At its best, weather forecasting is science-aided guesswork.
So why, one might ask, would anyone even attempt to tell us what the weather will be like for the entire continent several weeks, or even months, down the road?
Partly because we need to know, and partly because scientists have a bunch of new tools to help them make long-range predictions that were considered impossible just a couple of decades ago.
Giant Weather Patterns
In addition, what has changed in recent years has been our understanding of giant weather patterns that cover thousands of square miles and influence local weather around the planet. We've heard a lot about El Niño, also called the Southern Oscillation, in recent years, but that celebrated weather-driver is just the tip of the iceberg. Now we've got the Madden-Julian Oscillation, the Arctic Oscillation, and the Pacific Decadal Oscillation.
All of these have to do with variations in ocean or atmospheric temperatures and currents over immense regions, and they set the stage for weather conditions for an entire season. Scientists disagree among themselves over which of those oscillations are the most important, but there is considerable agreement that what happens in Florida this December will depend on earlier conditions as far away as the central Pacific Ocean or the Arctic.
So what are we in for this winter?
There is surprising agreement among scientists who looked at different sets of data. This winter, according to scientists at both the National Oceanic and Atmospheric Administration and the National Aeronautics and Space Administration, will be pretty much like last year's. So if you liked what you had last year, you should be happy this winter. But if you didn't, hang on for some tough sledding.
"I'm calling for a repeat of last year's winter," says William Patzert, an oceanographer at NASA's Jet Propulsion Laboratory in Southern California.
"For most of the United States, winter 2001-02 will feel like a sequel to last year's cold season," says a formal statement from NOAA.
Both forecasts are based partly on the absence of a strong El Niño, which consists of a warming of the waters of the Western Pacific Ocean, or its counterpart, La Niña. Both those systems are so powerful that they pretty well dictate much of the country's weather. But when they are inactive, as is the case now, something else will take over and set the stage.
Patzert is betting it's the Pacific Decadal Oscillation, and he has some very precise data from a historic satellite to base his forecast on. For decades some scientists have believed that the temperature of the Pacific Ocean has a profound impact on weather patterns over the North American continent, but it has been very difficult to get accurate measurements of an ocean that covers nearly half the Earth.
That changed nine years ago with the launch of the Topex/Poseidon satellite. The satellite bounces microwave pulses off the surface of the ocean. By measuring the incredibly brief time it takes for the pulses to return to the satellite, scientists are able to determine the exact height of the sea surface.
"That tells you how much heat is stored in the ocean," Patzert says, because heat causes the water to expand. Data collected during the 10-day period ending Oct. 31 revealed that the western and mid-Pacific regions were warmer than normal, and the Gulf of Alaska and Bering Sea in the far north were colder.
NOAA Hedges Its Bets