By Niki Morock, Meteorologist
I love the sound of thunder. I didn’t always, though. When I was a small child, I would hide under my bed during storms, and we used to get some good ones in Paducah, Kentucky. My mother would try to calm me by telling me that thunder was just the sound of angels bowling in heaven. I’m not sure that tactic worked as well as she wanted, but I still remember it.
As I got older, my fear turned into fascination. When I was in elementary school living in Mississippi, I thought that a tornado watch meant that I should run from window to window in the house watching for tornadoes. I’m not kidding. I took it that literally. I guess it gave me something to do on a summer day when I couldn’t play outside.
Now thunder gives me a strange joy – a combination of excitement over the power of the storm and an odd sense of peace that Nature is working the way she should. After all, thunderstorms are as much a part of our environment as sunny days are. We need the rain just like we need the sun.
I guess my attitude toward storms has changed since I was a child as much as our technology has.
The type of Doppler radar the National Weather Service uses was first designed in 1988 and went into wide usage in the 1990s as a means to track storms. There are currently 155 WSR-88D Doppler radars in the country and its territories. Improvements to the technology continue as scientists and engineers come up with new ways to listen to various aspects of storms using radar.
I say “listen” because radar stands for “Radio Detection And Ranging.” At the risk of over-simplification, the WSR-88D sends out radio pulses and listens for how much of them return and how quickly. Using specific algorithms, those signals are translated into a visual image for users to interpret as precipitation types, storm directions, wind speed, etc. It is quite the useful tool!
As much as meteorologists depend on it for nowcasting – meaning seeing what a storm is doing at the moment and projecting its path and strength into the extremely near future (minutes to a few hours) – Doppler Radar still has its limitations. Probably the most glaring one is how it is limited by the curvature of the earth’s surface. A radar’s beam is emitted in a straight line. While it will take several scans at multiple elevations above the earth, each scan is still a straight line. Imagine the lowest scan being directly out from the radar and basically parallel to the ground at the radar location. As the beam continues moving directly out along that path, the earth curves away from it. The effect is that the radar beam’s path goes higher in elevation as it gets farther from the radar site.
If you are in a location relatively close to the radar, the increase in altitude won’t matter much to you. If you happen to be in a place that is farther from the radar, it can make a big difference. While the radar is seeing what is happening in the top of the storm cloud, it could be missing what is going on at the bottom of the cloud.
For example, when I was a broadcast meteorologist on a network of radio stations in the middle third of the country, I once covered a tornadic storm in rural South Dakota. The cloud was so far away from the radar that it looked like there was just a little rain. I could not see any evidence of rotation on the velocity returns. I knew that it existed because the local National Weather Service office’s tornado warning stated that a trained spotter called in a tornado on the ground at a location under that cell. If it had not been for that ground-truth, that NWS office might have completely missed the severity of that storm and no warning would have been issued. Not only was that event an example of a rotation happening in the lower levels of a storm while not clearly indicated in the higher levels on radar, but it’s also a great example of how important trained Skywarn spotters are to our warning system.
Another more recent, relatively local example is what happened last week in Sampson County when straight-line winds up to 90 miles per hour hit an area hard. Below is the official word the NWS office in Raleigh gave on their Facebook page the following day:
Good morning, all. We appreciate all of your comments and questions regarding the results of yesterday's storm survey in Sampson County. This was a very rare event for central North Carolina; we tend to not get straight-line winds of this magnitude, though they are more common in other parts of the country. Most of our damaging wind events are spotty and relatively less impactful, and with that consideration, this was indeed not a "normal" event for our area. However, this shows the potential power of significantly severe straight-line winds.
We understand your concern, particularly given that this area was hit twice in the last week with damaging storms. However, please know that our assessment that straight-line winds led to this damage was determined based only upon the damage present. The magnitude of damage was indeed consistent with wind speeds of a weak tornado; however, the direction in which debris was scattered suggested straight-line winds were the culprit.
Not all straight-line wind events are created equally, and they certainly do not conform to conceptual models very well. A couple of additional items to note, addressing comments from yesterday: 1) straight-line winds can also sound like a freight train when they are the strength of a tornado and 2) in terms of changes in air pressure, tornadoes are associated with low pressure, while straight-line winds are associated with high pressure.
Our thoughts are with those of you affected by the recent storms in Sampson County. We are here to serve you. This was, unfortunately, the combination of a rare event with poor radar coverage in that area. While we were keeping an eye on the storm and thought wind gusts to 50 mph were possible, data available to us did not indicate a severe threat. We continue to strive to improve upon our warning service for all of central NC.
~US National Weather Service Raleigh NC, May 31 at 10:33am
I added the bold font in the last paragraph for emphasis.
Many commented on the post that we need an additional radar for better coverage in that area. Perhaps we do. Radars are expensive to site, establish, man, and maintain. There are other regions of the country with less radar coverage than rural North Carolina has. Of course, those regions also tend to be pretty sparsely populated, but I bet the people living there would like improved coverage for their families and properties.
Using technology like the WSR-88D, the National Weather Service provides a unique service to all residents of the United States. Its goal is the protection of lives and property, and it strives to reach that goal using (among other things) forecast models, satellite data, Doppler radar, trained volunteer Skywarn spotters and cooperative weather observers, and talented, well-educated, and passionate employees. It is a necessary expense line on any federal budget – something to consider if you are calling for additional radars.
Personally, I would love to see that particular improvement in the system nationwide. No one likes to be taken by surprise by severe weather, and I can’t imagine living some place where that could be the case. Or maybe I can – maybe I would feel more like the four-year-old me that feared every thunder clap because I was unsure of just how bad or benign each storm might be.