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Thunderstorms and Related Severe Weather

A thunderstorm, also called an electrical storm, is a form of weather characterized by the presence of lightning and its attendant thunder produced from a cumulonimbus cloud. Thunderstorms are usually accompanied by heavy rainfall and they can also be accompanied by strong winds, hail and tornadoes. In the winter months, snowfall can occasionally take place in a thunderstorm. Such is often termed thundersnow.

Classification

There are four main types of thunderstorms: single cell, multicell, squall line (also called multicell line) and supercell. Which type forms depends on the instability and relative wind conditions at different layers of the atmosphere ("wind shear"):

• Single cell storms form when the atmosphere is not strong enought, but there is little or no wind shear, meaning precipitation falls back down through the updraft that led to it, cooling it and eventually killing it. These storms are short lived, and last for less than an hour after becoming strong enough to produce lightning. Days with suitable weather conditions often see the repeated forming and dissipation of such storms, leading them to be known as "pulse" storms.

• Multicell storms are groups of cells in different stages of development which have merged into a larger system. The cloud becomes divided into updraft and downdraft regions separated by a gust front. The gust front may extend for several miles ahead of the storm, bringing with it increases in wind speed and atmospheric pressure, decreases in temperature, and shifts in wind direction. The storm itself will have different portions sequentially going through the various thunderstorm stages. In many cases the immature cells develop along a flanking line, resulting in what is known as a line multicell.

• Squall line or multicell line storms are formed as an organized line or lines of multicell storms frequently with a gust front. They often arise from convective updrafts in or near mountain ranges and linear weather boundaries, usually strong cold fronts or troughs of low pressure. Occasionally, squall lines are also formed near the outer rain band of the tropical cyclones. The squall line is propelled by its own outflow, which reinforces continuous development of updrafts along the leading edge. Squall lines tend to be hundreds of miles long, sometimes stretching across the Midwestern United States, covering five states at a time. These lines can move swiftly and in some parts of the line, bow echoes can form, bringing with it high winds, dangerous lightning, and possibly tornadoes. Heavy rain, hail and damaging winds scan occur in a squall line.

• Supercell storms are large, severe quasi-steady-state storms which feature wind speed and direction that vary with height ("wind shear"), separate downdrafts and updrafts (i.e., precipitation is not falling through the updraft) and a strong, rotating updraft (a "mesocyclone"). These storms normally have such powerful updrafts that the top of the cloud (or anvil) can reach miles into the air and can be 15 miles wide. These storms produce destructive tornadoes, sometimes F3 or higher, extremely large hailstones (4 inch—10 cm—diameter), straight-line winds in excess of 80 mph (130 km/h), and flash floods. In fact, most tornadoes occur from this kind of thunderstorm

Multicell or squall line systems may form within a meteorologically important feature known as mesoscale convective system (MCS) stretching for hundreds of miles. The mesoscale convective complex is a closely related phenomenon. They are large enough to have a pronounced effect on the upper-level and surface weather pattern, and may influence forecasts over a large area. MCS systems are common in the Midwest region of the United States and the Canadian Prairies during the summer months and produce much of the region's important agricultural rainfall. Prior to the discovery of the MCS phenomenon, the individual thunderstorms were thought of as independent entities, each being effectively impossible to predict. The MCS is amenable to forecasting, and a meteorlogist can now predict with high accuracy the percentage of the MCS that will be affected by thunderstorms. However, the meteorlologist still cannot predict exactly where each thunderstorm will occur within the MCS.

Severe thunderstorm

A severe thunderstorm is a thunderstorm with winds 57.5 mph or greater, 1.9 centimeter (¾ in) or larger hail, funnel clouds or tornadoes. These storms may contain frequent cloud-to-ground lightning and heavy downpours which can lead to localized flooding. An otherwise weak thunderstorm which produces a wind gust of the required strength would be defined as 'severe' whereas a very violent thunderstorm with continuous lightning and very heavy rain (but without the required wind gusts, hail or tornado/funnel cloud) would not. Many of the violent local thunderstorms which affect Florida so frequently during the summer months would not be defined as severe.

Severe thunderstorms may occur as supercell thunderstorms, although multicell and squall lines are the most common forms.

