precipitation in West Africa.

NCEP GEFS Model Forecasts Non-Bias Corrected Probability of precipitation exceedance Week-1: Valid September, 2010 Week-2: Valid September, 2010 For week-1, there is a high chance for precipitation to exceed 50 mm over Guinea, Sierra Leone, northern Cote d’Ivoire, northern Ghana, Togo, Benin, most of Nigeria, Cameroon, CAR, northern DRC and eastern Ethiopia. For week-2, there is a high chance for precipitation to exceed 50 mm within a belt covering from Guinea through Ghana, across southern Nigeria and Cameroon, all the way to CAR and northern DRC. Rainfall over a small portion of Eastern Ethiopia is also expected to exceed 50 mm.

1. There is an increased chance for above average precipitation over parts of West Africa: The anomalous westerly flow from the warmer parts of the Atlantic Ocean will feed moisture into the westward convective systems and result in enhanced rainfall activity. Confidence: Moderate Experimental Week-1 Precipitation Outlooks Week-1 Outlook 14 – 20 September, 2010

Experimental Week-2 Precipitation Outlooks Week-2 Outlook 21 – 27 September, 2010 No forecast – Climatology suggested.

Summary During the past seven days, much above average precipitation sustained moisture surpluses across western Sahel from Burkina Faso to Mauritania. The areas that received the heaviest rains include western Senegal with amounts averaging about 150 mm (100 mm above the mean) according to the satellite rainfall estimates. The persistent rains resulted in flooding in many localities. Rainfall was also above average over portions of the Gulf of Guinea region. However, light rains sustained moisture deficits over central and southeastern Nigeria. Rainfall was above average locally over parts of central Africa, but below average over northern DRC and eastern CAR. Rainfall was above average over western and central Ethiopia. During the past 30 days, rainfall was above average over most parts of West Africa except for local areas in western Niger, Sierra Leone, and most areas in Nigeria. In central Africa, rainfall was above average over southern Chad, northern CAR, but below average over southern Cameroon, and parts of Gabon, Congo, and DRC. Rainfall was also below average over western Uganda, but above average over western Kenya, southern Sudan and north- central Ethiopia. During the past 90 days, rainfall was above average over most parts of West Africa except for Sierra Leone, western Liberia, and northern Nigeria. Rainfall was above average over the southern half of Chad, CAR, southern Ethiopia and the northern edge of Congo. Rainfall slightly below average over eastern DRC and parts of Sudan and Uganda. Wet conditions resulted in above average rainfall over northern Ethiopia and local areas in southwestern Kenya. For week-1, There is an increased chance for above average precipitation over parts of West Africa