I have also notices a serious discrepancy in temps from website to website. I finally just stopped obsessing on the differences and chose one that is the most accurate to what I am experiencing from my local radio reports. I use
wunderground.com I stumbled on the site a month or so ago and I can tailor it to my area and it is easy to see high and low temps. I just got my colors organized and will be actually starting my quilt this week-end.