Our TV cable system has a channel that continuously shows the Nexrad weather radar while the audio is the drone of the NOAA weather broadcast. The radar display is a single-site from the Nexrad station at Grand Rapids.
Most Nexrad images we see are mosaics of the radar network. Over most parts of the U.S. the Nexrad coverage areas of the more than 150 radar stations overlap for both redundancy when a station is off the air, and to see greater detail by looking at storms from different angles and different ranges. The Nexrad images we see in the cockpit sent down by satellite, or now up from the ground by ADS-B, are all mosaic pictures that combine the data from all available radars.
What is revealing about the TV cable Nexrad display is that it is a fake. Well, fake may be too strong of a word, but it is misleading. The fake part of the display is an antenna sweep line that rotates once about every 52 seconds.
I’m sure the guys at NOAA put the fake antenna sweep mark on the display so people would think the radar is working, and not just static or broken. All of us expect to see a radar scanning when it is operational so the sweep line looks normal.
I have to say, I was taken in by the fake Nexrad sweep line. Or at least I didn’t really think about how disingenuous it really is. But this morning I had the radar channel on with the sound down. An area of convective activity kept brewing up new lines of extremely heavy rain over Lake Michigan. A chance of severe storms was in the forecast, and there was plenty of lightning and thunder nearby.
I was reading while watching the Nexrad with only glances when I noticed a cell increase from yellow to bright red and move at least six or seven miles as the radar sweep line passed. I thought, wow, that is sure a cell to watch if it can grow and move that fast.
Then it dawned on me what was happening. A Nexrad radar requires five to six minutes to complete a full 360 degree sweep depending on what mode the operator selects. The incredible growth and rapid movement of the cell I had noticed was the result of completion of that minutes long Nexrad sweep, not the 52-second rotation of the fake radar sweep line.
I hacked my watch and it took more than five minutes for anything to change again on the radar display. The fake sweep kept going round and round, but no cells moved, or changed shape or intensity until five minutes had passed since the last new picture.
I have written many times about the delay inherent in Nexrad, particularly the delay of receiving a Nexrad mosaic image in the cockpit. But this was the first real live example I have seen of how crucial that delay can be. Before my eyes a cell had grown from moderate to very heavy rain and moved many miles since it was last presented by the Nexrad. If you had been trying to weave around that cell using Nexrad you could have had a really big surprise.
Like I said, I understand the purpose of the fake sweep line on the TV presentation, but what a vivid demonstration of how Nexrad, at its best, is showing us weather history. When looking at a single site like I was the weather history is only five or six minutes old, but that still made a huge difference in cell development and movement. Add in all the delays of creating the mosaic and then transmitting it to the cockpit and we are really looking at old news.
Nexrad in the cockpit is the best thing to happen for pilots in a very long time. But we can only use it to miss by many miles, not to weave our way around storms that may no longer be where the radar shows.