In Friends S03E23, The One with Ross's Thing, there's a scene in which Ross shows Joey and Chandler the 'skin-like abnormality' he has on his lower back.
Rachel then enters the room and akwardness ensues. At that point, to ease the tension, Chandler says: 'Ok, well, it’s definite, two more weeks of winter'
Why is this funny?
Chandler is comparing Ross' rash to the groundhog's shadow.
He's saying the rash is the shadow and Rachel is the groundhog going back for another two weeks of winter atleast and it's a joke on the traditional Groundhog Day
Thank you to Harrison Payne for pointing this out.
According to folklore, if it is cloudy when a groundhog emerges from its burrow on this day, then the spring season will arrive early, some time before the vernal equinox; if it is sunny, the groundhog will supposedly see its shadow and retreat back into its den, and winter weather will persist for six more weeks.
As others have posted, it is a reference to Groundhog Day in the US.
Traditionally, it is explained that the groundhog is scared of his own shadow, and so, if he retreats into his burrow, it is because his shadow frightened him. The "result" of the groundhog seeing its shadow and retreating back to safety is that winter will continue for two more weeks (You just have to trust the science on this one).
In the Friends episode, Rachel's frightened reaction to the "thing" on Ross is similar to the groundhog's reaction to its own shadow. Chandler picks up on this similarity and makes his joke that winter will continue.
It is a reference to Punxsutawney Phil, the weather forecasting groundhog that predicts the length of winter.
Here's an excerpt taken from Phil's official site that explains how the tradition goes:
Groundhog Day, February 2nd, is a popular tradition in the United States. It is also a legend that traverses centuries, its origins clouded in the mists of time with ethnic cultures and animals awakening on specific dates. Myths such as this tie our present to the distant past when nature did, indeed, influence our lives. It is the day that the Groundhog comes out of his hole after a long winter sleep to look for his shadow. If he sees it, he regards it as an omen of six more weeks of bad weather and returns to his hole.
I guess that basically Chandler is saying that Ross's "skin-abnormality" is so big that it could even cast a shadow. Or maybe, since Joey and Chandler are caught staring at Ross's back, he tries to justify himself by saying that he was just using Ross to get a weather forecast (as he was the groundhog).
This tradition was also the starting point of the memorable and very funny comedy Groundhog Day starring Bill Murray.
I think a lot of the answers here are being a bit too literal.
In this scene the boys are caught in a position where they appear to be looking at - or even into - Ross's butt. Ross's embarrassment about the growth and Joey and Chandler's on their position leaves everyone involved unable to explain for a moment.
So, "using humour as a defence mechanism", Chandler cracks (pun most definitely intended!) a joke that this is just a folksy crackpot (and again!) method of divination, much like stirring entrails, reading tea leaves, or using a groundhog to predict the arrival of spring.
The idea that specific entities in the scene must directly map to participants in the groundhog day ceremony is too much of a stretch, and just not necessary for the joke.
That said, I can't help but mischievously speculate that Ross's butt represents the groundhog's burrow, and Chandler is jokingly suggesting that the groundhog has already retreated back inside.
