Among New Yorkers, there's a dark hypothesis about fall weather in the city, which is that there's only one "perfect" fall day with crisp sunshine and cool temperatures. For that single day, the city's harsh climate, with torrid heat in the summer and frigid cold and wintry mix events in the winter, is forgiven. 

But after that day, which typically occurs in mid-to-late October, the weather turns dreary and cold. And then comes the wintry mix. 

Well, that perfect day has come and gone, and the frigid air is knocking at the city's doorstep, poised to rush in overnight this Thursday, and really make itself known by Friday morning and into the first part of the weekend.

This Arctic blast means business. The frigid air mass first entered the lower 48 states on Wednesday, and on Thursday morning, air temperatures were in the single digits Fahrenheit in Minnesota, and below zero Fahrenheit in North Dakota. 

By Friday morning, wind chill readings in New York City and Boston will be in the teens Fahrenheit, the coldest they've been so far this season.