Regional tornado outbreak predicted today

The lower Mississippi Valley is bracing for what the National Weather Service Storm Prediction Center warns could be a “regional tornado outbreak” today, with widespread severe weather and the potential for a few strong tornadoes. It will be the second episode of severe thunderstorms to ravage parts of the South this November, bookending a month that began with deadly storms in Texas and Oklahoma on Nov. 4.

Hardest hit this time will be regions between northern Louisiana and southwest Tennessee, though anyone residing in Arkansas, Louisiana, Tennessee or Mississippi should remain on high alert. The storms could continue into the Tennessee Valley on Wednesday, although the risk of severe weather will lessen.

While severe weather is most common in the springtime, autumn often proves a muted “second season” due to periodic clashes between summer’s lingering warmth, fueled by Gulf of Mexico moisture, and surges of chilly Canadian air portending winter. The midSouth often becomes an atmospheric battleground as the jet stream strengthens each fall, energizing storms and encouraging some to rotate.

The Storm Prediction Center took the highly unusual step of issuing a level 4 out of 5 risk of severe weather, which is quite atypical for November. For that category to be drawn a full day in advance is even rarer. In fact, the agency has included moderate risks in its day-before outlooks only seven times in the past 20 years. They’re reserved for particularly dangerous severe weather events.

The level 4 out of 5 risk zone includes most of northwest Mississippi, extreme southeast Arkansas and the northeast corner of Louisiana. The zone includes communities mainly along and west of Interstate 55, including Greenville and Greenwood, Miss., and it extends as far east as Tupelo. Memphis is also clipped by the risk area, including some of its southern and western suburbs. Approximately 1.85 million people are encompassed within that zone.

A level 3 risk of severe weather surrounds the level 4 zone, casting a wider net and covering the rest of the Memphis metro area as well as Jackson, Tenn.; Jackson, Miss.; and Monroe, La.

A broader level 2 risk blankets areas all the way from the Gulf Coast into southwest Kentucky, including Nashville, Little Rock, Birmingham, Ala., and Shreveport, Baton Rouge and Lafayette, La.

Elsewhere, cities like Houston and Louisville are under a level 1 risk of severe weather.

The ingredients will be present for development of rotating thunderstorms or supercells that can spawn tornadoes. The Storm Prediction Center says “significant and/or long-track tornadoes” are possible, capable of reaching at least EF2 strength on the 0-to-5 Enhanced Fujita scale for tornado intensity. Whether intense tornadoes develop will depend on whether supercells remain discrete and unaffected by nearby storms.

During the late evening and overnight, the storms may merge into clusters and line segments, which would increase the risk of damaging straight-line winds while the tornado threat lessens some. Hail is possible too, but will not be the primary threat.

Moisture streaming northward will keep low clouds in place much of the day, with periodic showers.

It’s likely that storms will materialize by 3 or 4 p.m. local time, then survive until 9 or 10 p.m. before the supercells start to weaken some.