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Jun 01, 2023

How thunderstorms develop, organize and intensify

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Seeing cauliflower cumulus clouds bubble skyward and hearing distant rumbles of thunder are as much a part of summer in Virginia (and many other parts of the U.S.) as suntans, swimming pools and vacations.

We can often take it almost for granted without thinking much about the mechanics of how a sunny, sticky day with no hint of clouds can turn into one with menacing dark clouds, electrical currents dancing through the air, booming rumbles, tree-bending winds and huge drops of rain.

Southwest and Southside Virginia have had several rounds of thunderstorms this summer, varying from localized downpours to Friday’s windy squall line (possibly borderline derecho), over the past few weeks. So, as our August weather generally enters a period of fairly normal temperatures after last week’s brief heat spike (discussed later in this column), with intermittent thunderstorms the remainder of this week and beyond, here is a primer on thunderstorm development and organization with many interesting photographs from Cardinal News readers and staffers.

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Thunderstorm development requires, at the least, sufficient moisture and enough lift and/or instability for updrafts to elevate that moisture high in the atmosphere as billowing cumulonimbus clouds.

Atmospheric lift can occur with various kinds of weather systems – cold or warm fronts or outflow boundaries plowing air upward ahead of them like a bulldozer, for instance. Winds blowing upward over mountainous terrain is another potential source of lift.

Instability refers to the property of warmer air rising into cooler air above. If there is a layer of warm and/or dry air aloft, upward motion of warm, moist air can often be stymied – this is called a “cap” and can slow or block thunderstorm development. (Or, if the updrafts are strong enough to blast through the cap, that may be a sign of a higher risk of strong to severe storms.)

Just having enough moisture and a reasonable amount of instability can sometimes do the trick – these are summer “pulse” thunderstorms that go up with afternoon heating and come down quickly over one spot. So can having moisture and enough lift – thunderstorms in cooler seasons often form that way, with little instability but a strong cold front or similar feature lifting air. But, of course, the storms are more likely to form, and more likely to be stronger, if there is abundant moisture, strong lift and a high level of instability.

That brings us to the importance of atmospheric shear.

Shear refers to the changing of winds in direction and/or speed with height.

Moisture, lift and instability produce the updrafts. Shear is the principal mechanism for organizing those updrafts.

If there are light winds aloft with very little change in height, thunderstorm downdrafts, occurring once the moisture condenses aloft in cooler air and starts falling, plunge directly into the updrafts feeding the storm and quickly wipe them out, killing the storm in short order. All the lifted moisture rains out over one area, with some attendant gusts of wind and sometimes hail. These are those previously mentioned pulse storms, a common occurrence in Virginia summers, though not especially frequent this particular summer, with somewhat stronger upper-air flow than we often see.

The greater shifting there is between wind at the surface and winds in various layers aloft, the more updrafts and downdrafts become separated. This enables a storm to become stronger and have greater longevity.

Storms with a moderate amount of shear often develop into various forms of multicell clusters, in which multiple storms connect with updrafts and downdrafts at least partially separated. The shear is not sufficient to separate the storms into individual cells, but it is significant enough to keep the storms going longer than pulse storms.

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Much stronger atmospheric shear can create supercells, or long-lived discrete storms that continue to rotate. Well organized supercells can last hours, and because of their continuous, well-organized, rotating updrafts, the chances of the storm producing very large hail (golfball- to softball-sized) or tornadoes – particularly long-track, violent tornadoes – is greatly increased.

While there are clear-cut cases of storms obviously being supercells – and some extreme ones crossing multiple state lines like the one traveling from northeast Oklahoma to southwest Michigan in March 2006, and from Little Rock almost to Louisville in December 2021 – the delineation is often murkier, as storms with marginal shear, or briefly ingesting rotation from localized boundaries, can form supercell structures or act like a supercell for a short period of time even if the overall atmospheric environment can’t sustain those rotating updrafts. Such was the case around the Roanoke area on July 14, when several photos in this column were taken.

All updrafts in the Northern Hemisphere will have at least some tendency to rotate, usually counterclockwise, at least to some extent for a short time. There are cases where extremely high instability overcomes a lack of shear, causing rapid rotation as updrafts erupt upward very fast. The EF-5 Jarrell, Texas, tornado of 1997 formed in an environment of extremely high instability and very minimal atmospheric shear.

Forecasting thunderstorms is a difficult balancing act weighing the effects of moisture, instability, lift and shear. Rare is the situation where all four are obviously strong and high-end severe storms are nearly certain. More often, the factors vary in degree, and forecasters must determine if having one or more strong factors can overcome the lack of another, or if that lack will reduce the storm risk in intensity or areal coverage.

So this is only a highly generalized description of how thunderstorms develop, organize and intensify.

Thunderstorms are “thunder storms” because of the sound effects produced by lightning rapidly heating air that then cools and contracts rapidly, producing the rumbling noise we call thunder.

Lightning is the result of electric currents between oppositely charged particles. Positive and negative charges tend to separate as ice crystals rub against each other high in the atmosphere, where the updrafts have lifted moisture even when it is hot near the ground. Charged regions of clouds induce opposite charges on the ground. Lightning travels between oppositely charged portions of the same cloud, between oppositely charged portions of different clouds, or most dangerously to us, between cloud and ground.

