In the predawn hours of May 25, 2009, along the provinces of Antwerp, East Flanders, and Flemish Brabant (near Brussels), an astonishing 30,000 lightning flashes were recorded in a two-hour period–including 10,000 cloud-to-ground strikes. Wind gusted near 60 mph and hailstones measuring 2.4 inches across were noted in some areas. In addition to whole trees being uprooted and scattered across roadways like pick-up sticks, a moving train was blown from the tracks. So what was this dynamic phenomenon that wreaked a large swath of meteorological havoc across Belgium?
According to weather experts, supercell thunderstorms, such as the one in Belgium, can occur anytime from March to November, unleashing some of the most damaging winds, torrential rain resulting in deadly flash floods, and golf ball-sized hail known on Earth. A single supercell storm can cause hundreds of millions of dollars’ worth of destruction (or more) from flooding to dented cars, severe property damage, and loss of lives. The National Weather Service says these storms are responsible for nearly all of the significant tornadoes in the U.S., such as the one in Gainesville, Georgia, in 1936 that killed 203 people (though the term “supercell” was reportedly not coined until 1962).
Often called rotating thunderstorms, or mesocyclones, supercells are characterized by a “deep, rotating updraft,” the result of the tilting of an invisible horizontal vortex caused by wind shear. Both the least common and most destructive form of the thunderstorm classifications–squall line; multicell; single-cell–of which the supercell is a derivative–the weather phenomenon can persist over many hours with its updrafts and downdrafts each reaching speeds in excess of 100 mph.
On April 14, 1999, a supercell devastated the east coast of Australia’s New South Wales, its accruing hail causing an estimated $2.3 billion dollars’ worth of damage. The July 26, 2005 flood in Mumbai, India, was attributed to a supercell, whereby more than 37 inches of rain fell on the city in one day with 28 inches of that total falling in just four hours.
Further defined and distilled into several categories: Classic–seen a lot in arid regions; High precipitation–more likely to be found in humid climates; Low precipitation; and Mini- or Low-topped supercells, according to the National Weather Service not all supercells fit neatly into one of these categories. Many exhibit cross or hybrid characteristics, but regardless first show up on Doppler radar with a point or hook shape on the southwestern side, fanning out to the northeast.
Heaviest precipitation is seen on the southwest side, ending abruptly short of the rain-free updraft base not visible to radar. A rear flank downdraft, seen as a vaulted feature, is said to carry precipitation counterclockwise around the north and northwest sides of the updraft base, producing a pendant or “hook echo” that indicates the presence of a mesocyclone. With a cap or “capping inversion” forming a powerful updraft, an inverted–or cold above warm–layer forms above a normal (warm over cold) layer, resulting in an unstable moist layer beneath. When the cap weakens or moves, the results are often termed explosive.
In the U.S., the infamous Tornado Alley–a meteorological argot generally defining the region between the Rockies and Appalachian Mountains–sees it share of supercells each year. Stringent building codes that include stronger roofing materials, tornado warning sirens, and more secure connections between a building and its foundation are among the more positive outcomes spawned by marauding supercells, and forecasting technology continues to improve putting more time between the gusty perpetrator and its potential victims.