As storms (low pressure systems) move west to east across the country, we can usually identify a long "tail" of thunderstorms or heavy precipitation that curves southward and then southwestward from the center of low pressure. This is the clash zone between warm, humid air to the east and drier, cooler air to the west. In the Northern Hemisphere, winds swirl counterclockwise around the storm's center and thus move northward and northeastward ahead of the trailing front.
Along the Pacific Coast of the U.S., these winds pull in copious moisture from the ocean; when the low pressure is especially intense, this flow (referred to as an atmospheric river) can drop prodigious amounts of lowland rain and mountain snow on the Coastal States. Furthermore, if the storm is stationary or slow moving, extensive flooding may occur; as our climate and oceans warm, these events will likely become increasingly severe.
Such a scenario is currently developing off the California Coast, near San Francisco. The storm is intense enough to justify a Hurricane Warning with winds expected to gust over 70 mph atop the highlands. As the storm comes ashore, the plume of moisture will move southward, from the Bay Area to Southern California. Widespread, severe flooding is expected.
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