The storm system that caused the tornado had produced severe weather, including two weak tornadoes, in eastern Pennsylvania and New Jersey that morning. This was an unusual setup for a significant tornado, associated with a warm front near a low-pressure center. A thunderstorm cell formed south of Long Island around 10:20 am, and became a supercell sometime later after interacting with a surface low-pressure center. It turned north as a left-moving supercell, meaning it moved left with respect to the mean atmospheric flow. Left-moving supercells are very rare, as cyclonic storms usually turn to the right of the mean flow. It is unknown whether this leftward movement was due to an atmospheric interaction or terrain-induced movement, as the storm moved straight up the Connecticut River valley.