Almost like clockwork, winter storms swirl in off the Pacific Ocean with wind and rain that can lash West Coast communities with hurricane force and leave them with billion-dollar cleanup costs.
This season, the storms themselves are coming under the most intense scientific assault in more than a decade. From instrument-laden turboprops flying into comma-shaped cloud systems to a "picket fence" of sensors stretching from San Diego to Seattle, scientists are using tools and lessons from years of hurricane and tornado work to improve forecasts for these winter storms.
The project, known as CALJET, reflects a growing emphasis on boosting the quality of coastal weather forecasts. "These storms are as important as hurricanes or tornadoes" in terms of potential damage, says James Rasmussen, director of Environmental Research Laboratories at the National Oceanic and Atmospheric Administration (NOAA), in Silver Spring, Md.
In addition, he notes, storms that roll across the rest of the continent often start in the Pacific. And population growth along the West Coast, combined with phenomena such as El Nio - which is expected to bring more and bigger storms this winter - means foul weather will affect more and more people.
In particular, scientists are interested in providing detailed short-range rainfall estimates to West Coast emergency planners and water-resource managers up to 12 hours before a storm makes landfall. The hope is to provide these officials with the information they need to take actions to reduce flood damage.
"The damage from floods in 1995 and 1997 exceeds most hurricanes," says David Reynolds, the science operations officer at the National Weather Service office in Monterey, Calif. "Severe storms have occurred more often in the last three years than in the previous 20. We need to do a better job of predicting the quantity of rainfall."
Providing such shorter-range estimates is tough in any clime. But on the West Coast it can be fiendishly difficult. Rainfall there comes in bursts, and since storms come off the ocean, scientists lack data from ground-based weather stations that precedes storms as they move across the continent.
The ferocity of incoming winter storms can catch even the experts by surprise. Clifford Mass, an atmospheric scientist at the University of Washington in Seattle, recalls a bumpy research flight he took into one storm.
"We were flying at 1,500 feet over the Pacific when we went through a front," he says. "Life rafts, the coffee pot, everything hit the ceiling. You generally don't get that kind of motion when you fly through a hurricane." The pilots were very experienced, he says, "and even they were visibly shaken by the experience. These storms are not tame."
The key to understanding and forecasting rainfall patterns, scientists say, lies in a high-speed river of air that flows along the storm front. Known as a low-level jet, the wind appears at 2,000 to 5,000 feet. When a storm arrives off the Pacific, the jet collides with coastal mountains. Some of the flow is forced downward, generating intense winds. The rest is forced upward. Its moisture cools, condenses, and falls as heavy rain. "Low-level jets concentrate the transport of moisture and interact with the mountains in ways that focus the precipitation," says Wendell Nuss, an associate professor of meteorology at the US Naval Postgraduate School in Monterey. "The big question is: How?"
Buoys, beacons, flat-bed trucks
For answers, researchers are tapping hardware usually used for hurricane hunting and tornado chasing.
Next month NOAA's P-3 Orion hurricane hunter will begin searching for low-level jets, then release dropsondes - small clusters of instruments tethered to parachutes - to measure wind speed, temperature, air pressure, and humidity as they fall through storms.
Forty ocean buoys put offshore from Seattle to L.A. will relay data on surface temperatures and air pressure 12 to 36 hours before landfall. On land, 20 wind profilers spread between San Diego and Seattle are already beaming radio signals into the sky, gathering information on wind and temperature changes up to several thousand feet above sea level.
And a team from the University of Oklahoma will be traversing the coast in a pair of flatbed trucks bearing mobile Doppler radar to provide detailed, 3-D views of storms' wind patterns.
"Our whole capability of predicting weather will be improved as we extend good observations farther out into the Pacific Ocean," says NOAA's Dr. Rasmussen.