How about this for a weather forecast for the upcoming Bay Area winter: It could be drier than normal. It could be wetter than normal. It could be normal.
The experts aren't trying to be funny. Blame it on the chilly equatorial ocean condition called La Niña and the Bay Area's particular geographic location.
For months now, a vast network of buoys has detected cooling ocean waters all across the Pacific Ocean, signaling that La Niña is already here and getting stronger.
La Niña is a harbinger of extreme weather events worldwide. But if nature cooperates, the Bay Area could avoid the heavy rains considered likely to pummel the northern end of the state and the Pacific Northwest in the winter, or the dry weather that might parch the southern tip of California and southern states.
Just as easily, however, the atmospheric phenomenon could change, delivering to the area the same La Niña conditions soaking neighbors to the north, or providing the sunny days of a drought winter for those to the south.
The La Niña forecast for the Bay Area "is much more variable, almost a coin toss compared with what you get to the north or to the south," said Mike Halpert, deputy director of the federal Climate Prediction Center. The Bay Area, he explained, lies in an in-between zone that makes it difficult, if not impossible, to forecast La Niña's probable effect on rainfall weeks or months out.
In its annual "Winter Outlook," the center gives "equal chances" for the Bay Area and the surrounding region to be wetter or drier than normal in the winter.
"There still could be significant storms, and significant dry periods," said Warren Blier, science officer for the National Weather Service in Monterey. "It could also end up being something of a normal year. In a sense, that's what they're saying by saying 'equal chances.' "
The unsettled Bay Area forecast in the face of unquestionably cooling Pacific waters has to do with the influence of atmospheric forces like pressure systems oscillating far apart, and the path and strength of jet streams. And those can only be forecast, at most, two weeks ahead of time.
What's more certain is that Bay Area residents can brace for colder-than-normal weather, as can residents all along the West Coast. The Climate Prediction Center gives the entire coast 33 to 40 percent odds of chillier-than-usual winter temperatures, depending on the distance from the ocean.
Halpert said the cooler Pacific waters of La Niña, which might reach as low as 3.6 degrees Fahrenheit below normal, spike the odds of a chilly winter along and near the coastline. While La Niña starts in the equatorial region, its cool waters also typically "hug both the south and north American coasts," he said.
"If ocean temperatures are colder than average, then the land adjacent to it is likely to be colder than average," Halpert explained.
In addition, changes in atmospheric pressure systems favor the buildup of colder-than-normal air over Alaska and western Canada, which often penetrates into the northern Great Plains and the western United States, according to the center.
La Niña conditions return every two to five years, according to the Climate Prediction Center, and they're the opposite of El Niño conditions -- in which the waters along the equatorial Pacific are warmer than normal, and predicted rainfall and temperature patterns are reversed. There have been 20 La Niñas since 1949, and the one forming this year may be among the strongest yet. The most recent La Niña occurred in the winter of 2007-08. The Climate Prediction Center forecasts that La Niña will peak by January and then gradually weaken, exiting the world stage -- for now -- by the end of spring.
As a result of La Niña, the northern Rockies are also expected to have above-average snowfall. All southern U.S. states, meanwhile, have an increased chance of below-average rainfall. The southeastern United States, along with the East Coast, faces an increased risk of hurricanes. Higher-than-average temperatures are also expected in the nation's interior.
A strong La Niña in 1988 led to significant droughts across North America. Another one in 2008 spawned a brutal hurricane season, which included Hurricane Ike. That hurricane struck Texas and killed almost 200 people.
Contact Suzanne Bohan at 510-262-2789.