Superstorm Sandy will influence the U.S. economy for at least the next few months. Gauging the disaster’s effect requires assessing economic activity that might be lost entirely against activity that is substituted with other products or services (like when entertainment spending falls but hardware-store sales rise).

Here’s how some economic forecasters look at Sandy’s effect on the nation’s economy. Estimates for the storm’s influence on fourth-quarter economic growth range from a noticeable negative (as much as a 0.6 percentage point hit to annualized growth in the quarter) to mostly negligible once rebuilding efforts are taken into account. All of the analysts cautioned that storm-damage estimates are still early.

–On a national scale, $30 billion to $50 billion in economic losses would represent about 0.2% to 0.3% of nominal GDP. Part of these losses will eventually be made up by reconstruction activity, but it would be naïve to put forward the view that a hurricane is in some sense a stimulus for the economy. There’s no guarantee that reconstruction activity will be extra activity, on top of what would otherwise have occurred, rather than a substitute for that activity. … The effect on growth for the fourth quarter will not be catastrophic but might still be noticeable, especially in an economy with little momentum anyway. Suppose that the affected regions lose just 25% of their overall output for two days that is not recoverable later. That would knock about $25 billion annualized ($6 billion actual) off GDP, and could take as much as 0.6 percentage points off annualized fourth-quarter real GDP growth rate. –Gregory Daco and Nigel Gault, IHS Global Insight