The numbers of fast moving MCSs were as follows: April 11/56 (20%); May 46/103 (45%); June 50/173 (29%); July 33/160 (21%); August 27/125 (22%); September 8/61 (13%); October 8/67 (12%); total 183/745 (25%). The fractions represent (number of southward propagating MCSs)/(total number of MCSs), and the numbers inside the parentheses are the percentages of the southward propagating MCSs. We concluded that fast southward propagation of MCSs tends to occur when a trough exists over the Bay at the height of 600 hPa, and the wind shear between the surface and the 600 hPa height has a southward component. We also found that propagation towards other directions is related with eddy disturbances over the Bay.