During the spring and summer, severe weather rolls through a particular region, creates wind damage and depending on the severity, many may call the storm a tornado. But was it? When there is no striking funnel seen due to rain or the storm occurs at night, it can be hard to tell the difference between straight line winds and a tornado. In this instance, we cannot be fully confident a tornado has occurred until the National Weather Service sends a survey team to investigate, but meteorologists can help determine the difference using weather radar and experience.
Let's look at an example from June 23, 2015. On this day, strong winds aloft combined with a hot and humid unstable environment to create a volatile mix in favor of severe t-storms. A squall line of storms developed over south-central PA with the aid of a remnant boundary and front moving into the region. This line began to "bow" (just like a bow & arrow) heading into Philly, which tipped our meteorologists off that strong winds were associated with it (see below) and that wind damage heads-ups were needed. The storm relative velocity mode of the radar confirmed our suspicions and clearly represented the straight-line winds with particles moving quickly away from the radar site (shown in red below). Sure enough, this bowing segment produced wind gusts of 70+ mph in the Philly area and even 85 mph straight-line winds were surveyed by the NWS in southern NJ.
June 23, 2015 radar imagery courtesy of GR2Analyst, Tuscaloosa tornado from via NOAA/NWS.
Although many thought a tornado occurred in south NJ on the 23rd, a tornadic storm looks very different on radar. Instead of a "bow echo" appearance, tornadic storms typically take on a "hook echo" shape as shown above (radar from Tuscaloosa 2011). This occurs due to a specific storm structure, the most important element being storm rotation or a "mesocyclone." Storm relative velocity radar is key in this situation too, as you can see reds (traveling away from the radar) close by blues/purples (traveling towards the radar). This was a clear sign of cyclonic circulation and in this instance, where the two colors met (tornado vortex signature), a tornado was on the ground.
These are just a few of the tools meteorologists use to discern the severe threat from thunderstorms.
