google for weather underground - check Ely weather for the same dates last year - look for record high and low temps, and you'll have an idea of the good, the bad and the ugly.  Do the same for Atkokan, though weather underground does not have the record highs and lows,
so you might have to find another site for Atikokan; and/or do International Falls.

I've done that before to answer somone's similar question on here I think, so do a search for weather in september, or underground maybe,  or something like that, and you might find it.