Supercell storms produced last week’s tornadoes

Published 7:00 am Tuesday, February 14, 2017

By Skip Rigney

Last Tuesday a powerful tornado hit New Orleans East. The National Weather Service (NWS) later confirmed five other tornadoes in southeast Louisiana.

That afternoon a thunderstorm that had earlier produced a tornado in St. Tammany Parish moved eastward bringing hail to our county. Most pieces that I saw were the size of a nickel, but a few were as large as half-dollars.

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Severe thunderstorms dotted the radar for southeast Louisiana and south Mississippi much of last Tuesday morning through mid-afternoon. The NWS Forecast Office in Slidell, which has the warning responsibility for this area, broadcast 35 tornado warnings and 7 severe thunderstorm warnings in just eight hours.

The severe weather outbreak was unusual in a number of ways. Often severe storms that affect those of us living within 50 miles of the Gulf of Mexico are associated with lines of thunderstorms called squall lines.

However, last Tuesday almost all of the severe weather was produced by rotating supercell thunderstorms.

The supercells did not group together in a line, but instead showed up as distinct storms on weather radar as they streaked eastward. Supercells are more likely to produce strong tornadoes than squall lines are.

Supercells are more common farther away from the Gulf in central and northern Louisiana, Mississippi, and Alabama and north and westward into the Great Plains.

To have so many supercells is unusual at any time of year in south Louisiana or Mississippi, but for such powerful storms to form in early February is particularly uncommon.

That is because during winter the air near the surface is usually too cool to provide the instability necessary for strong updrafts. But, temperatures this winter have been much warmer than normal, and that was the case last Tuesday.

Another unusual aspect of the outbreak was that it was not set off by a surface cold front. Many severe weather events are associated with the clash of warm and cold surface air masses. The nearest cold front last Tuesday was in Oklahoma.

However, several miles above ground the southern end of a small, but distinct trough of low pressure was racing eastward across Louisiana.

At those upper levels, a cold pool of air associated with the trough on top of the warm surface air created an unstable atmosphere. Add a sharp increase of wind speed with height, and we had the ingredients for supercell storms.

Not only were the circumstances unusual for our area, the weather models last Monday were predicting that the highest risk of severe weather would be farther north in areas more typical for tornado outbreaks.

So, on Monday NWS forecasters were calling for only a “marginal” risk of severe weather in our area, which is their lowest risk category.

However, by 7:00 a.m. Tuesday NWS forecasters saw that dangerously unstable conditions were developing in Louisiana.

They heightened the threat level over southern Louisiana, Mississippi, and Alabama to the “elevated” category and issued a tornado watch. Thirty-five minutes later the NWS in Slidell issued the first tornado warning of the day.

This week we have a slight chance of severe weather late this Tuesday afternoon and night, although the weather pattern causing it is quite different than last Tuesday.

A surface low moving out of the Gulf into central Louisiana will destabilize the atmosphere over our area until a cool front sweeps through early Wednesday.