From Thursday, April 14, 2011 to Saturday, April, 16, 2011, devastating tornadoes rampaged across communities of the southern United States. Cities and towns from Oklahoma to North Carolina were assaulted by the deadly twisters.
The tornado outbreak led to a total of 289 tornado reports in 15 states over the three-day period. While this number reflects total reports, not total number of tornadoes (as multiple sightings of the same tornado are often submitted), this tornado outbreak will likely be ranked among the largest in history.
AccuWeather.com's Jesse Ferrell has more information on the outbreak in terms of numbers and ranking in history.
Tragically, the death toll has risen to 45 people so far with dozens of others injured. The number of fatalities could rise as investigations continue. This tornado outbreak already ranks as the most deadly outbreak since the 2008 Super Tuesday tornado outbreak.
In North Carolina alone, close to two dozen people were killed on Saturday, while seven people were killed in Alabama on Friday.
One of the reasons for the deadly outcome of this tornado outbreak is likely due to the fact that highly populated areas of the nation were hit. The weather pattern can explain why these highly populated areas were struck.
Jackson, Miss., and Raleigh, N.C., are among the large cities that were struck by large, devastating twisters.
Numerous homes, businesses, churches and even schools have been severely damaged or destroyed in the path of the tornadoes.
Setup for Deadly Tornado Outbreak
Three ingredients were in place to allow this violent tornado outbreak to occur from the southern Plains to the Southeast: a powerful jet stream, abundant moisture and a strong cold front.
A powerful jet stream helped to enhance the thunderstorm growth, while moisture surging in from the Gulf of Mexico acted as fuel for the thunderstorms.
A strong cold front plowing across the South was the trigger that initiated the thunderstorm development.
The difference in the direction of wind in the upper atmosphere, southwesterly, and the lower atmosphere, southerly to southeasterly, added a twisting motion that helped the severe thunderstorms to develop rotation. Thus, the wind difference with height, referred to as wind shear by meteorologists, made the severe thunderstorms more likely to produce tornadoes.
There are a couple of factors that are making this year a more active severe weather year compared to normal and compared to last year.
The first factor is that the water in Gulf of Mexico is warmer than last year. This means that there is very warm, moist air in supply for storm systems to tap into and provide fuel for severe weather.
The second is that we are in one of the strongest La Nina patterns in recorded history.
"The strong La Nina pattern means that tremendous contrasts in air masses, with cool and dry air to the north and warm and steamy air to the south, are occurring over the Mississippi Valley. This puts many highly populated areas in the path of dangerous severe weather," said AccuWeather.com Expert Senior Meteorologist Henry Margusity.
Ranking of Tornado Outbreak Compared to Historic Outbreaks
This mid-April 2011 tornado outbreak is likely to rank among the largest tornado outbreaks in history with 241 tornado reports.
"There has not been a tornado outbreak in history over three days with this many tornadoes spawned by a single storm system," according to Margusity.
The notorious 1974 tornado outbreak, which occurred on April 3-4, 1974, produced 148 tornadoes over the course of two days and is thought of as one of the largest and deadliest tornado outbreak in history from one storm system.
In the infamous May 2003 tornado outbreak sequence (May 4-10), 401 tornadoes occurred. However, multiple storm systems moving from the Southwest into the Plains triggered these tornadoes.
"In 2004, there was a two-day tornado outbreak which produced 170 twisters. In the 2008 Super Tuesday tornado outbreak, there was 131 reported tornadoes in one day," added Margusity.
At this stage, an exact comparison to historic outbreaks in terms of the strength and number of tornadoes cannot be made. It will take weeks for the tornado surveys, which determine the strength and exact number of tornadoes, to be conducted.
A low pressure system is set to deliver heavy rain to parts of the Southeast Friday and Saturday, bringing the risk of flooding to the area.
Watches and warnings issued to the public are based on different criteria. Knowing the difference between the two can prepare individuals for the necessary safety steps.
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West Palm Beach, FL (1942)
Deluge of 8.35" of rain in 2 hours.
Ft. Wayne, IN (1963)
Precipitation totaled 2.65": hail 1.75" in diameter, 2 tornadoes, $650,000 damage, 21 buildings destroyed.
Burlington, VT (1983)
15.6" snowstorm - April record.