Drilling seed before the greenup

I've got an alfalfa field that will be 5 years old this year, so it's on the tail end of good production. Going to drill some OG into it this spring, and was talking to my seed guy who advised getting it in the ground before greenup, so the next couple of weeks. Since this is my first time doing a spring overseeding, are there specific factors I should take into account before drilling it in? Soil temp etc? Any gotchas or other tips from people who have done this regularly?

 

Drilling seed before the greenup