Hi, based on very limited data, I think monarchs congregate and begin migrating earlier than the indicated longitude given in the migration timeframe for our area. I've been told by some old timers that they always saw large numbers in late August. Last year I saw the largest number I had ever seen about Aug 28 at Wolf Road Prairie--saw perhaps 200, tagged a couple dozen. This year, I heard reports of large numbers along the lakefront near Montrose harbor the last week of August.
I have seen only limited numbers the last couple weeks, supposedly our "peak" time. Yesterday I saw two on my butterfly bushes.