April 2011: historic U.S. extremes in rains, floods, tornadoes, and fires

(I have gifted Jeff Masters with some free paragraph breaks)

"April was a month of historic climate extremes across much of the United States, including: record breaking precipitation that resulted in historic flooding; recurrent violent weather systems that broke records for tornado and severe weather outbreaks; and wildfire activity that scorched more than twice the area of any April this century." Thus begins the April 2011 climate summary for the U.S. issued yesterday by the National Climatic Data Center.

The month featured very cold air spilling southwards from Canada, which gave Washington, Oregon, and Idaho top-ten coldest Aprils. Exceptionally warm air flowing from the Gulf of Mexico, which had near record-warm sea surface temperatures, gave Florida, Louisiana, and Texas top-ten warmest Aprils. The battleground where these two radically different air masses collided featured an exceptionally strong jet stream, which set the stage for the world's two largest tornado outbreaks in history: April 25 - 28 (201 confirmed tornadoes) and April 14 - 16 (155 confirmed tornadoes.)

Incredibly heavy rains also resulted, with six states along the Ohio River and Mississippi River watersheds recording their all-time wettest April in history. Eight other states had top-ten wettest Aprils, and the month was the 10th wettest April in U.S. history. Some areas along the Ohio River Valley received up to 20 inches of rain during the month, which is nearly half their normal annual precipitation.

April's extreme weather can be blamed in large part on the on-going La Niña episode in the Eastern Pacific. La Niña alters the path of the jet stream, and makes the predominant storm track in winter and spring traverse the regions that saw heavy precipitation. Climate change may have played a role in April's incredible U.S. extreme weather, though a preliminary investigation by NOAA's Climate Science Investigations (CSI) team concluded that "a change in the mean climate properties that are believed to be particularly relevant to severe storms has thus not been detected for April, at least during the last 30 years."

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1799

So stop thinking that!