Percent of U.S. that experienced either extremely wet or dry conditions (in the top 10% historically) between 1910 and 2011. Developed using NOAA’s Climate Extremes Index.

(updated Friday morning; originally posted 4:02 p.m. Thursday afternoon)

One month ago, I posted “Through November, 2011 has experienced the most extensive coverage of severe drought and abnormally wet conditions on record.” Now, it’s official. 2011 - through its entirety - was record-setting for extreme precipitation in the U.S. dating back 100 years.

Jeff Masters at Wunderground was the first to blog about this and offered the evidence:

The fraction of the contiguous U.S. covered by extremely wet conditions (top 10% historically) was 33% during 2011, ranking as the 2nd highest such coverage in the past 100 years. At the same time, extremely dry conditions (top 10% historically) covered 25% of the nation, ranking 6th highest in the past 100 years. The combined fraction of the country experiencing either severe drought or extremely wet conditions was 58%--the highest in a century of record keeping.

To me, as striking as the fraction of the country affected by these extremes was the close proximity between the downpours and the desiccation.

2011 precipitation ranks by state. (NOAA)

And while Texas suffered through its driest year on record and worst drought in history, a remarkable 20 major cities and towns to its northeast had their wettest year on record, as listed below.

Large towns and cities that experienced their wettest year on record in 2011. NOAA says: “This list should be considered preliminary and incomplete. ... When the data are complete, the final number of stations with new annual records will likely be larger.” (NOAA)

Just missing being amongst the record-setting wet cities? Chicago, which had its second wettest year on record with 49.41”, just over an inch shy of the 2008 record (50.86”).

The precipitation feast or famine situation was particularly pronounced during the spring, when Andrew Freedman discussed the startling contrast...:

I’m not sure I’ve ever seen such a prominent contrast between the precipitation “haves” and the “have-nots.” For perspective, all you’d need to do to travel from parched conditions to sopping wet ground would be to take a flight of less than two hours, such as from Austin to Little Rock, or New Orleans to Indianapolis.

Precipitation departures from average during 2011. Notice how close together the red shades (abnormally dry) are to the purple shades (abnormally wet) in the south central U.S. (NOAA)

It’s not an accident that there is such juxtaposition between droughts and floods this year. The prevailing weather pattern throughout the winter and spring, which was influenced by La Nina conditions that finally dissipated earlier this month, steered storms away from the Southwest, and doused the northern states with snow and rain. Also, precipitation extremes are already increasing as the climate warms....

The La Nina pattern, particularly, helped define the shape of the jet stream and prevailing storm track, which you can pretty well discern by looking at where the green shaded states border the white, yellow, and orange states in the U.S. state map above. The jet stream was often “super charged”, no doubt helping to enhance the power of the storms (hence the near-record tornado season) and the precipitation contrasts to its north (wet) and south (dry).

As Andrew alluded to, rising temperatures as a resulting of global warming also likely enhance precipitation extremes, by speeding up evaporation. That puts more water in the atmosphere for existing storms, but dries out places where storms are absent.

With La Nina hanging around through at least the first half of 2012, more precipitation extremes and variations are likely in the cards. It’s impossible to say if these will be as pronounced as those in 2011.