Prolonged heavy rain from low pressure storm systems and especially thunderstorms can pose a potential flash flood threat. Flash flooding is a serious threat to life and property requiring immediate action if flooding is occurring or imminent. Never drive across flooded roads. Move to higher ground if necessary. Be particularly careful at night when it is difficult to see or assess flood waters. NWS Louisville staff members monitor heavy rain and flash flood potential during significant storm systems. The WSR-88D Doppler radar can estimate rainfall. When Louisville forecasters believe that flash flooding is imminent or if a flash flood report is received, then they issue a flash flood warning for the area concerned. Below are some basic heavy rain and flash flood concepts.

A simple rainfall concept: The heaviest precipitation occurs where the rainfall rate is the highest for the longest period of time. Factors that affect heavy rainfall production: 1) Precipitation efficiency: How efficient is the thunderstorm in converting water vapor condensates into rainfall that reaches the ground. 2) Rainfall rate: How intense is the rainfall at any one spot.

Factors that contribute to efficient rainfall production and high rainfall rates: 1) Moist, deep-layered air mass: provides moisture needed for heavy rainfall and limits rainfall evaporation (if dry air were present aloft). 2) Deep low-level warm layer above 0 deg C: allows warm air mass to contain more moisture and enhances warm rain processes. 3) Strong inflow: produces rapid moisture advection and continual moisture source for storms.

To assess HEAVY RAINFALL POTENTIAL, one should consider: 1) Available moisture: surface, 850, and 700 mb dewpoints, precipitable water values, K index. (For more information, consult "Convective Season Parameters and Indices.") 2) Instability: CAPE, Lifted Index, Total Totals Index, Showalter Index. (For more information, consult "Convective Season Parameters and Indices.") 3) Average layer relative humidity: surface-700 mb or 1000-500 mb RH. 4) Strength of inflow: use surface, 850, and 700 mb upper-air charts and/or isentropic charts.

To assess FLASH FLOOD POTENTIAL, one must ALSO consider 3 other factors: 1) Topography: flash flooding is more likely in hilly and mountainous terrain than in flat areas. 2) Antecedent conditions: flash flooding is more likely from future rain if the soil is nearly saturated and/or streams are running high from recent past rain. 3) Storm propagation: extremely important in determining whether heavy rain will fall over a relatively large area (for moving storms) or across the same area (for stationary or regenerative storms). 3a) Forward propagation: heavy rain is progressive so that flash flooding is less likely unless topography and/or antecedent conditions dictate otherwise. 3b) Slow moving/Backward/Regenerative propagation: individual convective cells move forward but continued cell redevelopment upstream causes a thunderstorm complex to exhibit little or no overall (net) movement (i.e., some cell movement but little or no system movement); flash flooding is very possible in these situations.


Thunderstorms are rain storms accompanied by heavy rain, lightning and thunder. The are caused by updrafts of hot air. "As hot air near the ground rises to cooler regions in the sky, any moisture in the air condenses into cloud droplets, Strong updrafts promote the formation of rain, which can fall and drag air masses back down to the surface. Storms like this are short-lived." A thunderstorm is classified as "severe" when it contains one or more of the following: hail three-quarter inch or greater, winds gusting in excess of 50 knots (57.5 mph), tornado.

An average thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. At any given moment, there are roughly 2,000 thunderstorms in progress around the world. It is estimated that there are 100,000 thunderstorms each year. About 10 percent of these reach severe levels.

How does a thunderstorm form? Three basic ingredients are required for a thunderstorm to form: moisture, rising unstable air (air that keeps rising when given a nudge), and a lifting mechanism to provide the "nudge."

The sun heats the surface of the Earth, which warms the air above it. If this warm surface air is forced to rise -- hills or mountains, or areas where warm/cold or wet/dry air bump together can cause rising motion -- it will continue to rise as long as it weighs less and stays warmer than the air around it. As the air rises, it transfers heat from the surface of the Earth to the upper levels of the atmosphere (the process of convection). The water vapor it contains begins to cool, releasing the heat, and it condenses into a cloud. The cloud eventually grows upward into areas where the temperature is below freezing. Some of the water vapor turns to ice and some of it turns into water droplets. Both have electrical charges. Ice particles usually have positive charges, and rain droplets usually have negative charges. When the charges build up enough, they are discharged in a bolt of lightning, which causes the sound waves we hear as thunder.

The word storm has a broad meaning. It can refer to a number of different kinds of atmospheric disturbances. Thunderstorms often have upward drafts of warm air and downward drafts that carry the rain to the ground. "If winds are strong or their significant temperature difference and large amounts of moisture, very large clouds can evolve into thunderheads that acquire a vortex of circulating air. These upper cells can produce tornados and other violent weather."

Thunderstorm Life Cycle

Thunderstorms have a life cycle of three stages: The developing stage, the mature stage, and the dissipating stage. The developing stage of a thunderstorm is marked by a cumulus cloud that is being pushed upward by a rising column of air (updraft). The cumulus cloud soon looks like a tower (called towering cumulus) as the updraft continues to develop. There is little to no rain during this stage but occasional lightning. The developing stage lasts about 10 minutes.

