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    Brett Anderson

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    Any Signs of a Pattern Change?

    March 20, 2012; 2:53 PM ET

    Many have been asking when and if the crazy weather pattern will change. Before I get to that, let's officially welcome spring or should I say summer!

    The map below, which I quickly put together this afternoon, shows the temperature departures for this past winter. Clearly the warmth dominated, but what was most astonishing was the incredibly large area of much-above normal temperatures (>3 degrees Celsius). I probably should have put a third area of extreme warmth on the map (>6 degrees C.), which would have included the Prairies and into the Upper Midwest of the U.S. I have never seen such an extensive area of extreme winter warmth in my forecasting career.

    Why did this happen?

    It may be easy for someone to go and say that it was just global warming or La Nina, but it is not that easy. We are looking at several factors that just happen to come together for an extended period to produce this pattern. Some of those factors are....

    1. Unusually sustained, record positive phase of the Arctic Oscillation produced stronger west to east jet stream into southern Canada and the northern U.S. which spread Pacific air across the continent and trapped the Arctic air far to the north.

    2. North Atlantic Oscillation mostly in a positive phase (lack of blocking).

    3. Stronger western Atlantic high pressure ridge.

    4. Sea ice?

    5. Lack of snowcover from the Prairies to the eastern U.S. allowed cold air masses to modify as they tracked south and east. The darker, bare ground also absorbs more heat than snow/ice cover.

    6. La Nina played some role, but not sure it was as much as we thought.

    7. Warmer-than-normal sea surface temperatures over the Great Lakes and off the East Coast.

    In terms of global warming, we have to remember that this is just one season over a relatively small part of the Earth (North America). Yes, the planet is warming, but we just do not have enough data to link this past winter to global warming. Now, if we see a steady increase in winters like this from North America to Asia over the next few decades, then we may be able to establish a more concrete link.

    -----------

    It looks like the peak of this late winter/early spring heat wave across the central and eastern parts of the country will reach its peak Wednesday and Thursday before the massive East Coast high pressure ridge finally begins to break down. Yes, it will be much cooler by the weekend, but temperatures will still be above normal.

    Looking farther ahead.......

    --Pattern change toward the end of the month with warmer anomalies shifting toward the Plains and Prairies and away from the East. Temperatures trending toward normal in the East, but no signs of any Arctic air.

    --Significant pattern change (warmer) for Alaska for early April which means some of the colder air gets back down into southern Canada, but nothing that impressive.

    --Possible signs of a negative NAO the second week of April which would mean milder and somewhat wetter weather for northeast Canada, while colder air gets into Ontario. Again, this may be more typical April chill as the lack of ice/snow will allow cold air to modify more quickly than it normally would as it pushes south/southeast. Potentially chilly and wetter East Coast U.S.

    The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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