Based on geopotential height and wind field grid data from EC model, the relationship between the synoptic pattern over Europe-Asia area from Jun. to Jul. in 2003 and medium range forecast for torrential rain, which caused flood over Huaihe Valley,is analyzed. The results show that the heavy rain over Huaihe Valley that lasted for 14 days was caused by the interaction of the following factors:two waves maintained as extra-long wave over high level of 500 hPa,five or six waves appeared as long wave alternatively, blocking high established in the high latitude and the ridge of the West Pacific subtropical high stably located between 23～27°N that was north to its normal place.