What Do the Latest Climate Assessments Tell Us about Nor’easters?

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2906

By: Bob Henson , 4:00 PM GMT on January 29, 2015

An ever-lengthening procession of winter storms has marched across the mid-Atlantic and Northeast over the last few years. Even their names have grown more prolific and colorful, from Snowmaggedon of January 2010 to this week’s Blizzard of 2015, also known as Winter Storm Juno. Do these monikers imply the storms themselves are getting more fierce? The naming trend can be explained largely by the demands of social media. However, recent overviews of national and global climate indicate that, as a whole, the most intense rainstorms and snowstorms in the Northeast U.S. are growing even more intense. Our understanding of a warmer climate with wetter extremes arises from both observations of past trends and model-based projections of future climate. It’s also supported by basic physics: in a warmer global environment, more water vapor evaporates from oceans and lakes, where it can be steered into rain- and snow-producing storms.

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Within the United States, the region where nor’easters prowl is also where we find the most pronounced turn toward extreme rainfall and snowfall. The most comprehensive report to date on our nation’s climate is the latest U.S. National Climate Assessment, released early in 2014. It found that the trend toward intensified precipitation is stronger in the Northeast than in any other part of the country (Fig. 2.18 in the report, and Figure 1 above). For the period 1958 – 2012, this region saw a 71% increase in the amount of precipitation that fell on the wettest 1% of all days. “In the mid-latitudes, where most of the continental U.S. is located, there is an upward trend in extreme precipitation in the vicinity of fronts associated with mid-latitude storms,” noted the report. “Projections of future climate over the U.S. suggest that the recent trend towards increased heavy precipitation events will continue. This is projected to occur even in regions where total precipitation is projected to decrease, such as the Southwest.” (full report, Chapter 2, p. 37).

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Given the potential for disaster inherent in the worst winter storms, even a 13% increase in their frequency could pack a notable punch. As evident in Figure 2, however, there is sharp variability from decade to decade in the frequency of such extreme snowstorms, something that should come as no surprise to any longtime weather observer. Which brings us back to the last decade and its bumper crop of Northeastern storms. At New York City’s Central Park, five of the 10 biggest storm-total snowfalls on record have occurred since 2000; in Boston, it’s four of the top 10, and in Washington, three of the top 10. This 21st-century onslaught could be related to the longer-term boost provided to extreme rains and snows by a warming planet, as well as a shorter-term jump in the frequency of Northeast storms related to cycles in global and regional climate. In addition, a growing amount of research suggests that Arctic amplification and sea-ice loss could be influencing jet-stream behavior and fostering outbreaks of cold and snow in North America and Eurasia (see the subsection "Is the jet stream getting weird?" in this Jeff Masters post from last November).

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All storms are now unfolding in a warmer, moister global climate.

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