National Climate Report - May 2011

« National Climate Report - May 2011


The period through the end of May, 2011 was marked by numerous large severe weather outbreaks, causing a record-breaking tornado year for the year-to-date period. An estimated 1,217 tornadoes struck the U.S. during the January-May period, according to the Storm Prediction Center and 751 of those tornado reports were during the month of April alone.

Tornado activity during April was record breaking. According to data from the Storm Prediction Center (SPC), there were 751 tornadoes during the month, and the final tornado count surpassed the all-time monthly record of 542 tornadoes (May 2003). The previous April record was 267 tornadoes, set in April 1974. The 30-year (1981-2010) average for number of April tornadoes is 135. During spring (March-May) 2011 there were several significant tornado outbreaks, mostly across the Southeast, and more information on these individual outbreaks can be found in the State of the Climate report.

Environmental Conditions

During the month of April, environmental conditions came together to create the perfect scenario for severe weather across the eastern half of the country. A weather patternpersistent storm track across the central U.S. allowed frontal systems to interact with Gulf of Mexico moisture and initiate storms across the southeastern quadrant of the nation. During April, weather patternseven upper level troughs moved across the country. These upper level troughs provided dynamic forcing that allowed the development several severe weather complexes. Strong jet streams formed along and ahead of these upper level troughs providing diffluence aloft. Diffluence can facilitate rising air within thunderstorms, causing them to become stronger. Along the leading edge of the upper level troughs were surface cold fronts and surface low pressure systems. Southerly winds ahead of the surface cold fronts caused warm and moist air to surge northward. Behind the fronts, cool Canadian air swept across the central part of the country.

During the two largest outbreaks (April 14th-16th and 25th-28th), there was significant southerly flow ahead of the storm systems. Higher in the atmosphere, the wind tended to be more westerly in direction. This veering of the winds with height created a large spatial extent of vertical wind shear. This shear, under the right circumstances, can fuel long-lived supercell thunderstorms. Also at play was the difference in temperature between the warm southerly flow near the surface and the drier, cooler air aloft associated with the upper level troughs. This scenario is also favorable for severe thunderstorm development when warm moist air near the surface is lifted upwards by the surface cold front into the layer of cooler, drier air, releasing latent heat energy into the storm.

During the month of April other large scale phenomena played a role in the development of the record-breaking tornado outbreaks. Ahead of the cold fronts, warm and very moist southerly air invaded the eastern half of the U.S. from the Gulf of Mexico. A monthly analysis of winds using reanalysis shows that the southerly component of near surface winds across the Gulf Coast and southeastern U.S. more than 6.7 mph (3 m/s) above the 1971-2000 April average. This indicates that the southerly flow was persistent and strong across during April.

Another large scale feature which played a role in the development of these severe weather outbreaks was the Gulf of Mexico. Averaged for the month, the sea surface temperatures across the Gulf of Mexico were about 1 degree Celsius (1.8 degree F) above the long-term average. The warm water and atmosphere over the Gulf provided fuel for the severe weather outbreaks. The southerly winds ahead of the storms that moved across the country advected the warm temperatures and Gulf moisture over the continental United States. Many locations across the southern U.S. broke daily temperatures records when these southerly flow regimes set up across the region. The warm sea surface temperatures across the Gulf are a good indicator of the amount of moisture across the region. Warmer temperatures allow the atmosphere to hold more water vapor. The more water vapor in the atmosphere, the more readily storms form and the stronger storms can become.

Historical Perspective

Record Annual Tornado Fatalities
Year Fatalities
1925 794
(through May 31st)
1936 552
1917 551
1927 540
1896 537
1953 519
1920 499
1908 477
1909 404

On the January-May timescale, the number of tornadoes and the number of tornado-related fatalities were record breaking. It is likely that the entire country and many states will approach and/or break records for the number of confirmed tornadoes. For the year-to-date period, there were 1,217 preliminary tornadoes. Nationwide, the previous January-May tornado count record was 1,011 in 2008. In terms of the number of tornado-related fatalities, there were 561 during the January-May period. When analyzing the number of tornado-related fatalities, there were two separate comparisons — the comparison to the modern (1950-present) tornado record and the non-official (1875-present) record. The 561 fatalities is the most in the 1950-present period, and the second most in the 1875-present time period.

