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‘Significant’ tornadoes, storm outbreak possible in the South on Tuesday

Dangerous storms are forecast to erupt in Mississippi, Louisiana, Arkansas and Tennessee

Severe thunderstorm risk on Tuesday. (National Weather Service)

The lower Mississippi Valley is bracing for what the National Weather Service Storm Prediction Center warns could be a “regional tornado outbreak” on Tuesday, 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.” That’s due to periodic clashes between summer’s lingering warmth, fueled by Gulf of Mexico moisture, and surges of chilly Canadian air portending winter. The mid-South often becomes an atmospheric battleground as the jet stream strengthens each fall, energizing storms and encouraging some to rotate.

Areas affected

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.

Hazards

The ingredients will be present to support the development of rotating thunderstorms or supercells that can spawn tornadoes. The Storm Prediction Center says “significant and/or long-track tornadoes” are possible; these are 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. That’s not a certainty, but if supercells evolve, the risk of significant tornadoes will grow.

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. A few instances of hail are possible, too, but will not be the primary threat.

Timing

Moisture streaming northward will keep low clouds in place much of the day, with periodic showers. The sudden blossoming of thunderstorms or convective initiation won’t occur until a “cap” of warm air aloft is broken. That lid of hot, dry air, originating from the Desert Southwest, prevents humid, unstable surface air from rising — until it suddenly erupts upward and punctures the cap.

Weather models often struggle with simulating when convective initiation will occur. In this case, 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.

Preparation

On severe weather days, advanced preparation is key. Ensure that, wherever you go, you are no more than five minutes away from a sturdy tornado shelter — ideally something below ground and without windows. This includes if you are driving. Have multiple ways to be notified if a warning is issued for your location.

Tornado watches will be issued during the late morning or around lunchtime for broad areas. A watch means conditions are favorable for the development of severe thunderstorms and tornadoes. More targeted warnings will be issued for thunderstorm cells that, on radar, are exhibiting rotation or if a tornado is confirmed by a storm spotter or the presence of debris on radar.

If a tornado warning is issued for your location, that’s an immediate call to take shelter.

Discussion

The trigger for the storms is a pocket of cold air, low pressure and spin at high altitudes diving southeastward over the western United States on Monday. As it progresses toward the central states on Tuesday, it will destabilize the lower atmosphere, or encourage surface air to rise.

The parent upper-air disturbance is nestled within a dip in the jet stream known as a trough. That jet stream momentum aloft is contributing to a change of wind speed and/or direction with height known as wind shear. Any clouds that span multiple layers of the atmosphere in a sheared environment acquire rotation. That will enhance the tornado threat.

Most of the storms will be forming ahead of a cold front, although there are signs that a few supercells could form sneakily along a warm front in southern Alabama and the Florida Panhandle. Those could produce offshore waterspouts or brief tornadoes, too.

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