Authored by Robert Thompson, retired Meteorologist-in-Charge of the Boston/Norton National Weather Service (NWS) Forecast Office. He is now an adjunct professor at Anna Maria College and a Board Member of Blue Hill Observatory & Science Center.
Two tornado outbreaks in December 2021 have raised several questions even for us in New England. The first and deadliest outbreak in decades featured intense and long tracked tornadoes across the Mid-South and a portion of the Mid-West during the night of December 10-11. The second outbreak in the upper Midwest on December 15 was especially notable for its northern extent so far into the cool season with 19 tornadoes reported and a huge number of straight-line wind damage reports. These two outbreaks raise several questions. Just how extreme were these? What preparedness challenges existed? Was there a global warming connection? What takeaways might exist for New Englanders? As of this writing, damage surveys were still being conducted, and raw preliminary information was being processed.
Because data are still being collected and verified, the very preliminary information available limits a comprehensive picture of the meteorology behind these events and how these outbreaks fit into the historical perspective. The December 10-11, 2021 tornado outbreak (see Figures 1a and 1b) will likely go down as one of the deadliest on record and certainly the deadliest in the month of December to date. Up to 59 tornadoes have been reported across ten states in this one outbreak. One supercell thunderstorm traveled 250 miles and produced multiple strong and long tracked tornadoes. The death toll across the Mid-South and an adjacent portion of the Midwest stands at 88 at the time of this writing. Although the NWS forecasts, watches, warnings, and follow-up statements were accurate, timely, and clearly written, the deadly nature of this outbreak may stem from several factors related to warning reception and response as well as risk assessment. The Mid-South experiences tornadoes every year but only infrequently during the month of December. These tornadoes struck at night when visual detection by those at risk is very low and when people’s communication avenues may have been limited. The tornadoes wreaked havoc over a region more densely populated than in the classic tornado alley in the southern plains. A number of the reported tornadoes achieved at least a level 3 or 4 on the 0 to 5 Enhanced Fujita (EF) scale. There will likely be some refinement of intensities and track dimensions over the coming weeks.
Figure 1a. Preliminary tornado, wind, and hail reports for December 10, 2021. (Courtesy of National Weather Service Storm Prediction Center)
The second outbreak of December 15, 2021 (see Figure 2) occurred in a region accustomed to tornado risk but not so much in December. In fact, Minnesota recorded its first December tornado on record. Once again, the occurrence of “out of season” tornadoes may have caught some people unaware.
Figure 2. Preliminary tornado, straight line wind, and hail reports from the December 15, 2021 severe weather outbreak. (Courtesy of National Weather Service Storm Prediction Center)
There’s been considerable conjecture as to whether or not these tornado outbreaks are a consequence of climate change. Global warming may be a factor, but the linkage is not necessarily conclusive. Tornadoes can and have occurred throughout the United States and at all times of the year. One of the deadlier tornadoes in history occurred on June 9, 1953 in central Massachusetts. The “Worcester Tornado” tracked 86 miles and killed 96 people. Although the peak frequency for tornadoes occurs during spring and early summer, there is a secondary peak in the fall for some locations, and history contains examples of tornadoes occurring well outside peak season. One such out of season tornado laid a wake of devastation in the Windsor Locks, CT area in October 1978. The incidence of La Nina (anomalously cold surface water in the eastern equatorial Pacific) may have in at least an indirect way set the stage for these outbreaks as a consequence of influences on upper air wind patterns. It is possible as well that a warmer climate has set the stage for an environment more conducive to thunderstorms, including those storms with the capacity to spawn tornadoes. Tornadoes, especially violent tornadoes, are the product of strong wind shear acting in concert with a warm, moist, unstable air mass. The air mass was anomalously warm in the Mid-South December 10, and an accompanying extreme temperature gradient would have acted as a source of potential energy. Intuitively, one might expect such an environment to exist more frequently in a global warming scenario, but the science on this is still evolving and not considered conclusive. There also appears to be some evidence of a shift in the region of maximum tornado frequency farther east from the southern plains toward the more densely populated Mid-South. There remains uncertainty as to whether this is something of a pseudo-permanent change or just a long period variation. One of the challenges in tornado climatology studies stems from its incomplete data base. In past years tornado incidence was highly influenced by population density, and very limited data exist for tornado occurrences prior to 1950. A clearly articulated and rigorous linkage to global climate change has yet to be established. Thus, these at least somewhat anomalous tornado outbreaks may have a global warming link, but further research is needed.
A warm, moist air mass can contribute to an unstable (i.e., buoyant) air mass and set the stage for super cell (and sometimes tornadic) thunderstorms when strong wind shear is also present. The dewpoint is often a good measure of potential instability, since it is correlated with both temperature and moisture. Analyzing dewpoint climatology may help better uncover any influence of global warming on the frequency of severe thunderstorms and/or tornadoes. Greater instances of relatively high dewpoint air masses during the late fall, winter, and early spring months when upper level winds usually are stronger logically might set the stage for more frequent severe convection, but this conjecture requires more rigorous study before reaching any conclusions. For New England, we seem to have had a recent upward trend of tornado (albeit not violent or long tracked) incidences along our south coast in recent years. Warmer sea surface temperatures may be contributing to higher dewpoints and more instances of unstable environments coincident with problematic wind shear episodes. Once again, however, this is mere conjecture until a stronger linkage can be established through rigorous climate science research.
What about New England? Although this corner of the country is far from the traditional tornado alley of the southern plains, several tornadoes occur in Massachusetts on average every year. Intense tornadoes are rare but not unheard of. In June 2011, a Massachusetts tornado ripped a 28-mile path of destruction from Westfield to Southbridge up to a half mile wide with stretches of EF3 damage, and the Worcester Tornado in 1953 remains among the deadliest for the country. For years following the June 2011 event, the damage swath could be detected from satellite imagery, especially when the ground was snow covered. In 2017, a rare February tornado tore up a portion of Conway, MA. The moral of all this for New Englanders is that we should not let our guard down on tornado safety, because these storms can indeed occur here. Tornado and severe thunderstorm forecasts and warnings have improved over the past few decades, and evolving research promises additional progress on accuracy and timeliness. Although more climate research is needed, there is some logic to suggest that the presence of a warming global environment could increase the number of intense tornadoes, if not the frequency of tornadoes of any intensity at atypical times of the year and locations. Like preparedness for other infrequent hazardous weather, a small investment in time and attention could prove to be life-saving.
Finally, any increased risk of tornado and other hazardous weather impacts due to climate change requires investment in dedicated research and the availability of long duration, high quality data. The Blue Hill Observatory and Science Center constitutes a critical New England connection to climate research through its carefully managed long term observation program. Data from the Blue Hill observation program has been and will undoubtedly continue to be an integral component to our nation’s climate change research efforts.