Springtime Uptick in Severe Weather and Tornadoes

Springtime Uptick in Severe Weather and TornadoesBy Kailey Joyce, Lauren McCarthy | June 5, 2019

Severe weather has affected the Central and Southern Great Plains over the past few weeks, asevidenced by images of destructive tornadoes ooding social media and the internet.Climatologically, April-June is the most active time for severe weather in the United States.According to data from the Storm Prediction Center (SPC), tornado, hail, and strong wind reportstypically spike during these months.Prior to May 2019, this year’s tornado count was similar to the 2005–2015 average annual trend(Figure 1). However, the frequency of tornadoes in May has now pushed the 2019 year-to-datetornado count above average by over 200 tornadoes.

Compared to the past 5 years, the 2019 season again displays an above-average trend. The totalnumber of tornado reports from January-May is higher this year than for the same months in 2014-2018 (Figure 2)

The Tornado Alley region, which includes the Great Plains, the Midwest, and the SoutheasternUnited States, is famous for its high frequency of tornadoes.

Figure 3: Top 5 States, Storm Reports taken from SPC Annual Severe Weather Report Summaries(*2019 data taken through May 31, 2019)

But what is it about this time of year that makes severe weather outbreaks more common?It takes several atmospheric ingredients coming together to get the kind of severe weather we seein late spring and early summer. A major feature that drives severe weather is the jet stream, aregion of elevated winds about 5-7 miles above the Earth’s surface. Moving a lot like a wave, the jetstream dips toward the southern United States in the winter months then returns north towardCanada in the summer. As the jet stream begins to retreat in the spring, it pushes areas of upper-level energy back toward the southern United States. These features promote a rising motion in theair, a key factor in storm development. Additionally, the jet stream produces strong winds thatchange in speed and direction vertically with height — a factor called wind shear.Wind shear is another severe-weather ingredient because it promotes rotation in the atmosphere.Figure 4 (below) shows an upper-air analysis from May 21, 2018. A pronounced jet stream “trough”can be seen over the western United States with embedded jet streaks, or areas of high windspeeds within the jet stream. This analysis depicts an atmospheric setup that was a major driver ofthe severe thunderstorms, tornadoes, hail, and heavy rains observed over the Great Plains andMidwest on this day

Figure 4: Upper-air analysis from May 21, 2018. Image courtesy of SPC(https://www.spc.noaa.gov/obswx/maps/)

Rising temperatures and dew points of the springtime months provide the warm, moist air neededto produce severe weather. The warm maritime airmass in the southern US clashes with drier,colder air from the western states. These conditions create an “unstable” environment wheresevere thunderstorms and tornadoes can develop more easily. As we head into June, the locationand magnitude of these severe weather ingredients will continue to be major factors indetermining the frequency and strength of severe weather outbreaks