The Northern Hemisphere’s snowy spring

We wrote a lot about April’s unusual cold and snow in parts of North America (here, here, here, here, and here) as well as a bit globally (here), but now we have the actual data from Rutger’s University Global Snow Lab to confirm that the spring was indeed exceptionally snowy.

For the entire Northern Hemisphere it was the snowiest April since the middle 1990s, and for North America excluding Greenland, it was the snowiest since the early 1980s (over thirty years ago).

Here are the latest snowfall time series from Rutgers:

Most of the elevated snow cover compared to normal was found in the north central U.S., eastern Europe, and in parts of Asia as shown below (areas shaded in blue):

“The April snow cover extent for the contiguous U.S. was approximately 480,000 square miles, 209,000 square miles above the 1981-2010 average and the 5th largest April snow cover extent in the 47-year period of record,” reports NOAA’s National Climatic Data Center.

The elevated snow cover in both the Northern Hemisphere and North America this year runs counter to the long-term trend, which shows a steep decline.

(Rutgers Global Snow Lab)
(Rutgers Global Snow Lab)

What caused this year’s super snow expansion? A strong weather pattern known as “blocking” prevailed in March across the high latitudes such that NOAA recorded the strongest negative Arctic Oscillation (AO) reading on record for that month (a strong indicator of such blocking).  What that means is that unusually strong upper level high pressure sat around the Arctic Circle and flushed cold air down to the mid-latitudes to expand the snow-survivable space to larger-than-normal coverage areas.

You can see the prevailing March to April pattern below reflected in the difference from average in pressure at high altitudes (upper level height anomalies). The area outlined in blue, coinciding with lower than average pressures, was generally colder-than-normal:

In the U.S., April ranked as the 23rd coolest on record and the coolest since 1997.

As we moved into May, the Northern Hemisphere snow surplus finally receded due to changes in the global circulation pattern (even as parts of the central U.S. experienced record-setting late spring snow).

You can see the latest differences from average in snow cover (and also see the above normal Northern Hemisphere snow status for most of our preceding winter) at this website from Florida State University.

We are shifting from a winter that didn’t want to end to a summer that has been sluggish to get started compared to recent years.

(Jason Samenow contributed to this post)

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Jason Samenow · May 16, 2013

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