The cooler weather is coming from the Arctic. According to the Washington Post, using NOAA’s HYSPLIT model, which simulates the movement and origins of air parcels all around the world, we can deduce that this air mass actually began over the Arctic back on Aug. 3. It was north of Greenland, then traced a loop before sauntering south over Hudson Bay and settling over the interior Northeast and Mid-Atlantic over the weekend. Now it’s spread across much of the eastern United States.