SAN FRANCISCO (AP) — Rain and snow showers are falling on Northern California in the first wave of a wet weather pattern that is expected to spread throughout the state and extend through Christmas.

Forecasters warn that holiday travel, especially through high passes, will be affected by multiple rounds of rain, mountain snow and gusty winds.

Winter storm warnings are scheduled to go into effect in sections of the Sierra Nevada as early as Wednesday afternoon or by Thursday morning and last through Sunday. Chain requirements are already in effect on some stretches of routes through the Sierra.

Rain so far has been light to moderate around the San Francisco Bay Area.