The thunderstorm enters the mature stage when the updraft continues to feed the storm, but precipitation begins to fall out of the storm, and a downdraft begins (a column of air pushing downward). When the downdraft and rain-cooled air spreads out along the ground it forms a gust front, or a line of gusty winds. The mature stage is the most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes. The storm occasionally has a black or dark green appearance.

Eventually, a large amount of precipitation is produced and the updraft is overcome by the downdraft beginning the dissipating stage. At the ground, the gust front moves out a long distance from the storm and cuts off the warm moist air that was feeding the thunderstorm. Rainfall decreases in intensity, but lightning remains a danger. Types of thunderstorms

Single Cell and the Multicell Cluster Thunderstorms

The word storm has a broad meaning. It can refer to a number of different kinds of atmospheric disturbances. Single cell thunderstorms usually last between 20-30 minutes. A true single cell storm is actually quite rare because often the gust front of one cell triggers the growth of another. Most single cell storms are not usually severe. However, it is possible for a single cell storm to produce a brief severe weather event. When this happens, it is called a pulse severe storm. Their updrafts and downdrafts are slightly stronger, and typically produce hail that barely reaches severe limits and/or brief microbursts (a strong downdraft of air that hits the ground and spreads out). Brief heavy rainfall and occasionally a weak tornado are possible. Though pulse severe storms tend to form in more unstable environments than a non-severe single cell storm, they are usually poorly organized and seem to occur at random times and locations, making them difficult to forecast.

The multicell cluster is the most common type of thunderstorm. The multicell cluster consists of a group of cells, moving along as one unit, with each cell in a different phase of the thunderstorm life cycle. Mature cells are usually found at the center of the cluster with dissipating cells at the downwind edge of the cluster. Multicell Cluster storms can produce moderate size hail, flash floods and weak tornadoes. Each cell in a multicell cluster lasts only about 20 minutes; the multicell cluster itself may persist for several hours. This type of storm is usually more intense than a single cell storm, but is much weaker than a supercell storm.

The multicell line storm, or squall line, consists of a long line of storms with a continuous well-developed gust front at the leading edge of the line. The line of storms can be solid, or there can be gaps and breaks in the line. Squall lines can produce hail up to golf-ball size, heavy rainfall, and weak tornadoes, but they are best known as the producers of strong downdrafts. Occasionally, a strong downburst will accelerate a portion of the squall line ahead of the rest of the line. This produces what is called a bow echo. Bow echoes can develop with isolated cells as well as squall lines. Bow echoes are easily detected on radar but are difficult to observe visually.

The supercell is a highly organized thunderstorm. Supercells are rare, but pose a high threat to life and property. A supercell is similar to the single-cell storm because they both have one main updraft. The difference in the updraft of a supercell is that the updraft is extremely strong, reaching estimated speeds of 150-175 miles per hour. The main characteristic which sets the supercell apart from the other thunderstorm types is the presence of rotation. The rotating updraft of a supercell (called a mesocyclone when visible on radar) helps the supercell to produce extreme severe weather events, such as giant hail (more than 2 inches in diameter, strong downbursts of 80 miles an hour or more, and strong to violent tornadoes.

The surrounding environment is a big factor in the organization of a supercell. Winds are coming from different directions to cause the rotation. And, as precipitation is produced in the updraft, the strong upper-level winds blow the precipitation downwind. Hardly any precipitation falls back down through the updraft, so the storm can survive for long periods of time. The leading edge of the precipitation from a supercell is usually light rain. Heavier rain falls closer to the updraft with torrential rain and/or large hail immediately north and east of the main updraft. The area near the main updraft (typically towards the rear of the storm) is the preferred area for severe weather formation.

Upper Level Thunderstorm Features

Thunderstorms can look like heads of cauliflower or they can have "anvils". An anvil is the flat cloud formation at the top of the storm. An anvil forms when the updraft (warm air rising) has reached a point where the surrounding air is about the same temperature or even warmer. The cloud growth abruptly stops and flattens out to take the shape of an anvil.

If the thunderstorm has a very strong updraft, a small portion of the updraft air will poke through the flat part of the anvil — looking like a bubble of cloud above the rest of the anvil. This bubble is called an overshooting top. Most thunderstorms will have an overshooting top for a short time, but if you see a storm with a large, dome-like overshooting top that lasts for more than 10 minutes, chances are good that the thunderstorm updraft is strong enough and persistent enough to produce severe weather.

The anvil can provide other clues to the strength of the storm and how long it might last. If the anvil is thick, smooth-edged, and cumuliform (puffy, like the lower part of the storm), then the storm likely has a strong updraft and is a good candidate to produce severe weather. If the anvil is thin, fuzzy, and wispy like cirrus clouds, then the updraft is probably not as strong, and the storm is less likely to produce severe weather. If the anvil is large and seems to be streaming away from the storm in one particular direction, then there are probably strong upper-level winds in the storm's environment and the precipitation will be blown away from the updraft rather than fall through it.

