One important thing we always have to consider is what the weather is going to be at the actual location being surveyed, not just at the home office. That's usually a straight-forward matter of looking ahead, but sometimes what's important is what happened there previously.
I've left home on a warm day in the winter with no snow on the ground, only to arrive at a site with lots of snow cover - which is why I like to check the NOAA National Snow Analyses page before heading to a site this time of year.
It has 9 useful national maps, but the one I generally find most useful is the Snow Depth map, which shows current snow depths nationwide, updated daily. Looking at it today, I can see that most of Ohio is snow-free after after a few days of temperatures above freezing and rain, but the north half of Pennsylvania still has snow cover of 1-10 inches.
There are also zoomed-in maps of particular regions, like the Southern Great Lakes or Allegheny Front.