I can only speak to the fall migration south. (Can't remember much about spring.) I live in North Texas in a suburb between Dallas and Ft. Worth. Monarchs come through here in September and early October. I did see a few Monarchs in late October. This year my last Monarch eclosed on Thanksgiving day! The Monarchs may be coming through even later, though. Last year at my school in Arlington, we had Monarch cats on the tropical milkweed in early December which meant that Monarchs were passing through here in November 2011. FYI-Fall 2011 and 2012 were exceptionally warm. We have not yet had a killing frost in Bedford where I live. Maybe next week according to the weather reports...

That is interesting. The fact that they were on tropical milkweed is very interesting.
Thanks,
Elisabeth