Insurers are keeping a close eye on Hurricane Sandy as it churns north across the Atlantic Ocean, preparing to dispatch rapid-response teams to the hardest hit areas once the storm passes. The storm is so large that most of the Northeast appears poised to endure at least some of the storm’s wrath. No one knows yet where the hardest hit areas will be. Even after it passes, it will be days, or even weeks, before the full costs of the storm are known. A large part of the damage tab will be handed to the federal government in the form of flood-insurance claims.