Training: Identifying Rotation
Over the past couple of years I have submitted my video to the National Weather Service to use in their spotter training. Beginning last summer I ,also, began to go out on my own and teach civilians, public safety, and children on the ins and outs of severe weather. In this video I will outline what we are looking at and explain the key components of it. This video was shot from perfect position to illustrate where you, as a spotter, should be located when viewing a classic supercell. Hopefully you are able to apply some of these things to the knowledge you have already accrued.
First what I want all of you to do is to click play on the video above and watch the evolution a couple of times. What sorts of things do you notice? As an experienced spotter there are several storm features that should jump out at you. There should also be no question in your mind as to what is occurring. The experienced spotter should view this video and recognize every element that is occurring and be able to tell in real time what is about to occur. For the inexperienced spotter or novice, I will go into detail below using video captures and references to the video. Please feel free to use this in your presentations or leave comments and questions below. The following slides will cover a THREE MINUTE span and be labeled with the original photo along with a diagrammed image. Video sped up to 4x. 3:20 seconds down to :54 seconds.
Slide 1 and 1a – Slide 1: Real image – Slide 1a: Diagram
The first slide slows a standard wall cloud filmed from 1-2 miles away. Currently we are positioned in the inflow region of the storm where there is no precipitation falling and our visibility is unaltered. This is where you generally want to spot from. If you were viewing this from behind or in the core, you’re more than likely going to have impaired visuals due to heavy precipitation. The biggest danger to you right here would be lightning as it is very common throughout the storm. One way to tell how potent the storm is by the level of inflow you feel going into it. As you stand here, you’re likely to feel a brisk, warm wind at your back. This is what is known as inflow. Inflow is what feeds the storm. Many times it is warm and moist. As you look at the wall cloud at this point, you notice this long ragged looking cloud to the right. In the video it is feeding from right to left into the wall cloud. This is essentially a boundary between the warm, moist inflow and the cool, wet downdraft. Typically a lot of erratic cloud motions and rising will occur in this region. This in itself is NOT tornadic, but will signal the strength of the updraft. At this stage of the storm’s evolution, only broad cyclonic rotation is evident. The whole supercell is rotating, because by definition that is what supercells are. A long-lived rotating updraft. It is not uncommon to see the cloud base exhibiting rotation. In the case of this video capture, the areas marked with arrows is slowly rotating around a fixed point. At that fixed point, a lot of rising scud is evident. If you reference back to the video from 00:00 to 00:20, you notice a broad rotation, but nothing that has really tightened up.
What you are observing: Broadly rotating wall cloud
What you should report: Wall cloud with broad rotation, strong inflow, rising scud into the tail cloud and underneath the base
Slide 2 & 2a:
If you reference 00:24 – 00:40 in the video, you will see the next set of features that I am going to explain. We noticed in the first slide that inflow, tail cloud, and rotation were present. If you watched the video between the times posted above, you begin to notice a close point to this rotation now. Before it was very broadly rotating, but it was at 180 to 270 degrees. Now you notice that to left edge of the wall cloud there is a full 360 degree rotation. It is this area that we should be most concerned with. Also notice how clear it is behind the wall cloud. What happens is the supercell will ingest this warm, rain free, air and wrap itself around it. In the video you see the rain curtains behind the rotation moving in opposite direction. This is caused by dry air slamming down to the surface and essentially “parting the sea.” As the dry air wraps back around the circulation, it rises and eats away at the cloud base causing a clear slot. A clear slot is a big indicator that tornado genesis is occurring or about to occur.
What you are observing: Rapidly rotating wall cloud, clear slot developing
What you should report: Strong rotation in the wall cloud, strong inflow. Optional to report clear slot.
Slide 3 & 3a:
If you reference between 00:41-00:48 in the video above. As you are observing, this is when the most dynamic action takes place with this wall cloud. Remember, the video is sped up 4x speed. So we more or less went 2:00 minutes from broad rotation to violent rotation. The first thing we notice about this image is that there is no longer that clear air around the wall cloud. This phenomenon is known as “core-dumping.” There are many theories revolving around why this happens. There was a study done that says cloud tops will collapse above a tornado. As they collapse, their precipitation crashes down to earth (ex: microburst) which makes sense to me as rear flank downdraft winds are generally very strong like a microburst. The difference is these winds are rotating cyclonically around the wall cloud and not just slamming to earth and spreading outwards. Another theory is that as the clear slot develops from the rising dry air, cold moist air comes crashing down. What goes up, must come down. As the rotation tightens around a point, that downward traveling air gives that extra “push” needed to allow the funnel cloud and tornado to descend toward earth. Quite frankly I do not know the definitive answer as to why this occurs, but I do know it does occur on many of the tornadoes I have witnessed. Usually when you see a rapid descending of precipitation around the backside of the wall cloud, tornado time is not too far off. With that big clue set aside, we also notice the development of a funnel cloud. The most tell-tale sign a tornado is likely to occur.
What you are observing: Violently rotating wall cloud, funnel cloud, focused rain curtains wrapping around the wall cloud
What you should report: Funnel cloud
Slide 4 &4a:
The last four seconds of the video are of the tornado itself. Very obvious what you are seeing here. Classic tornadogenesis. Now your focus should be on movement and safety. I will get into that on another article at a later date. This, however, is your guide into recognizing storm features and what is actually going on in a microscale environment. As a storm spotter, safety and storm recognition should be your number one priority. If you observing a storm like this and are looking at anything other than this area, then you should not be out there spotting!
What you are observing: Tornado
What you should report: Tornado, it’s movement, your location, it’s estimated location. You can even estimate it’s speed.
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