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Eerie calm preceded violent swarm of super cells

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By Scott Sutherland
Meteorologist, theweathernetwork.com
@ScottWx_TWN
Monday, April 28, 2014, 9:30 PM

After a record was set for quietest start to the U.S. tornado season ever, extreme weather made a violent comeback.

While still unconfirmed, at least 16 deaths have been reported as a result of tornadoes sweeping through north of Little Rock, Ark., as well as regions of Oklahoma and Iowa on Sunday, and even more damage is being reported due to another tornado that passed through Tupelo, Miss., on Monday afternoon.

So, what’s been going on? Why were things so quiet leading up to this past weekend, and what happened to allow these violent storms and tornadoes to form?

When looking at the globe, March 2014 came up as the fourth warmest month of March in history. However, regions of Canada and the United States (east of the Rocky Mountains) struggled to get weather that would qualify as at least spring-like. The culprit here was the jet stream, which was plunging far into the southern United States, dragging chilly northern air along with it. This produced plenty of snow, and even a few ice storms, but it wasn’t warm enough to produce the right kind of weather for tornadoes to develop. This gave the U.S. their longest stretch of zero tornado-related fatalities for the start of the extreme weather season, for any year to date. 

However, this quiet was unfortunately not meant to last. As soon as the jet stream pulled further to the north, this allowed warm air from the Gulf of Mexico to flow on-shore, and it set up conditions that were just waiting for the first major cold front to sweep through. All that fast-moving cold, dry air coming down from the northwest met that mass of warm, moisture-laden air from the Gulf, and it set off a chain reaction. Towering thunderstorms lined up along the front and were pushed along to the east as they dumped their heavy load of rain and produced powerful winds.

From there, it was just a matter of time. When you have cold winds near the base of the storm blowing faster than the warmer air closer to the ground, this sets up a rotation in the warm, moist air flowing into the base of the storms. As the storm gains in strength, the rotation grows stronger and tighter, and thus faster, like a spinning figure skater drawing in their arms. At the same time, the powerful updrafts through the core of the storm force the rotating column of air to turn vertical, producing the familiar twisting vortex of a tornado. 

It takes some time to judge the strength of these tornadoes, as investigators need to survey the damage done along the tornado’s track. Two of this weekend’s tornadoes have already been rated as EF-2 on the enhanced Fujita Scale, with wind speeds of between 178–217 kilometers per hour. For the twister that tore through Vilonia and Mayflower, Ark., on Sunday evening, resulting in at least 16 deaths, it was estimated at nearly half a mile in diameter, and it cut a swath of destruction over 80 miles long (130 kilometers). It was at least EF-2, but based on the damage it caused, it could rate as EF-3, which would make it the first tornado of that strength the U.S. has seen this year.

The latest reports coming out of the state of Mississippi are saying that another tornado has struck the city of Tupelo, in northeastern Mississippi. There’s no word yet on the full extent of the damage.

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