Both these conditions are met in the storm systems that commonly form in the Plains or along the East coast in winter, as well as in storm systems that move onshore along the West Coast. Also, warm advection aloft can carry moisture northward over cold air. This northward moving air can rise and produce heavy precipitation even in the absence of a strong area of low pressure. Rain Rain is water droplets that form in clouds and fall to the ground. Wintertime rain is generally produced by nimbostratus or stratus clouds.
All precipitation starts out as ice or snow crystals at cloud-level.
If this warm air extends all the way to the surface of the earth, rain will fall at ground-level. Elevation plays an important role in determining precipitation type. For example, in the Pacific Northwest, cool moist air sweeps in from the ocean along and ahead of a cold front. Cities at low elevations near the coast, like Portland, Oregon, and Seattle, Washington, receive rain. Geography is also crucial along the East Coast in determining whether rain or snow will fall at a given location.