Cold weather in U.S.

http://www.climatecentral.org/news/for-first-time-in-20-years-cold-records-may-beat-warm-in-u.s-16790

December 2nd, 2013
By Andrew Freedman

With a large chunk of the U.S. having endured one of the coldest Thanksgiving holidays in years and even more brutally cold weather in the forecast over the next few days, 2013 is poised to have daily record lows outnumber daily record highs for the first time in 20 years.

That’s a stark reversal from last year — the warmest year on record in the U.S. — when record daily highs dwarfed record lows by a staggering 4-to-1 ratio. It’s also a stark reminder of the vagaries of short-term natural variability set against the backdrop of long-term global warming.

-----

December will be a crucial month for determining if lows outpace highs for the year. The last time daily record lows outnumbered daily record highs in the U.S. was in 1993. The Arctic air mass moving south from Canada, which is set to send temperatures plunging from Seattle to Texas by the end of this week, tilts the odds in favor of cold temperature records prevailing.

Even with the cold snap in the U.S., the world has continued to see above-average temperatures overall, with 2013 expected to wind up among the top 10 warmest on record. The last month to have global average surface temperatures below the 20th century average occurred in February of 1985.

While that would run counter to the trend that has been observed during the past several decades, which is thought to be due in large part to manmade global warming, it’s not surprising. Much as the stock market has short-term fluctuations amid long-term trends, it is to be expected that in individual years, temperature records will buck the long-term trend. What is remarkable is that it’s taken this long for it to happen.

While 2013 is unusual in the context of the past two decades, periods of relatively cold weather — including Arctic outbreaks — are still to be expected in a warming world, studies show.

As the climate has warmed during the past several decades, there has been a growing imbalance between record daily high temperatures in the contiguous U.S. and record daily lows. A study published in 2009 found that rather than a 1-to-1 ratio — as would be expected if the climate were not warming — the ratio has been closer to 2-to-1 in favor of warm temperature records during the past decade (2000-2009). That finding cannot be explained by natural climate variability alone, the study found, and is instead consistent with global warming.

-----

The large dip in the jet stream responsible for the cold air outbreak will initially allow milder-than-average conditions to envelop the East Coast, but there too, cold air will eventually win out, starting late in the week, according to the latest forecasts.

-----