From the Summary: "Storm-induced blackouts of the electric power grid are suggestive of the possible consequences of an electromagnetic pulse (EMP) attack, such as could be made by rogue states or terrorists detonating a nuclear weapon at high-altitude over the United States. Electric power grid failure caused by storms cascade through other critical infrastructures--such as communications, transportation, emergency medical services, food and water supply systems. Storm-induced blackouts provide an objective basis for extrapolating judgments about the threat posed by EMP to the civilian infrastructures that sustain economic, political, and social life. [...] Storm-induced blackouts are localized and last usually no more than a day or two. Yet they can momentarily return part of our society to technological primitivism and begin cracks in the social order. Compared to storms, the consequences of an EMP attack would be far graver. Compared to the worst storms, an EMP attack would probably destroy infrastructures more completely within a region and over a much larger region--perhaps over the entire continental United States. An EMP attack, compared to the worst storms, would probably inflict more lasting damage--requiring perhaps weeks or months to repair."
First EMP Commission: http://www.firstempcommission.org/