Because my first "hunt" for snows was back in 84 off of sweetwater and from about 1989 on I did nothing but live and breathe snow geese! My dad was never much of a hunter but his brother was. Combine that with he was from devils lake and even more so worked for Sioux manufacturing put my uncle on the forefront of snow goose hunting. It was because of Sioux manufacturing the north winds aka windsock decoys we're perfected with Tyvek since that's what sioux manufacturing makes.
So the reason I know they are going to come down in force is I have watched these birds migrate firsthand from the edge of the boreal forest north of the quill lakes in Saskatchewan all the way down to Oklahoma and back....still haven't hit the extreme ends of their migration yet. People think they are hanging up on us now days but the reality is their southern migration starts off at their still normal pace... But unfortunately for whatever reason they now start off the migration by doing a 400 mile lateral jog into Saskatchewan instead of coming down through southwestern Manitoba like they used to. The further west you go in Canada until you reach the mountains in Alberta the farther north the tree line is and since the migration only moves around 15 miles south a day it now takes considerably longer than it used to for the birds to get here. The snow goose migration whether it be north or south starts off very calendar based but somewhere along the line in both directions it eventually gets whether influenced especially on the northbound route. Simply put we already have somewhat huntable flocks down to the highway 200 corridor and now we have two days of stiff northwest winds and a full moon. So with all the ingredients like this combining the time of year with moon phase and migration patterns this is a recipe for a full on migration for the white devils.
Now all that bullshit said they are still snows ....so we are dealing with at best a bipolar schizophrenic of a bird so guarantees are pretty worthless because you never know exactly what they're about to do. But over the last several years this day has become my bench mark date because no matter how warm its been they just seem to arrive around the 27 of Oct and get to full force be a week later. Maybe it will hold true maybe it won't but so far the arrow points to a normal pattern so far.