The conditions of 30 blocking events were examined at the time of onset over the 1993/1994--1996/1997 cold season (November to March). The antecedent synoptic-scale eddies in 83.3% of the total events are shown to induce a large-scale low/high eddy forcing pattern upstream of the incipient block, and the mean basic westerly flow is weak, 76.7% of which possesses cyclonic shear. It appears that both the antecedent synoptic-scale eddies that induce a low/high eddy forcing pattern and the weak background westerly flow (zonal wave number 0) that allows quasi-stationary preblock ridge are two necessary preconditions for the onset of blocking anticyclone, but the cyclonic shear of the background westerly flow is a favorable preblock environment. In addition, a theoretical model, represented by the superposition of dipole and monopole envelope Rossby solitons with weak orographic forcing for zonal wave number 2, is proposed to confirm these observations. It is found that the dipole envelope soliton is dominant and exhibits a quasi-2-week oscillation during the interaction between an antecedent blocking anticyclone and upstream synoptic-scale eddies. The monopole soliton tends to break up, indicating that blocking anticyclone circulation is dominated by the amplification of dipole component associated with synoptic-scale eddies. Finally, the evolution of time-dependent eddy feedback is shown to control the direction of planetary-scale potential vorticity transports induced by transient eddies, which is an indicator of the onset, maintenance, and decay of blocking. Moreover, the changes of amplified blocking anticyclone and synoptic-scale eddies obtained theoretically are also in agreement with those of observed blocking and synoptic eddies. ¿ 2001 American Geophysical Union