The upwind "vector approach" is a method developed by Corfidi et al. (1996) to forecast MCS (or more specifically mesoscale beta element - MBE) movement. It is the vector sum of the mean flow through the cloud-bearing layer and the propagation component. The magnitude and direction of the propagation component is assumed to be equal and opposite to that of the low-level jet (850 mb).

Reference:
Corfidi, S. F., J. H. Merritt and J. M. Fritsch, 1996: Predicting the movement of mesoscale convective complexes. Wea. Forecasting,11, 41-46.