Your typical shelf clouds are usually formed by forward flanking downdrafts in multicell clusters and squall lines. At this point I would say the feature actually is a shelf cloud as the RFD is dominant. This is what happens when an updraft becomes occluded. It often continues to do so until the original wall cloud has fanned out into what looks like a regular shelf cloud. When a clear slot forms in a wall cloud, the rear flanking downdraft is in the process of punching through. So, I'd think that most anything on the leading edge of a big storm is rarely going to be properly called a wall cloud. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. *Wall Cloud - A localized, persistent, often abrupt lowering from a rain-free base. The glossary page for storm spotters (Norman) gives this definition for wall cloud: Wikipedia is a nice way to get a basic intro to a subject, or links to more definitive sources - but I wouldn't use them as my primary source. I'll let others with more experience answer, but I'll just put in my 2 cents.
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