A technical definition for Early Warning Systems (EWS) in hard to find. The work on classification of EWS requires a precise definition for EWS. The wikipedia definition fo EWS is what is quoted by the United Nations, very much human centric with natural disasters in mind. In my opinion EWS and Observer Controller systems have a lot in common (Figure 1). As they both try to predict and correct system's response. However, EWS can be broken down to a chain of systems that are distinctly a chain of communication systems.
I propose the following definition -
Definition "Early Warning System (EWS)": A chain of information communication systems comprising sensor, detection, decision, and broker systems, in the given order, working in conjunction forecasting and signaling disturbances adversely affecting the stability of the physical world, giving sufficient time for the response system to prepare resources and response actions in minimizing the impact on the stability of the physical world.
A paper in relation to the proposed definition as well as an introduction to the necessary and sufficient components can be found in the blog with title: "towards a definintion for EWS".