Posted by: JeffMasters

"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 its 3rd highest sea surface temperatures on record during April, 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 Niņa episode in the Eastern Pacific. La Niņa 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."

Figure 1. Six states along the Ohio River and Mississippi River watersheds had their all-time wettest April in history during 2011. In contrast, Texas had its 5th driest April on record, after recording its driest March ever. Image credit: National Climatic Data Center.

Great Texas drought of 2011 intensifies
April 2011 was the 5th driest and 5th hottest April in Texas history, going back 117 years. Exceptionally dry conditions have parched the soil and vegetation in Texas, which recorded precipitation of just 1.68 inches (43 mm,) on average, since February 1st. This is easily its driest February-April period on record for the state, nearly an inch less than the previous record (2.56 inches or 65 mm, Feb - Apr 1996.) The six-month period November 2010 - April 2011 was the 2nd driest such period on record. Based on the U.S. Drought Monitor, 94 percent of Texas is in severe to exceptional drought.

As a result of the great drought, an all-time April record of 1.79 million acres of land burned last month in the U.S., mostly in Texas. Much of the fuel for the fires came from dried underbrush and grasses which experienced ideal growing conditions during the summer of 2010, when there was abundant rain across the region. Nation-wide, the year-to-date period, January - April, has the greatest acreage burned in history, according to the National Interagency Fire Center.

Figure 2. The acreage burned in U.S. wildfire in April 2011 was by far the highest in the past decade. Most of the damage was done by a few huge fires. Image credit: National Climatic Data Center.

April 2011 smashes all-time tornado records
The largest tornado outbreak and greatest one-day total for tornadoes in history occurred during the historic April 25 - 28, 2011 tornado outbreak, said NOAA in a press release updated yesterday. They estimate 190 tornadoes touched down during the 24-hour period from 8:00 a.m. EDT April 27 to 8:00 a.m. EDT April 28 (134 tornadoes have already been confirmed, with several weeks of damage surveys still to come.) NOAA's estimate for the number of tornadoes during the three-day April 25 - 28, 2011 Super Outbreak, is 305 (201 are confirmed so far.) This is nearly double the previous record for a multi-day tornado outbreak of 155 tornadoes, set just two weeks previously during the April 14 - 16, 2011 outbreak. There were tornado outbreaks in May 2004 (385 tornadoes) and May 2003 (401 tornadoes) that had more tornadoes, but these outbreaks occurred over an eight-day and eleven-day period, respectively, and were not due to a single storm system. Prior to April 2011, the most tornadoes in a 24-hour period, and in an outbreak lasting less than four days, was the 148 tornadoes in the Super Outbreak of April 3 - 4, 1974. The final tornado count for April 2011 will approach the all-time monthly record of 542 tornadoes, set in May 2003. The previous April record was 267 tornadoes, which occurred in April 1974. The 30-year average for April tornadoes is 135.

Figure 3. The Tuscaloosa-Birmingham tornado of April 27, 2011 killed 65 people and injured over 1000. The tornado carved a path of destruction 80.3 miles (129.2 km) long, and up to 1.5 miles (2.4 km) wide. On May 4, 2011, the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite observed this segment of the tornado's track, near Birmingham, Alabama. Image credit: NASA Earth Observatory.

The death toll for the epic outbreak continues to fluctuate, and currently stands at 326, with 309 fatalities during the 24-hour-period from 8:00 a.m. April 27 to 8:00 a.m. April 28. The estimated 326 deaths makes this the 4th deadliest tornado outbreak on record. Only the great Tri-State tornado outbreak of 1925 (747 killed), the 1936 Tupelo-Gainsville tornado outbreak (454 killed), and a 1932 outbreak (332 killed) had more deaths.

Figure 4. The Tuscaloosa-Birmingham tornado as it headed toward the Univ. of Alabama on April 27, 2011 (video shot by Chris England, a Univ. of Alabama student).

Mississippi River crests at Memphis: 2nd highest flood on record
The Mississippi River has crested at Memphis, Tennessee this morning, reaching the 2nd highest level on record. The flood height of 47.79' was just below the all-time record height of 48.7' set in the great 1937 flood. Fortunately, the levees constructed by the Army Corps of Engineers are much taller and stronger than was the case 74 years ago, and the mainline Mississippi River levees are expected to hold back this record flood and prevent a multi-billion dollar flood disaster. However, flooding on tributaries feeding into the Mississippi is severe in many locations along the Mississippi, since the tremendous volume of water confined behind the levees is backing up into the tributaries.

Downstream from Memphis, flood waters pouring in from the Arkansas River, Yazoo River, and other tributaries are expected to swell the Mississippi high enough to beat the all-time record at Vicksburg, Mississippi by 1.3' on May 19, by 6' at Natchez, Mississippi on May 21, and by 3.2' at Red River Landing on May 22. The Mississippi is forecast to crest at 19.5' in New Orleans on May 23. The levees in New Orleans protect the city for a flood of 20.0 feet, so it is a good bet that the Army Corps will fully open the Bonnet Carre' Spillway 28 miles upstream from New Orleans this week. The Bonnet Carre' Spillway was partially opened yesterday, and has the capacity to take 250,000 cubic feet per second of Mississippi River water into Lake Pontchartrain. This may not be enough to keep flood heights from rising dangerously close to the top of New Orleans' levees, and the Army Corps may elect to open the final relief valve they have at their disposal--the massive Morganza Spillway, 35 miles upstream from Baton Rouge. The Morganza Spillway has been opened only once in history, back in 1973. Rainfall amounts of at most 0.75 inches are expected over the Lower Mississippi River watershed over the next five days, which should prevent flood heights from rising above the current forecast.

Good links to follow the flood:
Summary forecast of all crests on Lower Mississippi and Ohio Rivers.
Wundermap for Vicksburg, MS with USGS River overlay turned on.
National Weather Service "May 2011 Mississippi River Flood" web page