Something interesting I just noticed, the planes travel a fixed loop. They start in Eastern Asia and fly to Anchorage. From Anchorage they fly to Louisville or California. From there they return to Anchorage and then back to Eastern Asia. There are about 14 planes that take 2 days to complete the loop. The reason that some of the planes don't appear (especially on the return trip) is because they are not carrying any Apple Watches.
A word of caution: as long as there is 1 Apple Watch on a plane, it is likely to appear. While it is more likely there is a whole bunch on each flight (I would like to believe this as well), keep in mind they we may be looking at, as few as 7 Watches making their way to the states tonight.