Spring tornado season is coming to a close in the Tennessee Valley, but that doesn’t mean that severe storms go away. They just change character in the summer.

Through May 21st, the number of preliminary tornado reports in the U.S. had risen to 859, putting it #2 for most active start to the year to date, falling only behind 2011. Since then, that number has risen further to 1,008. States in the classic “Tornado Alley” of the Great Plains, as well as the Ohio Valley, have been most active so far this year. We eventually had our share of tornadoes here in the Tennessee Valley though, even though we haven’t been nearly as active as the Plains. On May 8th, a significant tornado outbreak produced at least 24 tornadoes in middle Tennessee and north Alabama, with at least three of them being of EF-3 in intensity. One person was killed, several other were injured, and there was severe damage to multiple communities.

“Tornado season” will continue on in the Plains for another few weeks and through the summer for the northern portion of the United States as the jet stream makes its annual migration northward as we head into the summer months. That means that, while the weather will probably be active for a while longer to our north and west, the spring “tornado season” is coming to a close here in the Tennessee Valley. Make no mistake though! That doesn’t mean that severe storms are going to go away and we won’t see them again until the fall. There can even still occasionally be tornadoes here in the summer, especially when a tropical storm or hurricane makes landfall and its remnants move near our area. However, the tornado threat does back down significantly and other thunderstorm hazards take more prominence during the summer months. Instead of big sweeping cold fronts and sharp troughs of low pressure that create high wind shear environments favorable for supercell thunderstorms and long-track tornadoes, severe weather during the summer months in our area comes from mainly one of three features: thunderstorm complexes originating from the Midwest with widespread straight-line winds and flooding, pop-up afternoon and evening storms that briefly grow strong enough to produce wind damage or hail, and quick developing spin-up tornadoes found in the outer rainbands of the remnants of tropical storms and hurricanes that have made landfall and track near our area.

The vast majority of severe storm damage during the summer in the Tennessee Valley comes from straight-line winds, either with thunderstorm complexes or from localized downbursts with pop-up storms in the heat of the afternoon. A downburst happens when cooler air from gathering rain and hail within a thunderstorm overpowers the updraft and that rain-cooled air becomes increasingly dense. That rain-cooled air then surges downward from the base of the thunderstorm before it hits the ground and then spreads outward. If it’s strong enough, it will cause straight-line wind damage both where it initially hits the ground and then where it spreads outward until it loses enough strength to no longer cause damage. This same general process is one of the main ones ongoing within those thunderstorm complexes too, but there are other processes at work that keep the thunderstorm line/cluster ongoing instead of just dying out upon the first downward rush of rain-cooled air. When that downburst of wind is focused over an area 2.5 miles wide or more narrow, it is called a microburst. If it stretches out greater than 2.5 miles, it is called a macroburst.

When these happen from pop-up air mass thunderstorms, there can sometimes be very little advanced warning before the damaging wind threatens. Because Doppler Radar only senses movement toward or away from the radar, and not wind movement upward and downward, it cannot detect the downward rushing wind until AFTER it has hit ground and started to spread outward horizontally along the ground. By then, a damaging wind signature may show up so that additional communities can receive a severe thunderstorm warning, but the initial location where the damaging wind begins often has very little warning. There are sometimes clues to watch for, such as signs that a thunderstorm updraft is beginning to collapse or horizontal winds in the mid-levels of the storm are beginning to converge together. Those are often clues that a downburst may be about to happen. However, it doesn’t always happen when those clues appear, and damaging downburst winds can also happen without either of those clues appearing ahead of time at all. This is how you can sometimes get wind damage during the summer without there being a warning issued first! It’s not laziness of the meteorologists watching the radar. It’s a limitation of the current state of the science. This is why it is always important to remain vigilant to weather conditions when thunderstorms approach and be inside away from windows, regardless of whether or not a severe thunderstorm warning has been issued!

Hail can also sometimes be a threat during the summer, especially with the localized pop-up thunderstorms, but it usually isn’t as potent of a threat as it sometimes can be during the spring months because the deep-layer shear to organize longer-lived thunderstorm updrafts is weaker during the summer months. Still, hail large enough to cause damage can sometimes happen during the summer with the strongest of storms. Hail is formed as raindrops are suspended upward into the thunderstorm by intense updraft winds. These raindrops encounter other supercooled water droplets and grow larger on their way upward, but as they get vaulted upward into the storm, they get into cooler temperatures that cause the raindrops to eventually freeze. They become heavier and begin to fall, but the intense updraft winds vault them back upward into the storm again. As they move downward and then upward, the hailstones continue to collect supercooled waterdrops that then freeze into layers as they are lifted back upward into the colder temperatures. This process repeats itself several times until the hailstones start to outweigh the intensity of the updraft. At this point, the hailstones fall to the ground.

Lightning is ALWAYS a major hazard with EVERY thunderstorm, regardless of whether it is “severe” or not. Keep in mind that EVERY thunderstorm, by definition, has lightning, and lightning does not make a storm “severe”. Only 58+ mph wind gusts, quarter size or larger hail, or a tornado make a storm severe. Lightning is a major hazard though, regardless of whether a storm has a warning issued for it and regardless of the time of year. However, summer months are a particularly dangerous time for lightning because of the frequency of thunderstorms combined with the frequency in which people are outdoors to encounter them.

Lightning is caused by the build up of static electricity inside a thunderstorm cloud from the bumping and tossing around of raindrops and hailstones. Those positive and negative charges then separate between the top of the cloud, the bottom of the cloud, as well as the natural electric charges found with objects at ground level. The static discharge of electricity between those positive and negative areas is the lightning strike.

