Automated vulnerability assessment and exploit generation for computing systems have been explored for decades. However, these approaches are incomplete in assessing industrial control systems, where networks of computing devices and physical processes interact for safety-critical missions. We present an attack synthesis algorithm against such cyber-physical electricity grids. The algorithm explores both discrete network configurations and continuous dynamics of the plant's embedded control system to search for attack strategies that evade detection with conventional monitors. The algorithm enabling this exploration is rooted in recent developments in the hybrid system verification research: it effectively approximates the behavior of the system for a set of possible attacks by computing sensitivity of the system's response to variations in the attack parameters. For parts of the attack space, the proposed algorithm can infer whether or not there exists a feasible attack that avoids triggering protection measures such as relays and steady-state monitors. The algorithm can take into account constraints on the attack space such as the power system topology and the set of controllers across the plant that can be compromised without detection. With a proof-of-concept prototype, we demonstrate the synthesis of transient attacks in several typical electricity grids and analyze the robustness of the synthesized attacks to perturbations in the network parameters.