WHEN IT RAINS . . .

A rapid-fire string of storms out of Asia has pummeled California. Among the victims: the elaborate web of rivers and reservoirs that drain the Central Valley.

REASON BEHIND THE RAINS

Northern California has been socked by storm after storm. Here’s why:

Jet stream funneling storms in from Asia

Cool, dry air moving down from Canada

What’s normal: This time of year, it’s typical for the jet stream to funnel storms this way into the state.

What’s different: The frequency of the storms lining up in Asia is far higher than usual. For two weeks, storm after storm has pounded the state.

What’s ahead: Cooler, drier air is moving down from Canada, replacing the moist winter storms. This will help slow the snowmelt. Only scattered showers are predicted this weekend.

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Rain Worse Than Snow

The rain is especially troublesome because it comes on top of already heavy snowfall. The rain melts the snow, increasing the amount of runoff. Normally, the snow would melt more gradually, during the spring thaw.

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No Break Between Storms

Nine-day rainfall totals for Blue Canyon reporting station in the Sierras:

WHY LEVEES FAIL

Levees are some of the most essential tools of any water control system. When they fail along rivers, lakes or reservoirs, water pours across residential and farm areas.

High water and unusually fast currents can breach levees. Surface erosion and tunneling animals can also weaken them.

EVACUATING YOSEMITE

Rangers led 900 visitors out of the park in a convoy over the Wawona Road.

Sources: Jon Erdman of WeatherData Inc., California Water Atlas, Corps of Engineers, staff and wire reports