[Dipper doesn't how the woman in the sparkly blue dress is making the snow and though as an investigator of the paranormal, it's very important to him that he figure that out, he's got something very important to do first: Pelt his sister (and anyone else who comes close enough) with snowballs. So he can be found stocking up snowballs or sketching out detailed plans for a fort he may or may not ever actually get around to building, depending on if he manages to form any alliances.
Eventually, though, when he's got enough snow on him that he might as well be a snowball, he will approach the woman and ask:]
SNOW DAY
Eventually, though, when he's got enough snow on him that he might as well be a snowball, he will approach the woman and ask:]
...how are you doing that?