Mike,
You are correct, the past few "systems" have cut-off, sat over the western U.S., and sheared off into the northern US, making minimal impacts on the central and southern plains weather. Most of the time, we look for a nice jet streak to round the base of an upper-level trough and nose itself out into the plains. For what it's worth, remember that transverse circulation about a jet streak results in divergence aloft over the left-front (left-exit) and right-rear (right-entrance) region of a jet streak. There was a publication by one of the SPC mets which found that the vast majority of signficant tornadoes occur in the exit region of jet streaks, and, in fact, there were more tornadoes in this study that occurred in the right-exit region (subsident region by transverse circulation) than the right-entrance region. Throw in the fact that many jet streaks are curved if it's rounding the base of the trough, and we tend to have enhanced upward motion in the left-exit region (due to transverse circulation and DPVA), enhanced subsidence in the left-entrance region (again, due to transverse circulation and DNVA), while the remaining two sectors (the right entrance and right exit regions) may experience negligible vertical motion due to the offsetting contributions from the transverse circulation and differential vorticity advection.
I'm glad you made the distinction, because many people tend to confuse divergence and diffluence. Divergence, as you correctly noted, combines both a speed influence and a directional influence. Winds may "fan out" over the plains (in a fashion known as diffluence), but there is often speed convergence over the same area. For example, we tend to expect diffluence downstream of a jet streak, yet if we are downstream of a jet streak, we had speed convergence. Sometimes these two contributions cancel out, other times directional divergence (diffluence or confluence) overweighs the speed convergence to result in net divergence aloft.
Because many of the big "systems" this year have not been progressive troughs, but rather giant upper-level cut-off lows, the flow in the plains has been unclimatological. With the cut-off lows like we have seen this year, divergence aloft tends to be negligible, since the flow about the low tends to be rather homogeneous in a circular sense. Typically, this isn't the case, and we do see at least a few events with the classic "jet streak coming out of the southwestern US and into the Plains". Alas, this hasn't happened much this year, and in fact there is a direct relationship between the lack of this upper-level setup and the lack of moisture in the plains. Istead of a deep western US long-wave trough, which usually induces lee troughing and broad return flow, we've had a persistent eastern US trough and Hudson Bay low (reminscient of January), which has allowed for these Canadian highs to invade the continental US east of the Rockies. The upper-air pattern has been such that these large continental airmasses have been able to push all the way through the Gulf and, in some cases, well into the Caribbean and Central America. Thus, we have an unfavorable upper-air pattern which has been a signficant cause of the lack of Gulf moisture in the plains. And, we see, the two big issues this year, moisture and the lack of progressive upper-level troughs and southwest flow aloft, are tied together.
EDIT: I was afraid to write "invade" since that's very similar to "INVADERS" in Corso-ese. :lol:
