Strong geomagnetic storms, caused by disturbances in the Earth’s magnetosphere, increase the chances of seeing the northern lights at lower latitudes. These storms are measured through the Kp Index.
The solar wind produced by the sun can be forecast to a degree by the number of sunspots. When scientists see increased activity in solar flares and coronal mass ejections from the sun, the potential rises.
While Maine feels very “north” to most of us in the continental United States, it’s still 2,600 miles from the North Pole. The solar activity and geomagnetic activity need to be robust to reach Maine. It’s just more likely in Maine than in more southern states, like Kentucky.
In addition to these factors, Maine has the benefit of having some of the darkest night skies east of the Mississippi. Even less dynamic northern light sightings are possible because there’s no light pollution getting in the way.