By DAVE NICOSA

After three consecutive mild winters in upstate New York and northeast Pennsylvania, the National Weather Service Outlook for Winter 2000-2001 calls for a return to more normal winter temperatures with above average snowfall, especially in the lake-effect snow belt regions. Reality is expected to set in this winter as many people may have forgotten what a "normal" winter feels like given the recent mild winters.

More normal temperatures are expected due to the lack of a pronounced El Nino or La Nina pattern in the Pacific Ocean. The last three winters have been dominated by either a strong El Nino (winter of 1997-1998) or a strong La Nina pattern (1998-1999 and 1999-2000 winters).

When a strong El Nino is in place, warmer than usual waters exist in the central and eastern tropical Pacific Ocean. This leads to an unusually strong subtropical jet stream flowing into the United States. As its name implies, the subtropical jet stream floods the lower 48 states with mild Pacific air as the frigid polar jet stream is diverted to our north. Therefore, during an El Nino, the majority of our air masses come from the milder Pacific Ocean instead of the icy cold Canadian landscape. During a La Nina, the waters in the central and eastern tropical Pacific Ocean become colder than normal. One important change during a La Nina dominated season is that large subtropical high pressure systems become stronger than usual. For our area, this means mild Bermuda high pressure systems tend to dominate over the cold Canadian high pressure systems. This also results in milder winters.

This winter there are no strong El Nino or La Nina patterns in the Pacific Ocean. Historically, our colder and snowier winters tend to occur under such circumstances. The polar jet stream and subtropical jet stream will duel for supremacy this winter, and the polar stream will win, allowing for more frequent visits of cold Canadian air masses to our region. This polar jet stream will also bring a greater chance for more snow. Our area is also expected to see average temperatures about 4 degrees Fahrenheit colder than the last three winters.

Another aspect of this winter season will be variability. There will be one or two periods of mild and almost snow-less winter weather, followed by one or two periods of cold and snowy conditions. This is due, in part, to another atmospheric and oceanic phenomena known as the North Atlantic Oscillation or NAO. The NAO is governed by two phases: a positive and a negative. When the NAO is positive, the jet stream becomes very strong over the North Atlantic Ocean and "bottles" up all the cold air over Greenland, northern Canada and northern Europe. The rest of Europe and the eastern United States see milder winter conditions because the bitter cold air of winter remains to the north. When the NAO is negative, the jet stream over the North Atlantic Ocean becomes weak. This allows frigid cold air to flow south into the eastern United States and Europe. In addition, the jet stream along the east coast becomes stronger than usual and nor'easters become more frequent. This brings heavy snows to our area in addition to the cold. Unlike El Nino and La Nina patterns which last one to three years, the NAO can change phase in one or two weeks, leading to rapid changes in winter conditions. This winter season the NAO is expected to be variable and lead to rapid swings in our winter weather conditions.

We must prepare for a little bit of everything. Tune into to NOAA weather radio, the voice of your local National Weather Service office. Up-to-the minute weather forecasts, winter weather warnings, statements and current conditions are continuously broadcast. NOAA weather radios are specially designed radio receivers which pick up the seven frequencies assigned to the National Weather Service. They can be purchased at most electronic stores.

For more information visit www.nws.noaa.gov/er/bgm or send me an e-mail at nicosia@noaa.gov.

[Dave Nicosia is a Warning Coordination Meteorologist, National Weather Service in Binghamton]