The start of winter rains in Northern California and another El Niņo year prompt dozens of questions, from how severe this year's rainy season will be, to where's the best location to book a ski trip.

"It looks like this year's El Niņo will be a moderate one," says Dr. John Monteverdi, SFSU professor of meteorology and fellow at the California Academy of Sciences. Monteverdi, who specializes in severe and unusual weather events in California, predicts "average rainfall to slightly above average in north-central California and above normal rainfall in Southern California, with most effects cropping up after mid-December."

For the Bay Area, it also means temperatures are likely to continue being milder than usual. But this El Niņo will not bring the heavy storms seen in winter 1997-98 during the last El Niņo episode, which Monteverdi classified a "moderate to strong" event. This time El Niņo will deliver a sizable payload of precipitation to much of California, but fewer of the torrential storms and extreme conditions seen last time.

The current pattern, according to Monteverdi, should create "the biggest snowfall in years" in Southern California's mountains, following a drought that has gripped the region for more than two years.

In addition to determining the conditions for skiing and other outdoor activities, the winter snowpack is an important source of fresh water for lakes, rivers and streams throughout the year.

Even though El Niņo will probably cause more precipitation than normal, it is not expected to cause significant flooding in the Bay Area. Unlike isolated heavy storms in dry years, which can quickly produce a deluge on the ground, El Niņo's persistent, ongoing rains tend to get absorbed at a steady rate.

El Niņo is a climate phenomenon caused by rising surface temperatures in the eastern Pacific Ocean around the equator. As the ocean warms the air above, it changes the wind direction and alters weather patterns throughout the world. On average, an El Niņo episode occurs every four to seven years.

Monteverdi has seen minor evidence to suggest El Niņos have been appearing more frequently and having more extreme effects since the mid-1990s. But he cautions that the evidence available is minimal, and that additional data could lead to different conclusions.