Crater Lake National Park's Frequently Asked Questions
Crater Lake Institute - http://www.craterlakeinstitute.com/
Answer: The major weather patterns at Crater Lake National Park originate in the Pacific Ocean. Storm events originate in the north Pacific and build in strength and moisture content over the ocean. Wind patterns at these northerly latitudes move storms from the ocean to the Pacific Northwest.
Over 100 inches (250 cm) of rain falls each year on the Oregon Coast. After crossing the Coast Range, storm clouds descend into the Rogue and Willamette Valleys, dropping about 30 inches (76 cm) of rain. As storms move eastward, the high mountains of the Cascade Range push the cool, moist air to elevations over 10,000 feet (3,000 meters) in many places. As the air rises, it cools further. Water vapor in the air condenses to form clouds, and snow crystals form within them. If there is enough moisture in the clouds, the snow begins to fall. If the temperature is warm enough, the snow melts before it reaches the ground and falls as rain.