Observations and modeling studies suggest that rear-inflow jets vary in strength from a few m/s for weak systems, to 10-15 m/s for moderately strong systems, to 25 to 30 m/s for the most severe systems such as bow echoes. These RIJ strengths are relative to storm motion, i.e., actual ground-relative winds may be much stronger.

Weisman (1992) quantified the dependence of rear-inflow jet strength on vertical wind shear and buoyancy for numerically simulated convective systems and confirmed that rear-inflow strength increases for increasing CAPE and increasing vertical wind shear.

See more below.

Yes. Imagine a synoptic pattern like this idealized scenario that is commonly associated with severe squall lines including bow echoes (Johns, 1993). For a squall line developing in the brown threat area, we can see that stronger flow in the region of the polar jet would enhance the rear-inflow jet associated with either the squall line or an embedded bow echo developing in the northern portion of the area.

Additionally, if the mid-level storm-relative winds are significantly stronger behind a squall line, these enhanced winds can also contribute to the generation of the rear-inflow jet, especially when the squall line is expanding rearward to produce a large stratiform precipitation region.

Of course, enhancements in the large-scale wind field behind the squall line need not be present for the production of a significant rear-inflow jet.