Extended U.S. Tornado Outbreak During Late May 2019: A Forecast of Opportunity
Vittorio A. Gensini David Gold John T. Allen Bradford S. Barrett
First published: 27 August 2019
https://doi.org/10.1029/2019GL084470 https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL084470 - open access
some excerpts and figures .. more within the paper
Abstract
The second half of May 2019 was an unusually active period for tornadic thunderstorms across the U.S. Great Plains, Midwest, and lower Great Lakes. While this period typically coincides with the peak climatological frequency of tornadoes, preliminary reports of tornadoes were over triple the expected 30‐year average. Multiple‐day outbreaks of tornadoes are not unprecedented in the United States; however, this event was perhaps the first to be forecast at subseasonal lead times (3–4 weeks) by the Extended Range Tornado Activity Forecast team. This forecast of opportunity was driven, in part, by anomalous convective forcing in portions of the tropical Indian and Pacific Oceans, causing subsequent changes in Northern Hemisphere atmospheric angular momentum. This manuscript analyzes the evolution of hemispheric‐scale circulation features leading up to the event, examines teleconnection processes known to influence U.S. tornadoes, and provides insights into the forecast process at subseasonal lead times.
1 Introduction
The period 17–29 May 2019 was among the most active periods of severe weather the United States has seen in years. While 2019 data are still preliminary, at least 374 tornadoes occurred during this 13‐day stretch, more than tripling the 1986–2018 average for this period of 107. In total, 757 tornado warnings were issued by the National Oceanic and Atmospheric Administration (NOAA)'s National Weather Service, with seven fatalities reported (Figure 1). This late‐May period contributed significantly to the second highest monthly (E)F1+ tornado count (220) on record for May since reliable tornado counts began in the early 1950s, behind only May 2003.
The 757 tornado warnings (red polygons) issued by the National Oceanic and Atmospheric Administration's National Weather Service from 1200 UTC 17 May to 1200 UTC 30 May 2019. Seven fatalities were reported during this period (locations marked by red +).
3.1 Event Summary: Synoptic Pattern
One of the most active periods of severe storms in U.S. history began on 17 May 2019 as a shortwave trough approached the Great Plains from the Great Basin. From 17 May onward, repeated days of severe weather, including several tornado outbreaks (Verbout et al., 2006), occurred as upper‐tropospheric southwesterly flow remained persistent over the Great Plains (Figure 2a), with average 300‐hPa winds during the period greater than 40 m/s. At 500 hPa, the mean negative geopotential height anomaly for the period was greater than 125 m over a large area covering the western CONUS. A persistent warm sector over the plains was characterized by vertically deep (≥2 km above ground level), anomalously rich boundary layer moisture (Figure 2c) and warm temperatures, both of which contributed to moderate‐extreme levels of convective available potential energy. In addition, sea surface temperatures in the Gulf of Mexico for the 5‐week period leading up to this event were at or above normal, providing a source of boundary layer moisture (Molina & Allen, 2019). Another notable feature of this persistent period of tornado activity was multiple days with relatively weak capping inversions associated with below‐average elevated mixed layer temperatures. This promoted high spatial concentrations of severe storms that, given the favorable atmospheric parameters, were able to produce a substantial number of tornadoes.
For the period 17–29 May 2019, average (a) 300‐hPa wind (m/s) and 300‐hPa geopotential height (m), average (b) 500‐hPa geopotential height (m) and 500‐hPa geopotential height anomaly (m), and average (c) 925‐hPa specific humidity anomaly (g/kg) and 925‐hPa geopotential height (m) as computed from the National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis. Anomalies calculated from the 1980–2010 climatology.
