Sweating in November? Forecasters at the National Weather Service say enjoy it while it lasts. The eastern half of the United States should brace itself for a colder than average winter, according to the weather service's Climate Analysis Center, which released its 90-day winter outlook yesterday.

While the science of guessing the weather 90 days in advance is in its infancy, government prognosticators say they have accurately predicted about 60 to 65 percent of winter temperatures. Winter temperatures are easiest to guess. Forecasters say spring and fall are a nightmare and that predicting precipitation is even trickier.

This winter, the forecasters are betting on cold temperatures in the east because ships and satellites have detected an immense bowl of warm water in the central equatorial Pacific.

This warm sea within a sea, some 4 degrees hotter than average, should cause storms and other perturbations in the region.

But more importantly, at least for the Washington area, these warm waters in the central Pacific should cause air to rise from the Earth's surface and slam into the jet stream, the high-speed river of air that swirls around the globe from west to east, playing havoc with the weather and acting as a curse or a blessing for airline pilots depending on their direction. The jet stream is caused when warm air from the equator migrates toward the poles but is deflected by the spinning motion of the planet.

"The warm waters cause more convection, a rising motion, like a spout of air that accelerates the jet stream and gives it a boost," said Huug Van den Dool, chief of the prediction branch of the Climatic Analysis Center, which is part of the National Oceanic and Atmospheric Administration.

This energetic injection of air, Van den Dool said, "makes it undulate."

The jet stream, while sometimes fickle, often takes one of two routes over the United States. It can cross directly over the Rocky Mountains. Or it can swing north over Canada or Alaska and then swing back down over the lower 48 states.

This winter, Van den Dool and his colleagues, including principal scientist Robert Livezey, believe the rising, warm Pacific air should send the jet stream swooping up to Canada and then howling down over the eastern United States, carrying frigid Arctic air with it.

The forecasters said mild conditions should dominate over the western half of the United States. There should be more rain in the Pacific Northwest and along the Gulf and south Atlantic coasts, including all of Florida.

Livezey predicted that the cold weather in the East should begin in earnest sometime in mid-December. It will take that long to perturb the system. Maybe. The weather service's 30-day forecast calls for a 50-50 chance of average temperatures for the Washington area. The immediate forecast calls for cooler weather over the weekend.

Last year, November was 1.1 degrees cooler than normal in the Washington area, according to the National Climatic Data Center. December, which brought a blast of Arctic air to the region when the jet stream was perturbed, was 11 degrees colder than normal. When the jet stream changed course again, January and February were considerably warmer, 8.4 degrees and 7.7 degrees respectively.

If you think it has been warm this month, James Wagner of the Climate Analysis Center says you're right. November looks like it will be about 3.5 degrees above normal. The last five days have been 11 degrees above normal.