Speaking of ice crystals aloft, hail forms as strong updrafts lift raindrops high into the sky where it is colder and they freeze into pellets of ice. If the updrafts are strong enough, these raindrops-turned-hailstones can make several trips to subfreezing layers, adding layers of ice – a sliced-open hailstone often looks something like an onion. In the strongest supercells with intense, long-lasting updrafts, hailstones can grow 3 or more inches in diameter. Some hailstones also become jagged, as multiple smaller hailstones partially melt and fuse into each other.

Strong downdraft winds are possible with just about any thunderstorms, at least briefly. A short-lived pulse storm can sometimes emit one huge damaging blast of wind as it collapses. Multicell storms forming a line or cluster often produce a “gust front” of strong winds blowing out ahead of the line – in an extreme case, this can become a derecho, traveling many hundreds of miles with an uninterrupted outflow of damaging winds.

There are also sometimes microbursts, intense downbursts of wind underneath a thunderstorm that fan out from a central point after hitting the surface. The worst microbursts can produce damage resembling a tornado, except in a diverging pattern rather than a converging one. In 1983, President Ronald Reagan landed at Andrews Air Force Base aboard Air Force One only six minutes before a microburst with clocked wind gusts of 149 mph.

Well-organized supercells have both forward-flank and rear-flank downdrafts surrounding and aiding the continuation of the rotating updraft, also known as a mesocyclone. The descent of the rear-flank downdraft can produce a “clear slot” near the back of the storm, with the wind wrapping around the mesocyclone to aid its intensification, sometimes leading to a tornado.

There is often a presumption that any structural damage beyond shingles or tree limbs being blown off must have been caused by a tornado, but various forms of thunderstorm downdrafts can exceed 100 mph and cause similar kinds of damage to that of a tornado, and even create the appearance of shifting or swirling winds to an observer as the storm moves relative to a location. Post-storm surveys by National Weather Service meteorologists focus on the pattern of storm damage to determine if it was the result of a tornado or some form of downdraft wind, often called “straight-line” winds.

A storm is considered “severe” when damaging wind (wind gusts of 58 mph or higher) and/or large hail (1 inch or larger in diameter) are observed or believed to be present in a storm, from surface reports or radar.

Frequent lightning and heavy rain do not make a storm severe, but each can be extremely dangerous and deadly.

A storm does not have to be fully severe to be a risk to life and property. Lightning occurs with the weakest and briefest of thunderstorms – the first bolt of lightning turns a shower into a thunderstorm (the word “thundershower” is sometimes used colloquially for a very weak thunderstorm). Sub-severe wind gusts in the 30-55 mph range are fully capable of blowing down trees with weaker root systems or in wet soil, or knocking off limbs across power lines or roads. And even a narrow column of heavy rainfall can flood a road to the point a vehicle cannot safely traverse it.

So while severe thunderstorm and tornado warnings should definitely be heeded by moving to the interior of a structure away from windows, moving inside when thunder is heard, skies suddenly turn dark or a weather threat is sensed in any way is always sound wisdom.

Last week’s incursion of the “heat dome” lasted only three days – July 27, 28 and 29 – for Southwest and Southside Virginia before a quick reversion to a similar weather pattern, dominated by northwest flow, that has kept most of this summer tolerable with regional temperatures, and appears likely to do the same for the next couple of weeks and possibly longer.

Apparently, the hottest temperature reported and officially recorded in our region was 98 degrees at South Boston in Halifax County on Friday, July 28. Danville hit 96 on Thursday and 97 on Friday, Roanoke 96 on Friday and Saturday, and Clarksville in Mecklenburg County 96 on Thursday and Friday.

A quick survey of regional stations in Southwest and Southside Virginia found no other location that went above 95 degrees, officially. It was notable that Blacksburg recorded its first 90-degree readings of the year on both Friday and Saturday. Having 90-degree weather is not a foregone conclusion for a Blacksburg summer – Blacksburg’s longest streak without a 90-degree temperature, 1,475 days, or a little more than four years, is fairly recent, ending July 23, 2016. Blacksburg also went almost two years without a 90-degree day before July 19, 2019.

At this point, there is no indication of the heat dome over the south and southwest parts of the U.S making as much of a move over us in coming days as it did last week, though we may start seeing it get a little hotter again by next week with a high building more westward out of the Atlantic. Interrupting any move to hotter temperatures will be occasional cold fronts and disturbances from the northwest, triggering intermittent rounds of showers and thunderstorms.

It’s a bit early to declare authoritatively that the hottest weather of the season has occurred, but considering the climatological peak of summer heat has passed for our region and there is no sign of a real heat wave on the horizon, we may very well have seen our hottest days of 2023.

The same atmospheric patterns that have shortened our heat dome experience are also, for now, deflecting any threat of tropical systems away from the U.S. into the open Atlantic.

One low-pressure system scraping the coast of the Carolinas did show some vaguely tropical characteristics over the weekend, but it has since moved well out to sea.

By writing about thunderstorms today, I broke a promise from last week about diving deeper into the topic of hurricane season this week. But that is still coming, next week.

Journalist Kevin Myatt has been writing about weather for 19 years. His weekly column is sponsored by Oakey’s, a family-run, locally-owned funeral home with locations throughout the Roanoke Valley.

Kevin Myatt wrote the Weather Journal in The Roanoke Times for 19 years. He has led students on storm... More by Kevin Myatt