Bring it to a mechanic asap to check the radiator, and the radiator hoses for leaks. Mechanic can do a compression test of the radiator for leakage. Steam is super hot and dangerous. Bad radiator hose can "bust" spraying super hot antifreeze all over the place causing burn injury.
When the engine is COMPLETELY COLD, feel and examine radiator hoses for damage, leaks, and soft spots. Old radiator hoses should be replaced, even if they are not leaking, as they eventually will in the middle of road. If Bar's Leaks are routinely used in a "good" radiator as a preventive measure, brown spots of Bar's shows leakage points; however, in this case, it is too late for Bar's, as the leakage is fairly severe shooting out steam. Otherwise, green antifreeze spots may or may not show possible leakage points. The mechanics use a pressure pump to find leakages, as they have the tools.