The severe weather outbreaks during spring 2011 caused significant property damage across the eastern United States on top of the unfortunate high number of fatalities. According to preliminary numbers from various disaster cost modeling and insurance risk reports, the estimated damage from the spring severe weather outbreaks will approach 24 billion U.S. dollars, with several of the severe weather events likely exceeding one billion U.S. dollars in damage each.

Significant Severe Weather Events

April 14th–April 16th Tornado Outbreak

A major severe weather outbreak impacted the southeastern U.S. between April 14th and 16th. A strong upper level low pressure system moved across the central plains, and ahead of the system, very warm and very moist air was advected northward from the Gulf of Mexico. As the associated surface cold front moved into the southeast, a series of severe thunderstorm complexes were initiated across the country. NOAA estimates the final tornado count for the outbreak was 176 tornadoes, marking one of the largest outbreaks on record, especially for April, which averages 135 tornadoes for the entire month. Although there were a large number of tornadoes, there were 14 tornadoes rated EF-3, and none were rated EF-4 or EF-5. A total of 38 people were killed from the tornadoes, 22 of which were in North Carolina alone. Nationwide, the tornado outbreak break was the deadliest since the 2008 Super Tuesday tornado outbreak that occurred on February 5th and 6th, 2008, when 57 people were killed. The 30 confirmed tornadoes across North Carolina broke the single-storm and single-day tornado outbreak record for the state. The previous record was from the infamous March 1984 Carolina’s tornado outbreak when 22 tornadoes killed 42 people in the state.

Record Daily Tornado Fatalities
Date Fatality Reports
March 18, 1925 747
March 21, 1932 332
May 17, 1849 317
April 27, 2011 316
April 3, 1974 310
May 27, 1896 305
April 11, 1965 260
April 5, 1936 249
April 25th–April 28th Tornado Outbreak

Another major and record breaking severe weather outbreak impacted the southeastern U.S. between April 25th and 28th. During the outbreak, there were 345 tornadoes in 21 states. The storm scenario was similar to the mid-April outbreak, with a potent upper level low pressure system moving across the Central and Northern Plains, but this low pressure system was a little stronger and had a path more northward. Ahead of the surface cold front, very warm and very moist air infiltrated the southern U.S. from the Gulf of Mexico. Temperatures across the southeast reached into the lower 90s (°F). A strong mid-level jet streak moved into the Tennessee Valley, providing strong shear and dynamic support for severe thunderstorm development. As the storm system moved towards the east, it initiated severe weather each day between the 25th and 28th. The most impressive period of the outbreak was on April 27th, and into the early hours of the 28th, when two severe weather complexes moved across Mississippi, Alabama, Tennessee, and Georgia. NOAA’s current estimate is there were 190 tornadoes during that time period. The previous single largest tornado outbreak to impact the U.S. was the April 3-4, 1974 Super Outbreak, when there were 148 tornadoes across the Southeast and Ohio Valley. According to the most recent estimates, there were 3 EF-5 rated tornadoes, 12 EF-4s and 21 EF-3s. There were an estimated 320 tornado related fatalities from the outbreak — approximately 321 of those occurred on the 27th. Alabama alone observed 235 of those fatalities. An EF-5 moved through northern Alabama and killed 78 people, the deadliest of the outbreak. Several major metropolitan areas were directly impacted by strong tornadoes including Tuscaloosa, Birmingham, and Huntsville in Alabama and Chattanooga, Tennessee, causing the estimated damage costs to sore. According to preliminary information, property damages might exceed six billion U.S. dollars. For more information see the monthly U.S. Tornado State of the Climate report.

April 27th Select Station Dewpoints
City April 27th Hourly Average Dewpoint 30-year average April Dewpoint
Birmingham, Alabama 66.2 °F (19.0 °C) 49.2 °F (9.5 °C)
Tuscaloosa, Alabama 66.0 °F (18.9 °C) 50.9 °F (10.5 °C)
Huntsville, Alabama 61.9 °F (16.6 °C) 48.3 °F (9.1 °C)
Joplin, Missouri Tornado May 22nd
Record Single Tornado Fatalities
Tornado Fatalities Date
695 March 18, 1925
Natchez, Mississippi 317 May 6, 1840
St. Louis, Missouri 255 May 27, 1896
Tupelo, Mississippi 216 April 5, 1936
Gainesville, Georgia 203 April 6, 1936
Woodward, Oklahoma 181 April 9, 1947
Joplin, Missouri 157 May 22, 2011
Amite, Louisiana/
Purvis, Mississippi
143 April 24, 1908
New Richmond, Wisconsin 117 June 12, 1899
Flint, Michigan 116 June 8, 1953

