Suppose that when the bell for lunch rings in your classroom everyone immediately jumps up and runs for the door as fast as possible to be first in the lunch line. (Your teacher doesn't allow this--and for a very good reason!!) You would end up with some kids climbing over other kid's backs. In other words, not all kids can fit though the door at the same time (since it is too narrow). Some would have to go UP -- kids climbing over other kids.
Now imagine two groups of kids running along the outside of your school building toward the same corner. You would probably get a similar effect. This could be called CONVERGENCE. In the atmosphere, we sometimes have the same thing happening.
On the day of our snowstorm (Tuesday, December 18, 1990), we had cool moist air streaming in from the West just before and during the time it was snowing. The eastward flow of air from the Pacific first meets the Olympic Mountains. The air must do one of two things--go over the mountains or around them. The air follows the easiest path, which is around the left (North side) and the right (South side) of the Olympic Mountains.