Percent of U.S. covered by extremely wet or dry conditions during the January-November period between 1910-2011. (NOAA)

NOAA’s Climate Extreme Index (CEI) reveals that (for the period covering January through November) 56% of the U.S. is experiencing either severe drought or extremely wet conditions, way above the historical average of 22% (hat tip, Jeff Masters,

Percentage of U.S. affected by extremely wet (top) versus extremely dry conditions (bottom) between January and November from 1910 to 2011. (NOAA)

2011 is somewhat unique in the historical record in that it ranks in the top 10 for both drought and heavy precipitation coverage. In many of the other extreme years, it was either usually wet or unusually dry, not both.

The prevailing La Nina pattern has supported the presence of a powerful jet stream slicing through the middle of the country, bringing bout after bout of stormy weather. But to the south and southwest of that jet stream, the moisture abruptly shutoff leading to historic drought.

Global warming may have something to do with the contrasts in this pattern. Added heat to the atmosphere juices up the wet extremes by making more water vapor available, while speeding up evaporation and drying in drought areas.

And in a new twist, researchers at the University of Wisconsin-Madison have identified the development of “superjets” in the Pacific ocean that may have fueled some of this year’s severe weather and heavy rains. These superjets form in the western Pacific when the subtropical jet stream lifts north and combines with polar jet stream. Jon Martin, one of the researchers, says these superjets can bring powerful storms to the Nation’s mid-section and Southeast 7-10 days after they form.

“If the subtropical jet stream is rearranged and superposed on top of the polar jet stream, it might be the mechanism that allows for this very long delay, a disturbance that can have discernible effect on severe weather thousands of miles downstream, and a week or more later,” he said.

Unusually strong jet stream winds were linked to some of the spring’s historic, deadly tornado outbreaks. An analysis of the frequency of these superjets has not been published, so it’s not clear if they were more common in 2011 or if there’s a global warming link.

“Historic weather data should tell us whether there has been a change in the frequency of these overlapping events, and whether that might be linked to a change in high impact-weather events,” Martin said.