The next winter's snowfall descends upon the remains of the previous winter's snowpack, compressing it eventually into ice. Over years, decades, centuries, the ice builds. Pulled by gravity, finally a glacier flows downhill. Its tongue may reach the sea, but its tail remains in the mountains, fed by the highland snows. Now, return to the cause of the soggy weather along those coasts.
Wet air comes off the North Pacific Ocean.
The air mass is comparatively warm, and warm air can hold more moisture than cold air. The oceanic air holds so much moisture that just a little cooling is all it needs to start dumping water. The air's lifting as it encounters the coastline can produce enough cooling to do that. Thus Juneau, Yakutat, and Valdez are deservedly known as rainy places. The water that doesn't rain out accompanies the air uphill into the colder zones high on the mountain slopes.