@Eragor14
It was the first snow because the unicorns were inadvertently causing it for the whole region with their new magic. At first it was only a normal amount of snow, but as they kept saying the phrase, it developed into the blizzard.

One question I have with this logic: if Maretime Bay never got snow, how did Sunny know how to snowboard? I would also like to know if the snow is tied to magic in any way. After all, the snow only seemed to come after magic was restored in Maretime Bay. Yes, I know in the real world snow is tied to weather patterns, notably cold fronts bringing over condensed water, which freezes before it hits the ground. However, Equestria’s weather has never been consistent with the real world, and it has always seemed to be very much tied to magic. So it is possible that there was snow in the area of Maretime Bay back in the days of old Equestria (IE G4) and the snow disappeared for several centuries after the loss of magic.
Obviously that is all hypothetical, but it could explain the highly unusual timing of ‘the first snow in Maretime Bay’s history’.