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Monday, May 30, 2011

Tornado Tutorial: Recognizing hook echoes on the radar images (by Shaun Tanner of the wunderground)

Quick Radar and Tornado Tutorial

by Shaun Tanner, wunderground, May 25, 2011

For the second straight day, the Storm Prediction Center in Norman, OK, is forecasting a high risk of severe weather (Figure 1). This time, the high risk of severe weather is directly over areas that have suffered through a record-setting flood of the Mississippi River of the past few weeks.

Figure 1. The convective outlook for Wednesday, May 25, 2011. Note the high risk in the Mississippi Valley.

If you are in the high or moderate risk area today, or even if you are just watching from the sidelines, the best way to prepare yourself for severe weather is to know what is coming well before it makes its way into your backyard. So, I have put together a quick and easy tutorial on how to read radar so that you know if a tornado is on its way toward you. I will try to use images from Tuesday's outbreak as an example.

Pinpointing exactly what you are looking at while looking at radar can be difficult, for sure. I was sitting with my fellow meteorologists yesterday watching the outbreak on radar while at the same time monitoring a TV station that was trying to cover the story. At one point, there were at least 3 tornadoes on the ground, and we were getting confused about which ones the TV meteorologists were talking about. And we are meteorologists! So if you get confused, don't worry. The bottom-line is to pay attention to local warnings and take cover when told to do so.

Storm Tracks
Weather Underground radar offers a unique feature that can let you track thunderstorms and potential tornadoes quick and easy. To see these storm tracks, simply go to the radar you are interested in and then choose something in the Storms dropdown menu. Then click Update Radar Map. What you get will look somewhat like Figure 2. The yellow squares represent thunderstorms that are producing hail, the yellow diamonds are mesocyclones (don't worry, another discussion), and the pink triangles represent possible tornado locations. Basically, the radar location "thinks" a tornado might there. This is where I usually start. If you see a pink triangle, or if there are really menacing reds and purples approaching your location, I would move onto the next step.

Figure 2. Storm tracks of various thunderstorms and possible tornadoes.

Hook Echo
A hook echo is the easiest way to recognize a tornado signature on radar. But, be wary, just because there is no hook echo present does not necessarily mean there is no tornado present. A hook echo looks a lot like Figure 3. Do you see the hook pretty much in the middle of the image? What you are looking at is the radar representation of a tornado. So, if you have a pink triangle for a storm track, and you see a hook echo, then you most likely have got yourself a tornado, and you should take cover if you are in its path. You can monitor the path of the tornado by watching the loop of the radar. GENERALLY, tornadoes follow a southwest to northeast path, but they can have a mind of their own sometimes. So, if you are close to the path of a tornado, then it is best to take cover, even if you think you are outside of its "path."

As a tornado gets "older," it may become "rain-wrapped" and harder to pinpoint. Figure 4 is a radar image from Tuesday, May 24, 2011, of a mature tornado that cruised north of Oklahoma City, OK. Admittedly, this tornado representation may be hard to see. But, it is there. See near the middle of the image where a swirl of deep red is next to a swirl of light green? That is the tornado. This tornado is almost "rain-wrapped." When a tornado becomes "rain-wrapped," it can be particularly dangerous because it becomes very difficult to see on radar and on the ground. If you can't see it coming on the ground, then you have no idea where it is or where it is heading. So, it is best to catch these tornadoes before they become "rain-wrapped," if possible.

Figure 3. This radar image was taken Tuesday, May 24, 2011. You can see the hook echo in the middle of the image, and it represents a strong tornado that approached Norman, OK. This tornado is the one that caused the evacuation of the Storm Prediction Center.

Figue 4. Radar of an almost rain-wrapped tornado from May 24, 2011, north of Oklahoma City, OK.

Radial Velocities
This might seem like hard-core meteorology, and therefore, might scare people off. But, in fact, it is a quite ingenious way of determining fast rotation, like a tornado. Figure 5 shows the radial velocities of one of the tornadoes that went past near Oklahoma City, OK, on May 24, 2011. Sometimes tornadoes can be hard to spot looking at this type of image, but this one is crystal clear. The actual radar site for this image is just off the lower right of the image. You can tell because this is where all the lines are converging. Using the scale on the left, the reds and oranges represent air that is moving away from the radar site, and the blues and greens are air that is moving toward the radar site. If no tornado were present, you would see blues and greens south of the radar site and reds and oranges north of the site. Because tornadoes are actually pretty small in scale and spin very rapidly, the tornado in the image will be represented by greens and blues right next to reds and oranges. Can you spot it? Pretty close to the middle of the image. Think about what that is telling you. It is saying that the air on the left side of the tornado is moving toward the radar site, while the air on the right is moving away. In order for that to happen logically, the air has to be rotating very quickly. In this case, the tornado is spinning counter-clockwise, which is the way that "most" tornadoes rotate.

Figure 5. Radial velocities of a tornado that moved past Oklahoma City, OK, Tuesday, May 24, 2011.

If all of this is confusing, which I have no doubt that it is, then never fear. The meteorologists at the NWS are very good at what they do. So, if they tell you to take cover in various Tornado Watches and Warnings, then do it. Stay safe everybody!

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