Winter Forecast Preview Part 2: This Winter's Factors | NBC 10 Philadelphia

Winter Forecast Preview Part 2: This Winter's Factors

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    NEWSLETTERS

    NBCPhiladelphia.com
    Will it be snow or not this winter?

    Just about anyone can tell you that we will see less snow than last winter. After all, it was an all-time record for this area. But how much less snow will there be?

    There are a lot of factors that go into forecasting for the entire winter season, and most of those factors are the exact opposite of what we saw last season. The first item is the critical ocean temperatures in the tropical Pacific. Last winter, they were unusually warm (a moderate El Nino), while they are unusually cold now (a moderate to strong La Nina). I can’t emphasize enough how important this change is -- it is likely to change the overall pattern over most of the world!

    Hurricane's Long-Range Winter Forecast Is Coming

    [PHI] Hurricane's Long-Range Winter Forecast Is Coming
    Tune in Thursday night to NBC10 News at 11 to find out what to expect this winter. (Published Thursday, Nov. 18, 2010)

    One of the main techniques in seasonal forecasting is through “analogs.” We look for past winters with similar conditions in the tropical Pacific and elsewhere around the world, and see how those winters turned out. No two winters are exactly alike, so there is no perfect analog.

    In this case, we first look at years that quickly shifted from a solid El Nino to a solid La Nina. There aren’t many. The winters were: 1964-65, 1973-74, 1988-89, 1998-99 and 2007-08. The 1964-65 winter’s La Nina was significantly weaker than the one now, so I gave it less weight than the others. (It also happened to be the snowiest and coldest of the group -- a possible sign?)

    Another factor we look at is the fall snow cover across the world in Eurasia (Russia, Kazakhstan, Mongolia, etc.). It might not seem logical, but an early development of a good snow cover helps the arctic cold to be more prevalent during the winter. Of the winters mentioned above, the 1988-89 winter stands out, since it had the LEAST Eurasian snow of any winter in the past 50 years. The winter of 2007-08 also had below average snowfall in that region. The winters of 1973-74 and 1998-99 were closest to the current year, with Eurasian snowfall a bit above average.

    Next: the PDO (Pacific Decadal Oscillation). It represents a pattern of ocean temperatures in the North Pacific. We’re now in a negative PDO, which has cooler than average ocean temperatures in the eastern Pacific (off the west coasts of the U.S. and Canada), and much warmer water in the western Pacific.

    We’ve been mostly in this negative phase for the past three years, EXCEPT for last winter, when it was positive. It’s another “opposite” from last year.

    Next: the QBO (Quasi-biennial Oscillation). This represents the change from easterly to westerly winds in the lower stratosphere. It’s a fairly regular shift, so it’s relatively easy to predict. This year, the QBO is positive, which usually leads to a more persistent and strong Pacific jet stream in the winter.

    Negative QBO’s, like last winter, lead to a weaker jet and more blocking patterns. If we get in the right (or wrong, depending on your viewpoint) part of the block, we can get persistent cold and snowy spells.

    Last (at least for this discussion): the NAO and its cousin AO. The North Atlantic Oscillation and Arctic Oscillation are saved for the last, since they are often variable and practically impossible to predict more than a couple of weeks ahead of time. These were probably the most important factors in last winter’s record snow (see the Part 1).

    A positive NAO has lower pressure near Greenland and higher pressure to the south. That allows for a fast jet stream over the North Atlantic, leading to faster-moving, weaker storms near the U.S. East Coast. The negative phase has the opposite pressure pattern, leading to stronger, slower-moving Nor’easters.

    We had a negative NAO virtually all of last winter. Since our big snowstorms are almost always related to a negative NAO, it has a HUGE impact on our winters here. Too bad it’s so hard to predict.

    The NAO has been negative for EVERY month in the past year. This has never happened in the 60 or so years of records that I looked at! So, you would expect it to go positive sooner rather than later.

    There are more scientific reasons to expect more of a positive NAO this winter. Some studies have correlated summer pressures in the tropical Pacific to the winter NAO. And the trend of cooling ocean temperatures near Greenland and rising temperatures to the south should lead to less favorable conditions for a negative NAO. The four best analogs for this winter also happened to have a positive NAO overall for the winter.

    The AO tends to go as the NAO, or vice versa. Last winter’s AO was the most negative EVER recorded, and in my view was the No. 1 cause of the monster snow season. It pushed the storm track farther south than normal, and allowed just enough cold air to come in to make the storms all snow. If the NAO trends positive, the AO will surely be more likely to be positive. And the positive QBO could also lead to a more positive AO.

    So, what does all this mean for my winter forecast? I can tell you that we will see MUCH less snow this winter but for the full forecast tune in Thursday night at 11 when I release the 14th Annual Hurricane’s Long-Term Winter Forecast.