Originally posted by Jeff Snyder+--><div class='quotetop'>QUOTE(Jeff Snyder)</div>
<!--QuoteBegin-Michael P. Morris
BL-6 km is important in determining the storm mode. Deep-layer shear vectors oriented perpendicular to the surface boundary are most favorable for discrete supercells.
IIRC, a normally-oriented deeplayer shear vector will favor, equally, left and right splits as they move off the convergence source. If we imagine a N-S dryline with an easterly shear vector, the right-splits will tend to move SE, while the left-splits will tend to move NE. If the dryline initiates a several supercells along its path, there may be significant storm collisions.
When there is a 45 degree difference between the convergence source (say, a dryline) and the deeplayer shear vector, right-splits tend to remain discrete longer since there will be a tendency for the right-splits to move more easterly while the left splits move more northerly or north-northeasterly. This can mean that the right-splits are able to "stay ahead of" the left-splits, thereby avoiding collisions. Regardless, the point that Michael made remains -- shear vector orientation that is more normal than parallel tends to favor discrete activity (rather than a congealing linear mess).
Slightly related to the shear-vector discussion, here's a paper highlighting the storm mode frequencies with different forcing boundaries. -->
Dial, G.L., and J.P. Racy, 2004: Forecasting Short Term Convective Mode and Evolution for Severe Storms Initiated along Synoptic Boundaries. Preprints, 22nd Conf. Severe Local Storms, Hyannis MA. [104K PDF][/b]