In general that would be a pretty big leak unless the outside temperature was hot and you drove a large number of miles. A pinhole leak in a hose or a deteriorating hose interface at the clamp can vent steam at a fairly large rate. Unfortunately you need to start tearing things apart top and bottom and try to find the telltale clues of a leak in a hose, overflow tank, heater core, multitude of heat exchangers or radiator. Look for scale and drips.
If you have a head gasket problem you need to stop driving ASAP. You may want to pop a valve cover to see if there is foam.
If you lost your inter-cooler under the supercharger you also need to stop driving ASAP.
Last edited by busaboy; Jun 3, 2014 at 10:51 AM.