Where thunderstorms occur

Thunderstorms occur throughout the world, even in the polar regions, with the greatest frequency in tropical rainforest areas, where they may occur nearly daily. Kampala and Tororo in Uganda have each been mentioned as the most thunderous places on Earth, an accolade which has also been bestowed upon Bogor on Java, Indonesia. In temperate regions, they are most frequent in spring and summer, although they can occur in cold fronts at any time of year. Thunderstorms are rare in polar regions due to the cold climate and stable air masses that are generally in place, but they do occur from time to time, mainly in the summer months.

In more contemporary times, thunderstorms now have taken on the role of a curiosity. Every spring, storm chasers head to the Great Plains of the United States and the Canadian Prairies in summer to explore the visual and scientific aspects of storms and tornadoes.

Watches and Warnings

A severe thunderstorm watch is issued when weather conditions are favorable for the development of severe thunderstorms. If the thunderstorms are forecasted to be so severe that they may produce tornadoes, then a tornado watch (which also automatically implies a severe thunderstorm watch) is issued. A severe thunderstorm watch can also be upgraded to a tornado watch as conditions warrant. A watch must not be confused with a severe thunderstorm warning.

A watch does not mean that the severe weather is actually occurring, only that conditions have created a significant risk for it. If severe weather actually does occur, a severe thunderstorm warning or tornado warning will be issued, and urgent action should be taken. It should be noted that while a severe thunderstorm watch does not imply in its name the risk for tornadoes, they can and occasionally do strike even when a tornado watch is not issued.

In the United States, the Storm Prediction Center issues watches for areas likely to produce tornadoes and severe thunderstorms. The watch boxes (or weather watches, WWs) are usually issued in the format of x miles north and south, or east and west, or either side of a line from y miles direction of city, state, to z miles another direction of another city, state. For example: "50 miles either side of a line from 10 miles northeast of Columbia, South Carolina to 15 miles south-southwest of Montgomery, Alabama". ("Either side" means perpendicular to the center line.) In addition, a list of all counties included in its area of responsibility is now issued by each NWS forecast office for each watch.

In the event that a severe thunderstorm watch is likely to lead to very destructive winds or hail, enhanced wording with the words particularly dangerous situation (PDS) can be added to the watch. This is rare with severe thunderstorm watches (since the tornado threat has to remain low enough to only warrant a severe thunderstorm watch); it is far more common with tornado watches.

A severe thunderstorm warning is issued when trained spotters or doppler radar indicate a strong thunderstorm is producing dangerously large hail or high winds, capable of causing significant damage. It does not account for lightning or flooding. A warning must not be confused with a severe thunderstorm watch.

In the U.S., the National Weather Service defines large hail as being at least ¾ inch in diameter and high winds as being 58 miles per hour or greater.
A severe thunderstorm warning means there is significant danger for the warned area. Occasionally, severe thunderstorms can and do produce a tornado without warning. However, frequently a severe thunderstorm will produce serious wind damage as severe as a tornado. A severe thunderstorm warning can also be upgraded to a tornado warning if a tornado is detected on radar or actually spotted. Generally, but not always, a severe thunderstorm watch or tornado watch will precede a warning.

Lightning

Lightning is an electrical discharge that occurs in a thunderstorm. It can be seen in the form of a bright streak (or bolt) from the sky. Lightning occurs when a charge is built up within a cloud. When a large enough charge is built up, a large discharge will occur and can be seen as lightning. The temperature of a lightning bolt can be hotter than the surface of the sun. Although the lightning is extremely hot, the short duration makes it not necessarily fatal. Contrary to the popular idea that lightning doesn’t strike twice in the same spot, some people have been struck by lightning over three times and skyscrapers like the Empire State Building have been struck numerous times in the same storm

There are several kinds of lightning.

• In-cloud lightning is the most common. It is lightning within a cloud.
• Cloud to ground lightning is when a bolt of lightning from a cloud strikes the ground. This form poses the greatest threat to life and property.
• Ground to cloud lightning is when a lightning bolt is induced from the ground to the cloud.
• Cloud to cloud lightning is rarely seen and is when a bolt of lightning arches from one cloud to another.
• Ball lightning is extremely rare and has no known scientific explanation. It is seen in the form of a 20 to 200 centimeter ball.
• Cloud to air lightning is when lightning from a cloud hits air of a different charge.

For an excellent reference and preparedness guide to all aspects of Thunderstorms, please see http://www.nws.noaa.gov/om/brochures/tstorm.pdf