Last night, a rare, catastrophic tornado outbreak occurred over portions of the Southern and Midwestern US. As of Saturday afternoon, 34 people were confirmed dead, with an estimated 75-100 deaths across Missouri, Illinois, Tennessee, and Kentucky and potentially 50-70 deaths in Mayfield, Kentucky alone. Most deaths were due to a series of tornadoes – or possibly a single tornado, from a single, long-track supercell thunderstorm that began in Arkansas and tracked approximately 250 miles through Missouri, Tennessee, and eventually, Kentucky, where it made a direct hit on the town of Mayfield. This tornado has been dubbed by some outlets as the “Quad State Tornado,” alluding to the “Tri-State Tornado” of March 18, 1925 that tracked 219 miles across Missouri, Illinois, and Indiana in a similar fashion.
In addition to having an exceptionally long-lived and powerful tornado, this outbreak was also notable for the time of year it occurred. The vast majority of US tornadoes occur during the spring, when the upper atmosphere is relatively cool, the lower atmosphere is warm and moist, and there is the instability and wind shear needed to create massive, rotating supercell thunderstorms. However, the southern US also sees a secondary tornado peak in the winter, when systems tracking across the continent interact with warm, moist, southerly winds originating from the Gulf of Mexico and create the necessary lift, instability, and shear to create supercell thunderstorms. Compare the tornado frequency by month over southern and central Plains, often called “Tornado Alley,” to the mid-south, which is sometimes called “Dixie Alley,” and note how Dixie Alley has a sizeable, secondary peak in the late autumn.
Compared to the Great Plains, the tornadoes over Dixie Alley tend to be faster-moving, longer-lived, and can be more violent due to the potential for extreme amounts of low-level wind shear, which is key for forming and sustaining tornadoes. Additionally, the REALLY big tornado outbreaks, like April 3-4, 1974 and April 25-28, 2011, tend to favor Dixie Alley instead of Tornado Alley.
Meteorological Synopsis:
The typical Dixie Alley tornado outbreak has two primary components: a strong, fast-moving trough that moves across the Western US, consolidates/strengthens over the Great Plains, and further strengthens & develops a surface low as it heads into the South & Midwest, and a warm, moist, unstable airmass over the Gulf of Mexico that can be entrained into the intensifying cyclone’s warm sector, ahead of the cold front. When this happens, you get three key things: (1) a favorable wind shear profile for strong, long-lived tornadoes due to the change in wind speed/direction between the southerly, moist flow at the surface and strong, westerly flow aloft, (2), a moist, unstable air mass with a lot of potential energy (CAPE) that can cause explosive thunderstorm development when released, and (3), a cold front that provides the dynamic forcing needed to lift air parcels and allow them to tap into this CAPE & convect, forming tall thunderstorms. The below two charts show the 500mb heights/vorticity and sea-level-pressure/CAPE at 10AM PDT Thursday 12/9 – note how there are a series of shortwave troughs embedded within a larger longwave trough over the Western US and ample amounts of CAPE over the Gulf of Mexico. I’ve also drawn the approximate trajectory of each of the shortwave troughs and how they would combine into one much stronger trough over the Great Plains.
By 10pm PDT Friday 12/10, the troughs had merged into one intense shortwave trough over the Central Plains, a surface low was undergoing rapid cyclogenesis ahead of the trough, and the moist, unstable air over the Gulf of Mexico was being pulled northward due to the deepening low pressure. All of the ingredients had come together for a major tornado outbreak.
When all of the ingredients came into place, convection exploded along and ahead of the cold front in a highly sheared environment, resulting in long-lived, violently-rotating supercell thunderstorms.
Satellite imagery showing evolution of last night’s #tornado outbreak. Violently rotating storms were triggered when a cold front interacted with a moist, unstable airmass originating from the Gulf of Mexico in a highly sheared environment, forming numerous long-lived tornadoes. pic.twitter.com/SniDFrL76y
— Charlie Phillips (@GeoduckChuck) December 12, 2021
The Quad-State Tornado
The longest-lived and most intense of these storms featured a near-continuous series of tornadoes, or possibly a single tornado, that traveled all the way from NE Arkansas through NE Kentucky, briefly passing extreme SE Missouri and extreme NW Tennessee in the process.
Credit: Wikimedia User TheAustinMan (check out his hurricane work as well!)
The tornado damaged numerous towns and had a particularly devastating impact over Mayfield, Kentucky, which is estimated to have seen as many as 50-70 deaths from the tornado, many of them employees of Mayfield Consumer Products, a candle-making factory that was destroyed by the tornado. The preliminary damage assessment for the tornado is at an EF3, and I wouldn’t be surprised if it was upgraded to an EF4 based on some of the damage in the Mayfield area. Radar imagery showed debris from the tornado extending an incredible 37,000 feet into the atmosphere – some of the highest heights ever recorded for a debris signature.
A terrifying, historic supercell. It’s not surprising, but all of the radar-based tornado intensity indicators, including a TDS up to 37,000 feet(!!!) and an accompanying debris plume suggest that a violent (EF4+) tornado impacted Mayfield, Kentucky this evening. pic.twitter.com/GNmH2J1iip
— Sam Emmerson (@ou_sams) December 11, 2021
Needless to say, it’s just heartbreaking to see some of the images coming out of the towns that were hit by these tornadoes. If you’d like to donate to tornado relief, there are many options… the American Red Cross is my go-to though.
Pacific NW Weather
A strong storm came through the Pacific NW today, bringing wind, rain, and mountain snow both west and east of the Cascades. Wind-prone Rattlesnake Mountain near Hanford, WA saw a gust of 102 mph, but many lower-elevation spots east of the Cascades saw very high winds as well, with the Tri-Cities gusting to 53 mph, Walla Walla hitting 61 mph, and a site on the Hanford Nuclear Reservation hitting 78 mph! Winds were a bit less west of the Cascades but many spots still hit 30-40 mph, with Paine Field at Everett hitting 53. Portland Airport hit 40, and one of my friends down there reported that the weather was “Forrest Gump sideways rain kinda stuff.” Not the kinda stuff you want to get caught outside in!
But while we’ve been having all this cool, rainy, blustery weather in the lowlands, it has been absolutely dumping in the mountains. Crystal Mountain opened on 12/10, Snoqualmie will open on 12/14, and Mt. Baker will open on 12/15. Snoqualmie Pass has seen over 18″ of snow since Thursday, and Baker has seen over 2 feet since then.
The extended forecast looks cool and damp with snow levels near 1000-2000 feet, perhaps dipping as low as 500 feet in the overnight hours. We should be in a cool/wet pattern through Wednesday and may dry out a little Thursday and Friday before more rain returns for next weekend.
I don’t see any major lowland snow threats at this time, but models are in pretty good agreement on continued cool and damp conditions for the next two weeks, and weather.com (which has very good forecasts) is currently forecasting snowflakes on Christmas! I’ll keep an eye on it and keep you posted!
