National Overview - May 2011

« National Overview - May 2011

Note: Data in this report are compiled from preliminary statistics

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Briefing Package presented by Tom Karl, Ed O'Lenic and Harold Brooks on June 15th (PDF)


The spring (March-May) of 2011, particularly April, brought extreme weather and climate events to many parts of the United States. Tornadoes, flooding, drought, and wildfires ravaged many parts of the country during the period, and each of these extremes broke long-standing records and have been compared to the 'worst such cases' in history. While similar extremes have occurred throughout modern American history, never before have they occurred in a single month. According to the Storm Prediction Center (SPC), there were 751 tornadoes during April alone, and the confirmed number of tornadoes surpassed the all-time monthly record of 542 tornadoes set in May 2003. Record rainfall along the Ohio River Valley, punctuated with snowmelt across the upper Midwest, caused record flooding along the mid and lower Mississippi River, with water levels surpassing the historic floods of 1927 and 1937. Above-normal precipitation and vegetative growth during 2010, followed by dry and windy conditions the first five months of 2011, created ideal wildfire conditions across the Southern Plains where millions of acres of land burned. According to the U.S. Drought Monitor (USDM), the same region experienced Extreme-to-Exceptional [D3-D4] drought following consecutive months that were record to near-record dry.

April brought an weather patternactive weather pattern across the contiguous U.S., with strong storms moving through the center of the country, tapping into moisture from the Gulf of Mexico as they matured across the mid-Mississippi Valley. These storms caused widespread severe weather across the Southeast and widespread heavy precipitation across the Ohio Valley. Both the number and magnitude of the severe weather events, as well as the amount of precipitation across the Ohio Valley, broke all-time records, according to preliminary data. Meanwhile, the storm track essentially blocked Gulf of Mexico moisture from entering the Southern Plains.

Across the Upper Midwest rapid melt of an above-average snowpack during late March through mid-April swelled rivers and caused near record river crests along the Red River in North Dakota and Minnesota. Farther east, across Minnesota and Wisconsin, a significant portion of the snowmelt water found its way into the Mississippi River and moved southward towards the Gulf of Mexico. Meanwhile, the above-average rainfall across the Ohio Valley, combined with snowmelt, caused the Ohio River to swell to near-record levels. At the confluence of the Ohio and Mississippi Rivers, the above-average water flow of each combined to cause the Mississippi River to crest at record to near-record levels from Illinois to Louisiana, flooding hundreds of thousands of acres. After judicial directives, the Army Corps of Engineers opened spillways and destroyed levees, flooding rural areas to save major population centers and infrastructure.

On a statewide level, during April, above-normal precipitation was widespread across the northern half of the country, while the Southern Plains and Southeast had near- to below-average precipitation. Below-normal precipitation was observed for the previous six months across the southern Plains, exacerbating drought conditions there. At the beginning of May, 73 percent of Texas was experiencing Extreme-to-Exceptional [D3-D4] drought conditions. Texas had its second driest November through April period, third driest January-April, and driest February-April and March on record. The prolonged dryness fueled several large wildfires, which burned 1.79 million acres (0.72 million hectares) nationally during the month, shattering the previous April record.