COLD? BLAME THE NORTH ATLANTIC OSCILLATION

THE BALTIMORE SUN

After a pretty cold December for Baltimore and a very cold start to January, some Marylanders have begun to wonder what's going on with the weather this winter.

"Why so cold this year?" asked Juan Damien, a reader of the Sun's WeatherBlog at MarylandWeather.com. "Jet stream? El Nino? Any indication that it will continue? Wasn't planning a break, but with these temps [I] may break up the winter with a trip to the Keys."

Meteorologists are blaming the cold spell on shifting patterns of atmospheric pressure to our east, called the North Atlantic Oscillation, which have brought cold and snowy weather from central North America to Northern Europe and northern China.

"The North Atlantic Oscillation is the reason for the recent cold," said Christopher A. Strong, a meteorologist at the National Weather Service Forecast Office in Sterling, Va. "It is a cycle that, to a large extent, governs how cold we are here at any given time."

Since the beginning of December, the NAO has been in a strong "negative" phase, meaning there are smaller differences between low pressure centered near Iceland and high pressure over the North Atlantic. This creates changes in the flow of weather patterns around the northern latitudes. It allows cold air outbreaks and snowy weather on the East Coast, and sends cold air into Northern Europe.

In Baltimore, after a very mild November, December turned colder in response to the negative NAO, averaging 2 degrees below normal. Early January has been nearly 4 degrees below the long-term average.

Along with the cold, the East Coast since autumn has been enduring a series of coastal storms, a pattern typical of an El Nino winter like this one.

The combination of coastal storms and cold temperatures brought the snowstorm that hit the Mid-Atlantic region Dec. 18-19, dropping more than 21 inches at Baltimore-Washington Thurgood Marshall Airport, a record for Baltimore in December.

As cold as it's been, Strong said, it might have seemed colder than it really was. "I would caution that we are grading 'cold' on the skewed scale compared to our relatively mild weather over the past several years," he said.

Baltimore hasn't had a colder-than-average winter since 2004. And no cold-weather records have been broken here this winter. Temperature records for Baltimore go back to 1871.

"November was over 4 degrees above normal (very mild), which will also alter people's impressions going into winter," Strong said via e-mail. "December was a few degrees below normal, but was just our 36th coldest on record."

For the winter so far, he said, "we are 27th coldest, with a 33.7 degree average."

And you don't have to go back too far to find much colder beginnings to a Baltimore winter. In 2000-2001, the city saw its fifth-coldest start to winter, averaging 29.5 degrees. The winter of 1989-1990 began at 27.8 degrees, the third-coldest on record.

This still may not be the weekend for a getaway to the Florida Keys, which are suffering the effects of a brutal arctic outbreak that has gripped most of the country. The weekend forecast for Key West calls for highs in the 50s and lows in the 40s.

But the cold here won't last, Strong said.

While the NAO's negative phase can last for several weeks, "the changeover can happen in a matter of days. After a cold weekend and early week, we will moderate to near-normal temps around here by midweek next week."

Forecasters at the weather service's Climate Prediction Center say those near-normal temperatures are expected to continue well into the third week of January.

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