A weak signal can be an early indication of a new interest among people, a change in behavior, attitude, idea, a growing hype or such. A weak signal can be expressed in all sorts of conversations, papers, blogs, magazines or other. It is what can be perceived as funny or striking: it is something different, something noticeable, it can spread like a virus, hopping conversations, drawing attention. By doing so, the signal gets stronger.
One signal does not mean a whole lot. It becomes valuable when there is a magnitude of signals pointing in the same direction. Weak signals are never alone. The signal becomes stronger as more expressions of the signal can be found. Detecting weak signals is about noticing its converging direction, where the signals point to and discover its underlying dynamics: what is it that people express with the signal, what perspective is it pointing to.
A signal becomes more valuable when more expression can be found, when the idea is used in multiple environments or products. It is like the lifting of the earliest signal up in a pyramid. The bottom of the pyramid might just be the earliest indicators concealed in much noise. Higher up the pyramid the signal becomes stronger as more expression of its meaning is given. The higher the signal, the more noise reduction, the clearer it becomes where the signals point to and what it expresses. The signal becomes a concept or perspective of underlying needs or wants.
I think, depending on your focus, you’ll want to be able to make a difference in whether the signals will lead to a new hype, or leads to more of a sustainable expression. Hypes come and go and might be interesting to spot for your business, but more interesting is the detection of signals that lead to a more permanent change. As for a lot of emerging things, there are overestimated in the short term, but underestimated in the long run.
If you are able to spot the signals that lead to a true change in society or business, you’ll be able to mark the signal as a waypoint for future-scenario. Since no man can predict the future, you can have a concept of it. The future is a point where a path of developments lead to. If you are able to set several waypoints on the road to the future, you’ll be able to test you concept of it. Over time your waypoints will be reached one by one. If not, your concept of the future will have to be adjusted.
In regard to detecting weak signals, I think you’ll need to have a concept of the future. Without it, I think it would be extremely hard to just stumble upon signal and interpreted them correctly.

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I’m not too sure about your last remark. I think, and agree with you, that Elina Hiltunen’s presentation on Weak Signals was a method for trendwatching. That is something different from scenario thinking. If you’re thinking about future scenario’s you need to have a vision and a concept of the future. But in trendwatching I don’t know if that is absolutely necessary. It is all about keeping your eyes open and coming to conclusions based on your observations of where things might be going next. In that sense it is much more extrapolating from the now into the future and not at all like counter factual thinking, the way we are used to approach that…