It does NOT have to be raining where you are in order for you to be at danger from lightning! Positive lightning strikes from the anvil of a thunderstorm can happen several miles away from the body of the storm where the rain and “rough weather” is happening. If you are close enough to the storm to hear thunder, you are close enough to be struck by lightning! Head indoors immediately away from windows, or get into a hard-topped vehicle with all windows shut and without touching metal objects in the car if stuck outside, and stay there until 30 minutes after the last rumble of thunder. Particularly dangerous places to be during a thunderstorm are near tall objects outside like trees, power lines, streetlights, near sports stadiums, and in wide open spaces like golf courses, fields, etc., where YOU are often the tallest object! It is also very dangerous to be out near bodies of water during thunderstorms because of the excellent conductive properties of electricity that water has. Since lightning strikes happen with little to no warning at all, the best protection is to be weather aware on days when storms are possible, and head indoors immediately for shelter when storms are approaching.

One of the biggest culprits of all of these together would be the lines or clusters of thunderstorm complexes that we often see moving into the area during our northwest flow patterns in the late spring and first half of summer. They can happen at any time of day or night, and they can happen at any time of the year, although they often form under different circumstances in the summer than they do the fall through spring. These QLCS (Quasi-Linear Convective System) and MCS (Mesoscale Convective System) events are responsible for most of our “big” severe weather days during the summer, especially the ones in which we see widespread straight-line wind damage. They are often responsible for our bigger flooding events that happen during the summer outside of tropical systems. These tend to be prolific lightning producers because of the amount of instability during the summer months. These complexes can also sometimes produce spin-up tornadoes embedded within the lines of storms, although they don’t do that quite nearly as often during the summer months in our area of the country as they do to the north in the summer or here in the fall through spring. What’s known as a shelf cloud will often accompany the leading edge of these storm complexes. This is formed by the condensation as warm, moist air rising on the leading edge of a line of storms interacts with rain-cooled air along the gust front. This forms a wide wall of low-hanging clouds that can often stretch from one side of the horizon to the next. Many people get these confused with wall clouds, since a shelf cloud looks like a wall of cloud, but these are formed on the leading edge of storm complexes and are usually a signal of a straight-line wind threat. Wall clouds, are an isolated lowering on the back flank of a supercell thunderstorm in its updraft region and can often be a precursor to a tornado forming.

When these lines and complexes of severe thunderstorms track for many miles and produce widespread intense wind damage along their path, they can sometimes be classified as a “derecho”. A derecho is a long-lived windstorm associated with a line or complex of thunderstorms. While there is some debate on specific criteria, a derecho generally has a path of wind damage that extends for 250 miles or more, has widespread wind gusts along that path of 58 mph or more, and has multiple well-separated gusts of 75 mph or greater along the path. Derecho events are most common in the Plains and Midwest, but our area here sees them as well. In terms of a long-term climatological average, we usually average seeing a derecho event in the Tennessee Valley once every year or two. Some summer seasons we have seen a couple of them in a single year, while others we have gone a few years without seeing a wind damage event that means that specific criteria. While environments supportive of derecho events can be forecasted in advance and you may see that word ahead of time in forecasts, a wind damage event is never officially classified as a derecho until after the damage has happened.

As mentioned above, those summer thunderstorm complexes can occasionally produce spin-up tornadoes during the summer months, but it’s not often. Generally speaking, we don’t see many tornadoes in our area during the summer anyway, but IF they are going to happen, it most often happens in association with landfalling and decaying tropical systems in which the remnants affect our area. This is because of the strong vertical wind shear that is present in the right half of a tropical system relative to the direction of its forward motion, and how this strong vertical wind shear often interacts with instability after dry air in the mid-levels of the atmosphere wraps into the circulation of the tropical system after landfall. While not all landfalling tropical systems produce tornadoes, it is exceedingly rare for a landfalling tropical storm or hurricane to not produce at least one tornado. While tornadoes can happen in the outer rainbands prior to the system’s landfall, tropical cyclones are usually more active with tornadoes in the day or two after landfall as the surface winds weaken but the winds a few thousand feet aloft stay strong, increasing wind shear, and dry air aloft works into the circulation to increase instability. When looking at the large overall database of tropically spawned tornadoes, the majority of them are quick spin-up EF0-EF1 tornadoes. HOWEVER, it is not rare at all for tropical-type tornadoes to sometimes get into the EF2-EF3 “strong” range, making them capable of significant structural damage. In the United States, there have only ever been two documented (E)F4 intensity tornadoes from a tropical system (both happening in the 60s in Texas and Louisiana), and there have never been any recorded (E)F5 intensity tornadoes spawned from a tropical system.

Flash flooding is a year-round threat that can be found in any of these storm types during the summer storm types. It takes only 6 inches of fast-moving water to sweep away a person, and only 12 to 18 inches of fast-moving water to sweep away most personal vehicles! If you come across a flooded roadway, please turn around and find an alternate path. You don’t know how deep or fast-moving the water is, and you don’t even know if the roadway is still there under the flood waters! TURN AROUND, DON’T DROWN! It is especially important to be mindful of the potential for flash flooding when slow-moving thunderstorm complexes or landfalling tropical systems are in the forecast, or when slow-moving pop-up afternoon thunderstorms try to sit around for a few hours.

As you can clearly see, just because spring “tornado season” is ending doesn’t mean you stop being weather aware!