SPECIAL REPORT: The danger of winter tornadoes in the Tennessee Valley

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TENNESSEE VALLEY (WHNT) -- Records on tornado activity in the Tennessee Valley have been kept for more than 60 years.  Only four years have had no reported tornadoes. The most recent year with no tornadoes on record was 1987.

Typically, tornadoes are most frequent between March and May.  April holds the highest number of tornadoes with 147 reported between 1950 and 2014. The spring months also feature the strongest and longest-track tornadoes.

With the right conditions, tornadoes can easily happen in the cold season. Most tornadoes in the winter are much weaker than those in the spring, but they present a myriad of challenges to forecasters.

Chris Darden, Meteorologist in Charge at the National Weather Service in Huntsville, admits "our biggest fear sometimes is in the winter."

Related: Alabama among states with most February tornado touchdowns

The atmospheric setup associated with winter tornado events often involves what is known as a conditional threat. In other words, only under certain conditions could tornadoes form. This means many times there is more uncertainty than in the spring.

"I really think it's trying to pinpoint that spatial window," says Darden, referring to determining where the best combination of shear, instability, and moisture will be.  Darden characterizes winter storm systems as having "the perfect kind of setup wind shear-wise," but moisture and instability are almost always limiting factors.

Many memorable winter tornadoes in the Valley have occurred during the overnight hours. One of the worst happened on February 16, 1995 around 5 a.m. when an F3 tornado tore through Joppa and Arab, killing six people.

"Tornadoes that occur at dark are certainly more dangerous, because people [are usually asleep or doing other things]," Darden says. Tornadoes in the winter "do tend to [happen] a little later. But one thing to keep in mind-- there's not nearly as much daylight."

Most tornadoes in the winter usually are not very strong, but Darden explains "every once in a while you will get an outlier event [like] the 'Super Tuesday' event in [February] 2008." In the Tennessee Valley, two EF-4 tornadoes occurred between 3 and 6 a.m. in Lawrence and Jackson Counties on February 6. The photo gallery below has images of damage in Lawrence County done by the storm.

Tornadoes in the winter often occur within lines of storms. In fact, more than 50 percent of all tornadoes in the Valley happen in quasi-linear convective systems (QLCS). Darden explains the challenge this presents to forecasters: "picking out the individual rotations within a line is a big deal. It's one of our biggest meteorological challenges." Areas of rotation within squall lines often form quickly, making detecting by radar and issuing warnings much harder.

Some other memorable cold-season tornado events include December 16, 2000; January 23, 2012; January 21, 2010; and January 19, 1988.

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