Tornadoes Rake Mississippi Delta; More Storminess Ahead

One of the longest-tracked tornadoes ever observed in December carved its way from northwest Mississippi into southwest Tennessee on Wednesday. The twister, likely to be rated at least an EF3 after damage surveys on Thursday, was part of an unusually far-flung year-end outbreak of severe storms that extended from the Gulf Coast to the Great Lakes. Although several tornadoes were reported as far afield western Illinois and central Indiana--and even Michigan experienced its first December tornado on record--the bulk of the 29 preliminary tornado reports received by NOAA’s Storm Prediction Center (SPC) came from northern Mississippi and adjacent areas. At least 10 deaths were reported by midday Friday, most of them tornado-related.

By far the most destructive storm of the day was the long-lived supercell that produced the long-track tornado noted above, as well as several others. Greg Carbin, warning coordination meteorologist at SPC, speculates that this storm was in a small region where instability from warm, moist surface air and very high vertical wind shear came together for nearly ideal supercell conditions. “Elsewhere, there were numerous supercell structures and fast-moving line segments producing damage,” says Carbin. “However, based on my interpretation of the character of radar reflectivity during the event, many of the storm updrafts were ‘stretched out’ by the intense vertical shear across the region.”


Figure 1. Tornado east of the Shack Up Inn outside Clarksdale, Mississippi, on December 23, 2015. This is probably the same tornado that stayed on the ground for 150 miles and later killed one person in Holly Springs, Mississippi. Image credit: Guy Malvezzi.


Figure 2. Vehicles blown off the road in Holly Springs, Mississippi on December 23, 2015. A seven-year-old boy was killed in the town by the tornado. This photo also illustrates the danger of being parked beneath an overpass when a potentially tornadic thunderstorm is approaching. Image credit: Dan Smith and Karla Fisher.




Figure 3. Radar reflectivity (top) and Doppler velocity (bottom) of the supercell thunderstorm that spawned the Holly Springs, Mississippi tornado as seen at 5:28 pm EST December 23, 2015.


How far can a tornado go?
Continuous tornado paths longer than 100 miles are uncommon, and they can be a challenge to confirm, especially in rural areas. In many cases, a long-lived supercell will produce several tornadoes in quick succession (also known as a tornado “family”), with brief breaks in between each damage path. The infamous Tri-State Tornado of March 18, 1925, was long envisioned as a single twister producing a 219-mile-long path, but more likely it was a tornado family with one dominant member. A thorough analysis led by former SPC forecaster Robert Johns found 32 gaps along the Tri-State path, each at least 1 mile long. The researchers concluded that a 151-mile segment from Bollinger County, Missouri, to Pike County, Indiana, was most likely to be the longest continuous path from this tornado family.

The longest confirmed tornado track during winter (December-February) is the 122-mile-long path from a tornado that plowed through northern Arkansas during the Super Tuesday outbreak on February 5, 2008. Looking throughout the year, there have been at least 26 tornadoes of at least F3/EF3 strength since 1950 with path lengths of more than 120 miles, according to Harold Brooks (National Severe Storms Laboratory).


Figure 4. Preliminary damage track of the violent tornado that cut a long swath across northwest Mississippi into southwest Tennessee on Wednesday, Dec. 23, 2015. An updated track will be produced after storm surveyors assess the damage. Image credit: NWS/Memphis.

2015 no longer the nation’s safest tornado year
As a result of Thursday’s activity, the year 2015 to date now has close to 20 tornado-related deaths. This puts 2015 out of the running for the least number of U.S. tornado fatalities for a given year in records going back to 1875. The year with the lowest death toll remains 1910, which saw just 12 tornado-related fatalities. The year 1986 is in second place, with 15, and in 2009 there were just 21 tornado-related deaths.


Figure 5. Thunderstorms (mostly non-severe) are lined up along the jet stream from southern Louisiana to the mid-Atlantic at 1630Z (11:30 am EST) on Thursday, December 24, 2015. Image credit: NOAA-NASA GOES Project.

More storms in the offing through the holiday weekend
Thunderstorms will pose less of a threat on Thursday and Friday than on Wednesday, although in some areas they’ll accentuate the presence of a record-warm, record-moist air mass. SPC has only a marginal risk of severe weather for Thursday from the lower Mississippi Delta to the Delmarva region, with a smaller marginal risk from northeast Texas to southwest Tennessee. Another marginal-risk area on Friday extends from northeast Texas to most of Kentucky and Tennessee. More intense activity may develop over the weekend, with SPC’s Day 3 outlook for Saturday already including a slight risk for a large part of Texas, and parts of east Texas and Louisiana outlooked for possible severe weather on Sunday. A strong cold front will be plowing across the Southern Plains by then, which could shift the odds toward wind-packing squall lines as opposed to supercells.

Off-the-charts record warmth for Christmas
The well-advertised holiday warm wave continues to astound, with “instant” record highs set overnight in many locations from the Great Lakes to the Northeast. Readings at midday Friday were already into the 70s Fahrenheit from southeast New York to the Gulf Coast, with widespread 80s across Florida. Some of the daily record highs along and near the East Coast on Thursday will be 10°F or more beyond the warmest Christmas Eve in more than a century of recordkeeping. Breaking a longstanding daily record by more than 10°F is noteworthy in itself, and the intense zone of high pressure off the southeast U.S. coast is uncannily similar to the Bermuda highs common in midsummer! Given the intense interest in holiday weather and the many family gatherings under way, we can expect this bizarre weekend to spur countless dinner-table conversations about climate change and “global weirding.” A warm wave like this doesn’t “prove” climate change; it is one manifestation of the weather that results from natural variations such as El Niño playing out in a global atmosphere that is being warmed, moistened, and shifted by ever-increasing amounts of greenhouse gases. Like the spectacular warm wave of March 2012, which brought 90°F readings to Michigan, the tropical Christmas Eve 2015 could serve as an excellent candidate for attribution research--the attempt to unravel how much long-term climate change raises the odds of a particular weather event.

Have a great holiday weekend, everyone!

Bob Henson and Jeff Masters