I can't say as I have a great deal of experience (and frankly none with Northern Milkweed) but I think you might want to wait until spring to start your seeds. This past spring I started seeds of about 11 species, and there was quite a variation between species in growth rate. Swamp milkweed grew quite rapidly, whereas the A. hirtella I started from seed in May is still very small by comparison, even now in August. I presume Northern Milkweed is like most of the temperate milkweeds and needs to be vernalized before sowing. This means that before you plant the seeds, they should be refrigerated for 4 weeks or so, which puts you into October before the seeds would germinate. I think your best bet would be to wait until spring.
I counted back 14 weeks from the average date of last frost for my area. That is when the seeds went into the refrigerator. Four weeks later, I planted in seed starting medium, and grew them indoors for 8 weeks. I hardened them off for two weeks, and transplanted them outside about the time of the average date of last frost. Based on what I learned this year, I am probably going to do a couple of things differently next year. For starters, I will likely try not to be so anxious and will move my starting date later in the spring. This is so I can have the newly germinated seeds outside in natural light as much as possible. The seeds I started indoors under my grow lights (flourescent tubes) tended to be pretty spindly and I am hoping to avoid that next year by exposing them to as much natural light as possible. The other thing I am going to try is to keep some of the seedlings in deep plug pots (6" or 8" deep seed-starting pots) through the spring and summer and not transplant them until the fall. The deep pots will allow for good root development, and by planting in the fall I hope to avoid some of the problems I faced this year trying to keep things watered during the summer. I am told that the fall is a good time to transplant perennials, so I will give that a try next year, at least with some of my starts.
I hope this is of some help. Like I said at the beginning, I am no expert...I only know what has worked for me. Good luck!
Lawrence Family Garden
Monarch Waystation #163