Has Spring Sprung?

The temperature hit 75 degrees in Denver yesterday, the third record high this week.  Washington Park, seen in the above photograph, was crowded with people enjoying the unusually warm weather.  Is winter over for the year already?  Probably not.  March and April are considered Denver's snowiest months, and usually when we get such high temperatures, the following day or week we are punished with a nasty storm. Besides, Punxsutawney Phil saw his shadow on Groundhog Day earlier this month and therefore predicted 6 more weeks of winter.  But what I want to know is why it is that if a groundhog in Pennsylvania  sees its shadow on Groundhog Day, it means the entire country gets 6 more weeks of winter?  Explain that one to me.