What are Storm Surveys?

With the stormy weather we’ve seen of late, plus the small tornado that hit in the Cross Hill area on Monday, May 4, you may wonder how the National Weather Service and meteorologists in general determine after the fact whether or not a tornado caused damage somewhere, as opposed to “straight-line winds”. The National Weather Service routinely sends a storm survey crew out to any area where storm damage is severe enough to warrant a look and to answer that question.

Here are some answers as to just what is going on out there concerning the surveys:

Question: If there were people who saw a tornado or a funnel cloud, why does someone have to come out and verify it was a tornado?

Answer: Two reasons. First, tornadic thunderstorms will often produce strong winds, even in the absence of the tornado itself. Unless it was clear that a tornado was actually on the ground, it has to be determined if the damage was caused by the observed funnel clouds making contact with the ground, or from other strong winds associated with the storm. Second, even if it was an obvious tornado, a determination is made as to how strong that tornado was, based on the type of damage that is observed (in almost all cases, there are no actual wind speed reports taken from near or in a tornado).

Question: How do they know if it was a tornado or not?

Answer: A few things go into that equation, such as eyewitness reports/pictures, what radar showed over the area at the time, and the damage pattern observed in the area. For example, the spinning winds of a tornado can leave damage spiraling away from the center of a distinct tornado path, straight-line winds will push all the damage in one general direction over a more widespread area, downburst winds (which can cause rather severe straight-line winds as rain-cooled air rushes straight down from the clouds and into the ground) will show damage radiating outward from a point where the downburst hit. Sometimes, clean-up will already have begun on damage by the time a surveyor arrives; this can make some of the weaker events a little more difficult to classify. This is where comparison to the radar images and to spotter reports can really help out.

Question: How can the damage indicate how strong a tornado was?

Answer: Studies have shown relationships between how well-constructed buildings, trailers or outbuildings, and trees will behave based on a range of possible wind speeds. This matches up with the “Enhanced Fujita Scale”; the EF rating that is assigned to tornadoes. Click here to find more information about this scale.

Question: I know someone who saw a tornado pass by and they never issued a warning. What gives?

Answer: Chance are that if you are seeing something that looks like a tornado, so are many other people, some of whom are storm spotters or local authorities. These people are making sure that those “scary-looking clouds” are just that, and not anything more sinister. One thing they make sure of is whether any low hanging clouds are showing signs of consistent rotation…and that they’re not just hanging there and very slowly sort of, kind of, vaguely swirling. It can be determined relatively quickly if that is something to be worried about, or just a low-hanging cloud…and nearly every thunderstorm will have those low clouds. That isn’t to say the system is 100% foolproof, but many “tornadoes” are misidentified by witnesses and are often harmless…occasionally even storm spotters will be fooled if they don’t have a clear view. Basically, there are a lot of people who see things they’re concerned about, but a relatively small number of them actually pan out.

Question: How can I become a spotter and learn about what to look for?

Answer: The National Weather Service will hold occasional spotter training courses, and many of them will be open to the public. When such classes are available you’ll be able to find them by clicking here.

Question: I have a question you didn’t answer here. What do I do?

Answer: Post a comment, I’ll reply as soon as I can…