Update 3:05 p.m. EDT: The death toll from the southern storms is now at least 250, reported the Associated Press.
Last night, more than 100 reported tornadoes may have claimed 200 or more lives in Dixie Alley. The historic tornado outbreak contributes to what is likely to be a record-breaking month for twisters. Some incredible facts and records from the 2011 tornado season so far:
Where they hit
During the spring and early summer, thousands of tornadoes will strike the United States.
Textbook tornadoes form in the United States where warm, moist Gulf of Mexico air collides with cool, northern air, creating massive storms. Most of the Earth's tornadoes form in Tornado Alley, bordered by the Dakotas to the north, the Gulf Coast to the south, the Rocky Mountains to the west and the Appalachian Mountains to the east.
This year, the tornado season has targeted Tornado Alley's southern neighbor, Dixie Alley. Home to the deadliest tornadoes, Dixie Alley spreads from the Lower Mississippi Valley to the Upper Tennessee Valley, including Arkansas, Mississippi, Louisiana, Alabama, Georgia and the Florida panhandle.
Though these two spots are the most tornado-prone, tornadoes can strike anywhere at any time. Earlier this year, two reported weak tornadoes struck Colusa County, Calif., just north of Sacramento, which did not have a single tornado report from 1950 to 2010.
When all the damage is analyzed, April 2011 will almost certainly set a record for the number of tornadoes in the month of April. It could also set a record for any month, though storm reports will have to be combed through before meteorologists will know this for sure.
The Weather Channel has reported the confirmation of 292 tornadoes in the United States so far this month, besting the previous April record of 267 in 1974. Storm survey teams continue to assess the damage from this month's storms and could change the number of confirmed tornadoes. The average for April is only 116, according to the Storm Prediction Center (SPC) in Norman, Okla.
Counts of tornadoes and tornado damage today are higher than ever due to more people reporting tornadoes, and more buildings being hit as the country's population grows and its cities and towns expand. Storm chaser videos of funnel clouds are posted daily to YouTube, and rarely does a small tornado go unnoticed anymore.
If April 1974 were adjusted to account for unreported tornadoes, an extra hundred or so could be added to its total, said Greg Carbin of the SPC.
The record for most tornadoes in any month (since modern tornado record-keeping began in 1950) was set in May 2003, with 543 tornadoes. That easily broke the old mark of 399, set in June 1992. With yesterday's massive outbreak, the month April 2011 could challenge those months with 620 tornado reports so far (though these are not the same as actual tornadoes, as several reports could concern the same tornado).
Second "Super Outbreak"
None of the outbreaks this month comes close to matching the "Super Outbreak" of April 3-4, 1974, when more than 148 tornadoes were confirmed and 330 people were killed. That outbreak had seven storms that were rated F-5 intensity and 23 that were rated F-4 on the old Fujita damage scale. [The Tornado Damage Scale in Images]
Yesterday's deadly outbreak is likely the deadliest since 1974, with early estimates of 200 killed.
The deadliest outbreak of all time is believed to be the Tri-State Tornado of March 18, 1925, which killed 695 people in Missouri, Illinois and Indiana.
What's caused the outbreaks
A lingering La Niña pattern may be behind the steady march of storms across the region in recent days.
Along with creating dry weather in the Southwest and contributing to the historic wildfires in Texas, La Niña tends to guide the jet stream north through the Pacific Northwest and Great Lakes, trapping cold air on the northern side and the warm, humid air needed for thunderstorms on the southern side.
That means that the South stays wet because cold fronts that would normally dry out the atmosphere are blocked.
This year in North Carolina, 28 tornadoes have been confirmed for April 16, a record outbreak for any single day in the state.
In Wisconsin, 14 tornadoes struck on April 10, the biggest outbreak for any April day in the state's history.
Longest tornado track ever
The Tri-State Tornado of March 18, 1925, traveled 219 miles (353 kilometers) across southeastern Missouri, southern Illinois and southwestern Indiana.
That Tri-State tornado was on the ground for about 3.5 hours. A huge storm yesterday is thought to have been on the ground for at least 2 hours, and the supercell system that spawned it is thought to have lasted for 300 miles, which would set a new record for longest-lived tornado system.
Dangerously close video shows a possible mile-wide tornado in Tuscaloosa.
Reports rising, deaths dropping
The 2000 to 2009 average for annual tornado–related fatalities is 62, according to the National Climatic Data Center.
Mike Smith, chief executive officer of Weather Data Services, a part of AccuWeather, believes that at least 100 lives were saved by the warnings before a massive EF-4 tornado -- the strongest of the year so far -- struck near St. Louis on Good Friday (April 22).
Overall, tornado-related deaths have been decreasing over the last few decades as forecasts and warnings improve.
Why Dixie tornadoes are deadly
A Dixie Alley tornado does not need to be big to be deadly.
Unlike the flat, grass-covered plains of Tornado Alley, tornadoes are hard to see in Dixie Alley. Trees and hilly terrain obscure funnel clouds, a problem made even worse by the region's high rate of nighttime tornadoes.
Often, tornadoes can be cloaked in rain, hiding even the most massive twisters.
To make matters worse, Dixie Alley is home to many manufactured houses and mobile homes that have weak walls and poor -- or nonexistent -- foundations. Before last night, more than half of this year's tornado-related deaths had occurred in mobile homes.
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Reach OurAmazingPlanet staff writer Brett Israel at email@example.com. Follow him on Twitter @btisrael.