The cold season (Oct-Mar) Climate Extremes Index (CEI) for heavy 1-day precipitation events shows that these events, on average, have affected about 10% of the Northeast over the past century (black line). But four of the last six years have seen levels above 40%, and up to 77% in 2010, highest in the record. Source: National Climatic Data Center (hat tip: Nick Sundt).

Heavy (highest 10th percentile) one-day precipitation events in the Northeast (including Maryland the District) have been the most extreme on record (since 1911) in four of the last six years. 2010 was the most extreme. With the weekend’s Snowtober-fest, the 2011 cold season is off to a fast and furious start no doubt, but it has yet to be entered into the record.

The weekend’s paralyzing storm precedes the release of a major report from the United Nations (UN) Intergovernmental Panel on Climate Change (IPCC) on climate extremes in a warming world. Seth Borenstein of the Associated Press received a draft copy and summarized some its findings today in a new story published today. His article begins: “Freakish weather disasters — from the sudden October snowstorm in the Northeast U.S. to the record floods in Thailand — are striking more often. And global warming is likely to spawn more similar weather extremes at a huge cost.”

2011 has been a record year for billion dollar weather disasters in the U.S. - although the majority resulted from tornado outbreaks - which have not (yet) been linked to global warming. Three of the ten billion dollar disasters have resulted from heavy precipitation events: the Groundhog Day Blizzard from Jan 29-Feb 3, the Mississippi River flooding in the spring and early summer, and Hurricane Irene - which caused massive flooding in New England.

That heavy precipitation events such as those witnessed in 2011 have markedly increased is nothing new. Nor is the projection that these events are very likely to keep increasing as increasing greenhouse gases from human activities juice up the atmosphere.

The U.S. Global Change Research Program reported in 2009 that “the amount of rain falling in the heaviest downpours has increased approximately 20 percent on average in the past century, and this trend is very likely to continue, with the largest increases in the wettest places.”

It further noted:

* One of the clearest precipitation trends in the United States is the increasing frequency and intensity of heavy downpours.

* While total average precipitation over the nation as a whole increased by about 7 percent over the past century, the amount of precipitation falling in the heaviest 1 percent of rain events increased nearly 20 percent.

* During the past 50 years, the greatest increases in heavy precipitation occurred in the Northeast and the Midwest.

Whether this increase in heavy precipitation manifests itself in more crippling snowstorms such as just experienced in the Northeast is more controversial. As Borenstein’s AP story today explained:

The snow-bearing Nor’easter cannot be blamed on climate change and probably isn’t the type of storm that will increase with global warming, four meteorologists and climate scientists said. They agree more study is needed. But experts on extreme storms have focused more closely on the increasing numbers of super-heavy rainstorms, not snow, NASA climate scientist Gavin Schmidt said.

The UN IPCC “Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX)” - finds there is at least a 2-in-3 chance that climate extremes have already gotten worse from man-made greenhouse gases according to Borenstein. It is due out November 18.