Here's a picture of a radar reflectivity image from a supercell thunderstorm from this afternoon in North Carolina that was producing a tornado at the time: Radar 0.5 degree Base Reflectivity image from KRAX, 2040Z, April 16, 2011. The tornado is completely wrapped in rain so it looks no different from its surroundings on the precipitation image. Most tornadoes are relatively weak and short-lived, and you'd never know they were there if you were looking at a regular radar image showing precipitation. Values close to 100% mean that everything in the area is uniform — all of the precipitation is the same size and the same shape, indicating that it's likely all rain. Here's the same view, but with the base velocity image telling us what the winds are doing inside of the thunderstorms. You can see that the red and green colors are very close together and relatively bright, showing strong rotation within the thunderstorm. when there was nary a blip on the reflectivity (precipitation) image. From left to right: classic supercell (North Carolina), low precipitation supercell (Nebraska), high precipitation supercell (Iowa). That's where the tornado was located, and this base velocity image is just screaming out "rotation!" Follow The Vane on Facebook (and the author on Twitter) for more weathery goodness. This is especially true of tornadoes that occur in landfalling tropical systems, squall lines, and storms very far away from the radar site. Can you spot it? Since not everyone is a weather geek, one of the most common questions people asked was "what am I looking at?" All radar images were archived using the Gibson Ridge Level 2 Analyst (GR2Analyst) radar software package. Both visually and on radar, the rain free updraft area and precipitation core are separate. B.A. As a thunderstorm develops, strengthens and begins to rotate, a hook shape can appear on the edge of the storm on radar… }, Very strong dual-pol correlation coefficient with #tornado northeast of Tupelo MS at 2:54 PM CDT #mswx pic.twitter.com/daOKmZg29D, — Tornado Quest (@TornadoQuest) April 28, 2014. Same concept as the “hook echo” explained above, but we generally look for this particular feature using a vertical cross-section. One of the dual-pol products that helps meteorologists detect the presence of a tornado is called the "correlation coefficient," or CC for short. The bounded weak echo region, BWER for short and nicknamed the “elephant trunk,” occurs when precipitation wraps around the warm, moist updraft. If you have a small area of very low CC values within the strong rotation showing up on the base velocity images discussed above, there's a good chance that you're looking at the radar beam reflecting off of debris swirling around inside of a tornado. The following two tabs change content below. The base velocity function of the radar does not show precipitation intensity but instead speed and direction. They often display certain radar characteristics. The previous radar images are base reflectivity images. Even though the image is static, the couplet is so intense that you can almost see it spinning. Dual-pol technology was used to confirm the presence of the Mississippi tornado that I've used as an example throughout this post. On the other hand, take this image from south-central Mississippi back on April 7 of this year. That same night, my wife’s boss had her husband and dog, along with her house all carried away, possibly by the same funnel. You just have to know what to look for, and that is a “hook echo.” The hook echo is formed when precipitation is caught by the mesocyclone, sucking The wall clouds and funnels will form right beneath them, so check there for any organized rotation. Where should you go in your house during a tornado? Fear not — here's how you can see a tornado using weather radar. Velocity is often symbolized in reds indicating winds moving away from the radar site (think “red shift”) and greens indicating winds moving toward the radar site. If the positive and negitive areas are oriented perpendicular to the radar beam, then you're looking at a rotation couplet which indicates a tornado. No? The base velocity product is usually measured in knots, but in my radar software I've added code to convert it the imagery to MPH, so that's the scale used in the images in this post. When tracking storms on radar, some of the most visually impressive and complex looking storms are tornadic supercells. This “hook-like” feature occurs when the strong counter-clockwise winds circling the mesocyclone (rotating updraft) are strong enough to wrap precipitation around the rain-free updraft area of the storm. Right: Supercell with clear debris ball in Kansas on April 12th, 2012. Starting in 1988, the National Weather Service rolled out a new type of radar, which you'll sometimes see referred to by its official name — "WSR-88D," which stands for "Weather Surveillance Radar-1988, Doppler.". Check out this example of a storm approaching Culpeper, VA on April 8th, 2011: Storm approaching Culpeper, VA on April 8th, 2011. Visually you can often see the entire updraft. The ability to see the winds inside of a thunderstorm is a huge deal. Left and middle: Two supercells on March 2nd, 2012 with confirmed tornadoes on the ground at the time these radar captures were taken. blockquote.twitter-tweet { A couplet is when red and green colors show up side-by-side within a thunderstorm on the base velocity image. It is actually possible to spot a developing or existing tornado on Doppler radar. Recognition of the hook echo has been around for decades; even before Doppler radar was invented and instituted in forecast offices, forecasters issued tornado warnings solely based on the visual appearance of a hook echo on radar. Tornadic supercell approaching Birmingham, AL on April 27th, 2011. U.S. tornadoes that occur outside the U.S. … the continental U.S. that is! As mentioned, not all storms exhibit that obvious hook echo and may appear considerably more harmless on the base reflectivity, when in fact they have strong rotation. The latest advance in weather radar technology is the relatively new "dual-polarization" — called dual-pol for short — that the NWS just finished installing across the country. Using base velocity is extremely important when determining if a supercell is strongly rotating and poses a tornado threat. The embedded Tweet above shows the CC image for the EF-4 tornado that struck Tupelo, Mississippi last week. It could see where rain was falling and how heavy it was, but that was it. It is important to stress that tornadic supercells could display all, one, or none of the above signatures on radar. Look at the image in the link below. It can be tricky to figure out where the radar site is most of the time, but you don't need to worry too much. The vast majority of tornadoes that touch down in the United States don't come with the (in)famous hook echo that people are familiar with. Below I provide some examples of common tornadic radar signatures and what they mean. A low precipitation supercell (LP) is simply a supercell that does not have a lot of precipitation surrounding it. When bright reds and bright greens are next to each other, that can indicate rotation in a storm. This is an animated example of the couplet produced by the tornado in Mississippi that I used as an example above. The most recognized and well-known radar signature for tornadic supercells. At the time this image was captured, there was a tornado just as strong but three times larger (1/3 of a mile wide) tearing through a few small farm communities. Birmingham, AL supercell from April 27th, 2011 and Raleigh, NC supercell from April 16th, 2011 both showing well-defined hook echo features. Debunking the myth. A very cool signature if you see it on radar! A couplet is when red and green colors show up side-by-side within a thunderstorm on the base velocity image. When couplets are large and relatively weak, it's indicative of broad rotation within a thunderstorm that needs to be watched but may not show a tornado on the ground.