Shelf cloud question...

[Todd Friskey]

Is it possible for the shelf cloud to exhibit a wall cloud as well?

 

[Jeff Duda]

Given the definitions and processes by which each are created, I would vote "no". However, this is a mixed issue since it is an issue with both objective and subjective aspects. A wall cloud is generally a lowered base of a thunderstorm that is formed when relatively cool, more moist air is pulled up into a storm via a strong updraft. Thus, you tend to see wall clouds under rain free bases associated with strong thunderstorms. The air parcels forming a wall cloud typically come from the forward flank area of a supercell, or just some rainy area of an ordinary storm (there's nothing meteorological that says non-supercellular storms can't produce wall clouds). Shelf clouds, on the other hand, are formed when warm moist air from the environment (i.e., from outside the region being impacted by the storm) rises over a surging cold pool produced by the storm. A shelf cloud can still be lower than other nearby clouds associated with the storm, and a shelf cloud is also the area where warm moist parcels that comprise the storm's fuel enter into the updraft. However, shelf clouds are usually much larger than wall clouds since they are forced by an expanding cold pool with environmental air flowing in from several directions, whereas wall clouds are typically small and concentrated in the area where the strongest part of the updraft is able to pull parcels up. However, I have seen tornadoes spawn from shelf clouds, and wall clouds can morph into shelf clouds due to storm scale processes (such as a cold air surge from the rear-flank downdraft area of a supercell).

Again, however, these types of clouds exist on a continuum, so there can be confusion as to exactly where the separation between shelf cloud and wall cloud is. If you're paying close attention to a storm, however, you can more easily determine the cloud type by simply observing the evolution of the parts of the storm. This just takes practice/experience.

 

[Dan Robinson]

In rare cases, a segment of a shelf cloud can transition into an updraft base, where a wall cloud/tornado can eventually develop. This can happen if a storm is cycling between outflow and inflow dominance, where inflow gets re-established on the eastern side of the outflow's gust front. Gene Moore has a good example of this happening on his web site here: http://www.chaseday.com/lewp.htm

 

[Skip Talbot]

Just a couple visual examples to illustrate some of the points made above, and keep in mind this is the rear flanking downdraft/core of a supercell. Most shelf clouds do not have wall clouds on their northern ends, such as your typical forward flanking gust front shelves on an MCS/squall line.

Looking NNW:

Looking SSW:

It's very common to have a wall cloud under the rain free base, and when the rear flanking downdrafts punche through it with an RFD clear slot, it starts to fan out like a shelf cloud. The RFD often takes over and the base fans out into a big shelfy gust front. In the middle of the transition, however, you often have what looks like a wall cloud on the northern apex of a shelf cloud. The wall cloud portion is being driven by inflow and the shelf cloud by the RFD outflow. A general rule of thumb which I love to use to diagnose these cloud types is to ask, "What is the pointy end pointing toward?" Toward the rain, and it's usually a wall cloud, away from the rain and it's a shelf. The reasoning behind this, as mentioned in the above posts, is that the wall is feeding off the rain cooled air so the tail cloud forms pointing toward the rain, while the shelf is being fanned outward by outflow and is feeding on the warm air ahead of the storm, so it points away from the storm's core.