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Wednesday, April 13, 2011

James Hurrell, WaPo: Putting this winter in context: How severe was it?

Putting this winter in context: How severe was it?

by James Hurrell, guest, Capital Weather Gang, The Washington Post, April 13, 2011

Temperatures over the last six winters relative to 1961-1990 average compared to three winters in the late 1970s. (University Center for Atmospheric Research)
For all the talk of this past winter being one for the record books, the reality is much different. While extremely cold conditions gripped some sections of the country at various times during the past few months, the winter of 2010-2011 actually served up temperatures that were just a little cooler than average over the entire contiguous United States. Records from thousands of weather stations across the lower 48 states from December through February show the past season did not even crack the coldest one-third of winters since 1895, when very reliable recordkeeping began.
So why was there so much talk about it being so cold?

Decadal average winter temperatures relative to 1961-1990 average. (University Center for Atmospheric Research)
Part of the answer, it seems, is that we have become so accustomed to warmer temperatures that a more typical winter now feels extreme. Looking back at these long-term temperature records, kept by NOAA’s National Climatic Data Center, it is clear that our winters no longer have the same bite that our parents and grandparents experienced.
Most of our coldest winters on record occurred in the first few decades of the 20th century. In stark contrast, almost all our winters since 1985 have been warmer than average, and the winter warming has been considerable – about 1.8 °F over the continental United States over the past 100 years.
Even with overall warming, however, temperatures can vary significantly from one year to the next and, especially, from one region to another. For instance, the lower 48 experienced its coldest winter on record in 1979. That winter, the nation as a whole was colder than average by more than 5 °F, and temperatures over the northern Plains States were more than 10 °F below average. The following winter, however, was relatively warm, as have been most winters since then. This past winter, temperatures over the contiguous United States were only 0.3 °F colder-than-average, making winter 2011 only the 39th coldest since 1895.
Undeniably, parts of this winter were brutal, especially over regions very sensitive to cold. Florida and Georgia, for instance, experienced their coldest December on record, generating many headlines about devastating impacts on citrus crops. Major media markets in the Northeast also experienced unusually wintry weather in January, generating epic traffic jams on snow-packed roads.
Far from this spotlight, however, warm temperatures in the Southwest were at or near all-time records early in the winter. And to our north, much of Alaska and Canada faced unprecedented warmth all winter, with the Hudson Bay failing to freeze up until mid-January—far later than normal.
There is one more reason that this winter felt exceptional: we got hit with some massive snowstorms. New York City, for example, set a new record for snow in January. But that is a very different matter than cold. Some of the heaviest accumulations occur when air temperatures are warm enough to hold plenty of moisture but just at the freezing point. That is why even a winter with close to average temperatures can deliver very large amounts of snow.
It’s easy to dismiss a changing and warming climate when our hometowns get hit with Arctic blasts and snow. But cold weather in one part of the country is often balanced by warmth elsewhere. In fact, the big picture shows that our winters are not only warming, but they also reflect global climate as a whole. Just as our winters have warmed in the past few decades, so has the world, although with considerable regional variations.
The past winter may have felt cold—and at certain times and places, it truly was. Overall, though, it was unusually chilly only when compared to the increasingly warm climate that has enveloped our world.
James (Jim) Hurrell is a Senior Scientist at the National Center for Atmospheric Research (NCAR) in the Climate Dynamics Division (CGD) of the NCAR Earth System Laboratory (NESL). Presently, he is Chief Scientist of Community Climate Projects in CGD, which includes the Community Earth System Model.

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