The theoretical conditions that would form one might be occurring right now in the Northeast where an Alberta Clipper, with arctic air pushing it, is colliding with a tropical rain front sliding up the east coast. So you jam some heavy moisture sucked up off the ocean right into a dense arctic air mass. Lucky for us that these Alberta Clippers have been moderate in size. Next winter they could become rather large if solar activity remains low.
It is has been confirmed, a few areas in Northern Minnesota bottomed out at - 40 F this morning (one at -42). That was 20 lower then forecast. So already this first arctic blast is much worse then even predicted. Quite a few places in Minnesota set record lows for the day.