PJM Interconnection LLC, which serves 67 million people from New Jersey to Chicago, issued a grid emergency for Tuesday and warned demand may exceed record levels for winter. With another burst of frigid air expected Friday, electricity demand could continue to test the system through the end of the month. Freezing temperatures now cover vast swaths of the US, after a weekend storm canceled flights, blanketed roads with snow and left more than 1 million homes and businesses without power.