WRAL WeatherCenter Blog


Posted March 4, 2009 5:37 a.m. EST
Updated March 4, 2009 6:11 a.m. EST

Tornado!  Perhaps no meteorologically-related term strikes as much fear into the human mind than this word.  And, with good reason.  In terms of raw wind power potential, there is nothing the atmosphere produces that can rival it.

Tornadoes can occur at any time of year, but are most common in March, April, and May. Interestingly enough, North Carolina leads the nation in having the greatest percentage of killer nighttime tornadoes. Of all the tornado related fatalities in our state since 1950, a whopping 81% have occurred at night!

While North Carolina is not in the proverbial "Tornado Alley", we are certainly not immune to these monsters. We only have to look back to the wee hours of the morning on November 15 when, like a thief in the night, tornadoes struck Kenly and Elm City. The Kenly tornado was rated an EF2, which means winds topped 130mph!

You can see the core of this deadly storm in this Doppler radar image taken from the National Weather Service Doppler Radar shortly after 3am on November 15. This was just minutes after the tornado struck Kenly. Doppler Radar measures the wind speed both away from and toward the radar. As you can see, winds were blowing at more than 80mph away from the radar, while on the other side of the circulation, winds were blowing toward the radar at about 10mph. When you combine those numbers, you get more than 90mph of circulation, and that's just what the radar sees. It can't resolve the smallest of details that resulted in those 130mph winds.

Other tornadoes come to mind that resulted in tragedy. The Raleigh Tornado which occurred in the wee hours of the morning on November 28, 1988 left 4 dead and tremendous destruction. The worst tornado outbreak in North Carolina History took place on March 28, 1984, when an intense area of low pressure traversed the Southeastern States, and 44 North Carolinians lost their lives. It was a night I will never, ever forget.

Tornadoes grow from thunderstorms, and there are two main elements that contribute to their development. First, instability. If you have a ball at the top of a hill, and you give it a nudge, it will start rolling faster and faster down the hill. So it is with parcels of air. If the atmosphere is unstable, and something comes along to give that parcel a nudge, like a trough of low pressure or a front, that parcel will accelerate upward, growing into a large thunderstorm. The second ingredient is wind shear. When the winds change direction in a clockwise manner as you go up through the atmosphere, and also increase in speed, conditions become very favorable for rotation to develop within a thunderstorm, and once that happens, severe weather is almost a guarantee even if a tornado doesn't form. When the degree of instability is high, and you have strong vertical wind shear, the stage may be set for a major outbreak of tornadoes.

The current state of technology allows us to be more specific in warning for tornadoes, but we still have a long way to go. The probability of detection is very good, but the false alarm rate is still not acceptable. Radar does not tell you what is going on at the ground, but rather what is going on above the ground wherever the beam happens to be sampling the atmosphere, and the farther away from the radar you are, the higher up that beam is. Spotters on the ground are still the best way of determining exactly what is transpiring on the ground. That's why the Skywarn network is so important!

Remember, the safest place to be during a tornado is in a basement, or a small interior room on the lowest floor of your house, like a bathroom or closet. Stay away from windows, and cover your head to protect yourself from flying debris. The chance of a tornado coming down your street during your lifetime is very small, but the one time you ignore the warnings may be the one time you needed to take action. Stay safe!