It’s likely that you’ve seen some of the commercials over the last 5 or 6 months, and if you have, you know that a lot has been made about 5 minutes. A relatively short length of time, but in the case of a severe weather situation, it could seem like an eternity, especially if someone is witholding, or doesn’t have access to what could be some life saving information. I guess in that sense you’ve probably raised your eyebrows once or twice when you’ve seen the commercials as it has been made to appear that there is a big discrepancy in the weather information being delivered to television.

Whether it is a lack of understanding or a concious decision to mis-lead, I’ll leave that up to you. I’m going tell you what is really happening with the National Weather Service Radar (NEXRAD) when severe storms threaten our area. Storms exist in the atmosphere in 3 dimensions. They are so many miles wide, so many miles long and so many miles tall (think of it as the x,y and z axis from your high school geometry class). When thunderstorms are present, NEXRAD scans at many different angles to the horizon, or what we call “tilts”. The image below depicts how increasing angles of the radar beam can give a 3 dimensional picture of the atmosphere.

In severe weather, or scan mode 21 (below), NEXRAD sends out and receives radar signals at 9 different angles to the horizon. The scheme begins by doing a 360 degree scan at half a degree above the horizon, it then performs a 360 degree scan at 1.5 degrees above the horizon and repeats this process by ending with a 360 degree scan at 19.5 degrees above the horizon line. This produces a volumetric or 3D picture of the atmosphere in a process that takes 5 minutes to complete. Each of these tilts arrives independently in our Live HD Doppler as they occur, we just don’t typically show you these individual scans as storms start to not really look like storms at the higher tilts and the reflectivity doesn’t represent precipitation that is falling from the storm.

So why do we need all of these tilts? By scanning in 3 dimensions we are able to ascertain depth of rotation (knowing that a storm has a tight circulation from its base through a 20,000′ layer makes you more concerned about a potential tornado), hail core positions and height within an updraft before they fall out the storm, intense updraft and storm height (typically the higher the storm top the greater chance of severe weather occuring). In contrast, the image below shows how a television station owned radar operates. It only scans one level. Scanning like this gives us very little to no information about the severity and potentially life-threatening imapact a storm may have.

In the image on the right, a bounded weak echo region (BWER- depicted below right in what would be the space between the high heel and the rest of the shoe) will show a tornado’s location when a hook echo is present. Note the difference in seeing the entire storm structure in 3D versus one slice on the left. Just under the 13 in our logo, you can see an overshooting top, also an indication of enhanced storm strength. In the single slice, you have no idea that one is present.

In the image above,  our sister ABC station in Houston, TX uses 3D NEXRAD imagery to show the intensity of rain bands in Hurricane Dolly as it came on shore near Houston. Note under the abc13 logo how a band that looks pretty intense at the surface (typical radar view you see on TV looks intense), but the slice shows that precipitation isn’t as intense higher up, so rainfall rates are weakening within that band. Below, you see a 3D radar image that allows us to see the condensation funnel of an actual tornado that moved through Moore, OK in a significant tornado outbreak that occured on May 3rd, 1999. A 2D radar image at one tilt just doesn’t give us all of the information, storms don’t exist in 2D.

Do you want the whole story or just a slice of it? You decide.