A leading mode of subseasonal variability capable of giving rise to synoptic patterns favorable for enhanced tornadic activity is the MJO (Barrett & Gensini, 2013; Baggett et al., 2018; Thompson & Roundy, 2012; Tippett, 2018). As an MJO event evolves over a 40‐ to 60‐day cycle, tropical convection along the equator propagates eastward from the Indian Ocean toward the Pacific. Such propagation was clearly evident in outgoing longwave radiation anomalies (Figure 3) over the 4 weeks leading up to this event. Dynamically, latent heat release results in the formation of an anomalous anticyclone to the northwest of the convection, leading to an intensification of the upper‐tropospheric zonal winds to the anticyclone's north (Moore et al., 2010). As the MJO perturbation propagates eastward forcing convection, the net result is an extension of a strong upper‐tropospheric jet into the midlatitudes of the central Pacific. When a blocking anticyclone is in place over the eastern North Pacific ocean, this jet extension eventually leads to wave breaking over western North America. It is this wave breaking, and subsequent troughing over the U.S., that links the tropical MJO to U.S. tornado frequency (Barrett & Gensini, 2013; Baggett et al., 2018; Thompson & Roundy, 2012; Tippett, 2018). The predictability of the MJO convection as it moves east from the Indian Ocean across the Pacific Ocean (Lim et al., 2018) allows it to serve as a leading indicator of upcoming tornadic activity once the MJO convection moves into the eastern Pacific Ocean (Baggett et al., 2018).
Fig 3. Average 300‐hPa geopotential height (contours) and wind speed (color fill) for (a) 19–25 April, (b) 26 April to 2 May, (c) 3–9 May, (d) 10–16 May, (e) 17–23 May, and (f) 24–29 May 2019.
Multiple time and space scales contribute to subseasonal and low‐frequency variability, including El Niño and the MJO. The former was lingering from boreal autumn 2018, while the latter was becoming active from latter April into early May. While El Niño is known to be less favorable to tornado occurrence (Allen et al., 2015; Cook et al., 2017), its modulation of the large‐scale circulation is not necessarily unfavorable to tornadic potential. This paradox arises due to its role in the development of a subtropical jet stream over the central and eastern Pacific extending into the Americas. This influence, as illustrated by (Cook et al., 2017) for the later winter months, can be favorable to the development of tornado outbreaks, particularly over the southeastern United States. Indeed, this type of subtropical jet signature was evident in the 4 weeks leading up to the May 2019 event, and the subtropical jet merged with the North Pacific jet for the duration of the event (Figure 4), suggesting that ENSO contributed favorably to this anomalous period of tornado activity.
Average 300‐hPa geopotential height (contours) and wind speed (color fill) for (a) 19–25 April, (b) 26 April to 2 May, (c) 3–9 May, (d) 10–16 May, (e) 17–23 May, and (f) 24–29 May 2019.
4 Summary and Discussion
The question of why some periods record anomalously above‐climatology tornado frequency has troubled many in the U.S. forecasting community for the past few decades (Barrett & Gensini, 2013; Lee et al., 2012; Marzban & Schaefer, 2001; Thompson & Roundy, 2012; Tippett et al., 2015). The period 17–29 May 2019 stands as one of the most active in history, and was characterized by more than three times the climatological number of tornadoes for that time of year, occurring over 13 days and encompassing a wide region of Great Plains and Midwestern CONUS. Here, we have illustrated that a persistent upper‐level synoptic trough over the western CONUS, with a downstream ridge aloft over the eastern CONUS, were the main synoptic features of interest. Attribution of such synoptic‐scale features to larger‐scale, and therefore more predictable signals (Grazzini & Vitart, 2015), remains challenging owing to the complex manner in which processes interact to produce coherent, and therefore potentially predictable, subseasonal evolutions. In the present case, time scales associated with the propagation of the MJO from the equatorial Indian Ocean to the central Pacific Ocean (∼20–30 days, or roughly half a cycle) helped create an anomalous North Pacific jet stream extension and retraction sequence that aligned favorably with a transition in AAM from a relatively high to a low state. Previous research indicates that such MJO and AAM/GWO events can lead to favorable atmospheric conditions for tornadic storms in the U.S. Here, with careful monitoring of such features as they emerged both diagnostically and in NWP‐derived RMM phase space, forecasters were able to use signals within both the MJO and AAM/GWO to anticipate the potential for an extended period of favorable severe weather conditions nearly four weeks in advance. While the forecast metric of above‐normal, normal, or below‐normal (tercile) levels of tornado activity over a subjective spatial region is among the more simple methods available (Klemm & McPherson, 2017; Hartmann et al., 2002), this is a unique example of how understanding tropical convection's role in modulating extratropical dynamic processes can be used to identify a forecast of opportunity for an extreme weather event. The event also offers a pathway for developing operational predictions of U.S. tornado activity across a portion of the subseasonal timescale. Finally, this manuscript represents a single case of a successful subseasonal tornado forecast. More cases, including potential null events, should be examined in future work.
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