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Thunderstorms
THUNDERSTORMS

A thunderstorm is defined as a regional storm that produces lightning and thunder. On average, a thunderstorm lasts for approximately 30 minutes and has a diameter of 15 miles. There are an estimated 2,000 thunderstorms currently occurring throughout the world, which averages to about 100,000 a year in the United States. Around 10% of these storms are classified as severe, producing hail 3/4 size or larger, winds greater than 58 mph, and even tornadoes.

Thunderstorm Hazards
  • Thunderstorms provide essential and beneficial rain, however may also bring hazards to life and property.
  • Lightning – Lightning occurs in all thunderstorms and tends to strike the tallest object in a particular area.
  • Strong Winds – Strong to severe thunderstorms may produce damaging straight-line winds, which may exceed 100mph.
  • Flash Floods – Flash floods are the #1 cause of deaths associated with thunderstorms.
  • Hail – The size of hail may range from pea to grapefruit size or even larger.
  • Tornadoes - The high winds and flying debris can cause significant property damage and loss of life.

Thunderstorm Life Cycle
The evolution of all thunderstorms, whether they are severe or not, are broken down into three main stages: developing stage (also known as the cumulus or towering cumulus stage), mature stage, and dissipating stage. During the development stage, only updrafts (air rising upward) are dominant throughout the cumulus cloud. The developing stage typically lasts for about 10 minutes with periodic lightning and virtually no rain. Once the updraft reaches the upper-levels of the atmosphere, precipitation within the storm begins to form. This initiation signals the beginning of the mature stage. As the precipitation begins to fall, a downdraft (air moving downward) forms. The downdraft is created when the falling precipitation creates a downward drag and is further aided by the rain-cooled air since colder air weighs more than warmer air. Eventually, the cooler air will hit the ground and spread outward creating a weather phenomenon known as a gust front. Throughout the mature stage, the updraft and downdraft coexist equally, allowing the storm to strengthen. Due to the storm’s strong organization, this is the most likely time for hail, heavy rain, frequent lighting, strong winds, and tornadoes. The mature stage lasts on average for 10 to 20 minutes, but may persist for longer periods of time if the storm is strong enough. Ultimately, the downdraft will become the dominant feature, which will cause the storm to weaken and enter the dissipating stage. Even though the dissipating stage weakens the storm, it still produces lighting and occasionally strong burst of winds.

Thunderstorm Formation
In order for any thunderstorm to develop, three key components are required: moisture, instability, and lift. As warm, moist air in the lower to mid levels of the atmosphere begin to rise, it starts to cool and condenses the water vapor, forming clouds. Once the warm air begins to rise more rapidly, it initiates the formation of an updraft. The air will continue to rise as long as it weighs less and stays warmer than the surrounding air. This is known as instability. The final ingredient needed for thunderstorm formation is a lifting mechanism, which forces the air upwards and helps create thunderstorms. Typical lifting mechanisms include frontal boundaries, mountains, sea breezes, and daytime heating.

Types of Thunderstorms
Thunderstorms are classified into four broad categories based on their physical characteristics: single cell storms, multicell cluster storms, multicell line storms, and supercell storms.
  • Single Cell – These storms typically last on average of 20 -30 minutes. They generally consist of a weak updraft and are poorly organized.
  • Multicell Cluster – They are the most common type of thunderstorms. Multicell cluster storms may consist of either weak or strong updrafts and contain a group of cells that move along as one unit. Each of these individual cells is in a different stage of the thunderstorm life cycle. On average, multicell cluster storms last for about 20-30 minutes, however the whole line may persist for several hours.
  • Multicell Line – This particular thunderstorm classification is more commonly known as the “squall line.” The squall line may consist of both weak and strong updrafts that contain a continuous gust front at the leading edge of the system. As the gust front surges forward, this enables warm air out ahead of the line to be pushed up into the updraft, allowing the squall line to persist for hours.
  • Supercell – Out of all of the classification of storms, the supercell is the most severe and dangerous category of thunderstorms. One reason why these storms are particularly severe has to due with the way they are organized. Supercells contain a strong, rotating updraft called the mesocyclone and are usually isolated from other thunderstorms, which allow them to feed on the abundance of warm, moist air (fuel) without having to compete with other storms for energy. Even though supercells are rare, they pose a significant threat to life and property. Supercells may last for several hours and often produce heavy rains, large hail, strong straight-line wines, and even tornadoes.

Thunderstorm Organization Features
One method commonly used in determining a thunderstorm’s potential strength and structure is to visually evaluate the thunderstorm at different levels and look for certain clues’ telling what kind of environment the storm is in.
  • Upper-Level Features – Thunderstorms that are strong or even severe will have an anvil shaped cloud with sharp and well-defined edges instead of wispy, thin features. In many instances, there will be a large overshooting top and may last for more than 10 minutes if an intense updraft is present.
  • Mid-Level Features – The area of focus within this section of the storm is the main storm tower area. Solid updraft towers, which have a “cauliflower” appearance, indicate a strong updraft and the possibility of severe weather. If the updraft has a fuzzy appearance, then the updraft is not very organized. Another component commonly found at the mid-level is the flanking line. More organized thunderstorms have a row of small cumulus towers (flanking line) that build up to the main storm tower, typically on the south or southwest side of the storm.
  • Low-Level Features – Evaluating storm structure at this level is the most critical area in determining a storm’s severe weather potential. The easiest area to identify within this area is the rain-free base. This is a low, flat cloud base that has virtually no falling precipitation and the most intense updraft region of the storm. If the storm is severe enough, an isolated lowering attached to the storm may form known as the wall cloud. If a wall cloud begins rotating, and is persistent over time, a tornado may be forming.

Thunderstorms are a force of nature that occurs every single day. They can range from your everyday, garden-variety storm to an explosive supercell capable of being a major threat to life and property. Thunderstorms are categorized by their physical characteristics and provide indications of the environment they are in. They are also crucial for bringing moisture and precipitation.
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