Bangladesh is likely to witness another spell flood from July 20-21, warned the Bangladesh Water Development Board.   This information was provided in a report published by the board on Saturday.   According to reports, heavy rain is likely to occur in Assam, West Bengal and northeast India for four days from July 20-21. Due to the onrush of upstream water and heavy downpours, the north, northeast and central parts of the country, may witness another flood.   Arifuzzaman Bhuiyan, executive engineer of the government's Flood Forecasting and Warning Center, said that according to the forecast data, about 800mm of rain is likely in four days from July 20, which will increase the water level in the country's rivers.   However, even if the water level rises, the water level in the Padma River may start to recede continuously from July 26-27. The overall flood situation may become normal in the last of July or the first week of August.