On May 22nd a severe weather outbreak occurred across the central Plains and the Midwestern United States, and several of the storms generated tornadoes across the region. The most destructive of the storms tracked from southeastern Kansas into southwestern Missouri during the late afternoon. This supercell thunderstorm spawned a strong tornado over Joplin, Missouri, resulting in devastating damage. The National Weather Service rated the tornado an EF-5, with winds in excess of 200 mph (320 km/hr). The tornado was on the ground for approximately 6 miles (9.7km) and had a maximum width of 3/4 mile (1.2 km). The tornado struck the heavily populated southern part of Joplin, resulting in at least 157 deaths and 1,000 injuries. The tornado surpassed the June 1953 tornado which killed 116 people in Flint, Michigan as the deadliest single tornado to strike the U.S. since modern tornado record keeping began in 1950. The deadliest tornado on record for the U.S. was the 'Tri-State Tornado' which killed 695 people across Missouri, Illinois, and Indiana in March 1925.

The image on the left shows the base level reflectivity from the Springfield, Missouri NEXRAD radar site of the Joplin, Missouri tornado Sunday May 22, 2011 22:43 GMT (5:43 pm local time). The high reflectivity values occurring over southern Joplin indicate the mostly likely location of the EF-5 tornado.

The image on the right shows de-aliased storm relative velocity from the Springfield, Missouri NEXRAD radar site of the Joplin, Missouri tornado Sunday May 22, 2011 22:43 GMT (5:43 pm local time). Green colors (negative values) indicate wind moving towards the radar site (located 60 miles [97km] to the east), and red colors (positive values) indicate winds moving away from the radar site. The tight couplet of reds and greens over southern Joplin show the strong rotation occurring within the EF-5 tornado. The large scale green and red couplet indicates the larger scale rotation occurring within the supercell thunderstorm.

3-D NEXRAD Animation of Joplin Tornado

Note: Data in this report are compiled from preliminary statistics
Updated May 26, 2011

The movie shows the vertical composite of radar sweeps from the Springfield, Missouri NEXRAD radar site of the Joplin, Missouri tornado Sunday May 22, 2011, giving a 3-dimensional image of the tornado structure. The Google Earth KMZ file used to create this video can be downloaded here.

Images created with NCDC's Weather and Climate Toolkit and GRLevel2.

Preliminary* Statewide Torando Records

Annual Tornado Records
State 2011 Preliminary Tornado Reports* Old Record / Year
Alabama 160 94 / 2008
Kentucky 52 39 / 1997
Mississippi 136 109 / 2008
North Carolina 89 67 / 2004
Tennessee 75 46 / 2009
Jan–May Tornado Records
State 2011 Preliminary Tornado Reports* Old Record / Year
Alabama 160 72 / 2009
Arkansas 66 62 / 2008
Georgia 56 49 / 2008
Indiana 48 46 / 1990
Kentucky 52 34 / 2008, 1974, 2003
Louisiana 70 44 / 1990
Maryland 14 13 / 2002
Mississippi 136 56 / 2008
Misssouri 85-tie 85 / 2003
North Carolina 89 53 / 1998
Ohio 31 28 / 1973
Tennessee 94 45 / 2003
Virginia 38 33 / 2008
Wisconsin 35 33 / 2005
Entire U.S. 1371 1,224 / 2008
April Tornado Records
State 2011 Preliminary Tornado Reports* Old Record / Year
Alabama 140 35 / 2009
Arkansas 46 29 / 1979
Georgia 46 25 / 2009
Kentucky 41 29 / 1974
Louisiana 36 24 / 2000
Maryland 11 5 / 2002
Mississippi 121 26 / 2005
Misssouri 30 26 / 1994
Nevada 3 1 / 2005, 1964, 2001
New York 8 4 / 1991
North Carolina 85 24 / 1996
Tennessee 50 42 / 1974
Texas 69 - tie 69 / 1957
Virginia 35 18 / 2008
Wisconsin 16 11 / 1984
Entire U.S. 875 273 / 1974

*2011 numbers reported are preliminary tornado reports. These numbers are subject to change as more up-to-date storm surveys are completed.

Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: National Climate Report for May 2011, published online June 2011, retrieved on December 14, 2018 from