1. SEASONAL FORECAST JAS 2009 : revisiting its evolution
Ousmane NDIAYE
PhD student Columbia University

2. The consensual seasonal forecast of JAS 2009 issued back in end of May indicated wet over the Gulf of Guinea and dry over western Sahel consistent with the SST pattern over the Atlantic as well as predicted Nino condition in the Pacific.
The SST dipole in the Atlantic (warm over the Gulf of Guinea and cold over the north-eastern tropical Atlantic) maintains the ITCZ further South in the Gulf of Guinea favoring wet conditions for bordering countries and dry conditions over the Sahel.

3. OBSERVED RAINFALL CONDITIONS
August
July
June
May
Forecasted conditions were observed from
May to July : wet over the Gulf of Guinea (green) and dry over the Sahel (grey).
The stations in Niamey and Dakar show such tendencies until beginning of August.

4. Special case in Ouagadougou, Burkina-Faso
The capital of Burkina-Faso, Ouagadougou, did show predicted conditions up to July (see deficit in grey) but few events changed the whole situations (look at the event in September) and had tremendous impacts.

5. BURKINA-FASO : OUAGADOUGOU
Courteously by Nakoulma Guillaume

6. SENEGAL : DAKAR
Courteously by Moustapha Ciss

7. GAMBIA : BANJUL
Courteously by Lamin M. Touray

8. Rainfall in Rosso : August 27- 28, 2009 176 mm in 24h
MAURITANIA : ROSSO
Rainfall in Rosso : August , mm in 24h  
3 died, many injured and more than 4000 families relocated
Courteously by Sidi Mohamed

9. TROPICAL SST CHANGE OVER THE ATLANTIC FROM MAY TO AUGUST 2009
This year from May to August 2009 the SST over the Atlantic has completely reversed. The dipole in May which favored dry conditions (North wind) has completely reversed during August to condition which favors wet conditions (South-Westerly flow).
This SST reversal and consequent wind changes can explain the situation of this year which started with dry conditions in the Sahel and became wet starting end of August to September.

10. What did we learn and the way forward
Need a better forecast of the SST over the Atlantic,
Consider ENSO : strength, location and timing
Continuous updating the seasonal forecast as SSTs do change
This year rapid change occurs in end of July
One GCM was able to project correctly Aug rainfall initialized with July SST
keep the users updated : explain what did happen and ‘keep in touch’
Separate weather (few events in time) and climate (seasonal trend)