Mid-Level Thunderstorm Features

Things you might notice in the middle levels of the storm are usually associated with the storm's main updraft tower. If the clouds in the main updraft area are sharply outlined and look like a cauliflower, then the clouds are probably associated with a strong updraft that could produce severe weather. If the clouds in the updraft area have a fuzzy, mushy appearance, the updraft is probably not as strong. If the updraft tower is almost perfectly upright, the storm probably has an updraft strong enough to resist the upper-level winds blowing against it. If the updraft leans downwind, then the updraft is usually weaker.

Thunderstorms with good storm-scale organization usually have a series of smaller cloud towers to the south or southwest of the main storm tower. These smaller towers are called a flanking line and usually have a stair-step appearance as they build toward the main storm tower.

Some supercells during their development will show signs of rotation in the updraft tower. You may see streaks of cloud material that give the storm tower a "corkscrew" or "barber pole" appearance (called striations) and strongly suggest rotation. A mid-level cloud band may also be visible encircling the tower like a ring around a planet. This is another sign of possible rotation within the storm.

As a storm grows in size and intensity, it will begin to dominate its local environment (within about 20 miles). If cumulus clouds and other storms 5-15 miles away from the storm dissipate, it may be a sign that the storm is taking control in the local area. Sinking motion on the edges of the storm may be suppressing any nearby storms. All of the instability and energy available locally may focus on that one storm which could result in its continued development.

Low-Level Thunderstorm Features

Some of the most critical cloud features to determine if a thunderstorm is severe and whether it could produce a tornado are found at or below the level of the cloud base. These features can be confusing and frustrating.

An easy feature to identify is the rain-free cloud base. It is an area of smooth, flat cloud beneath the main storm tower with little or no falling precipitation. The rain-free base is usually just to the rear of the precipitation area, and marks the main area of inflow where warm, moist air at low levels enters the storm. The rain-free base is sometimes called the "intake area."

Inflow bands are ragged bands of low cumulus clouds extending from the main storm tower to the southeast or south (usually). The presence of inflow bands suggests that the storm is pulling in low-level air from several miles away. If the inflow bands have a spiraling nature to them, it suggests the presence of a rotating updraft.

The beaver's tail is another significant type of cloud band. The beaver's tail is a smooth, flat cloud band extending from the eastern edge of the rain-free base to the east or northeast. It usually skirts around the southern edge of the precipitation area. The beaver's tail is usually seen with high-precipitation supercells and suggests rotation in the storm.

A wall cloud is an isolated cloud lowering attached to the rain-free base. A wall cloud forms as the storm intensifies, and the updraft draws in low-level air from several miles around. Some of the low-level air is pulled into the updraft from the rain area. The rain-cooled air is very humid, and the moisture in the rain-cooled air quickly condenses at a lower altitude than the rain-free base to form a wall cloud. The wall cloud is usually to the rear of the visible precipitation area. Wall clouds are usually about two miles in diameter and mark the area of strongest updraft in the storm. To determine if the wall cloud may be tornadic, it will have four basic characteristics. First, the wall cloud will be persistent — lasting for 10-20 minutes (it may change shape) before a tornado appears. Second, the wall cloud will rotate consistently, and often violently before a tornado develops. Third, strong surface winds will blow in toward the wall cloud from the east or south-east (inflow). Surface winds of up to 25-35 miles an hour are often found near tornadic wall clouds. Fourth, the wall cloud will show rapid vertical motion in the form of small clouds in or near the wall cloud and will quickly rise up into the rain-free base. However, not all tornadic wall clouds will have these characteristics, and some tornadoes do not form from wall clouds!

Shelf clouds or roll clouds are examples of other clouds that you may see beneath the cloud base of a storm. Shelf clouds are long, wedge-shaped clouds associated with the gust front. Roll clouds are tube-shaped clouds and are also found near the gust front. Shelf and roll clouds can form anywhere where there is outflow. Shelf clouds typically form near the leading edge of a storm or squall line. A shelf cloud can form under the rain-free base, and look like a wall cloud. A shelf cloud may also appear to the southwest of a wall cloud and is associated with phenomena called the rear-flank downdraft.

To tell the difference between wall clouds and shelf or roll clouds, remember a wall cloud 1) suggests inflow and an updraft, 2) maintains its position with respect to rain, and 3) slopes upward away from the precipitation area. In contrast, shelf clouds 1) suggest downdraft and outflow, 2) move away from rain, 3) slope downward away from the precipitation area.

Thunderstorm Detection and Rainbows

Satellites show us pictures of the clouds before they become big enough to be thunderstorms. We can watch these pictures over an hour and notice that the clouds are growing rapidly. Satellites also can tell us the temperature of the clouds — and we can tell if a cloud has grown tall enough to be a thunderstorm. We can even see the thunderstorm anvil from satellites.

Doppler radar sends out pieces of energy that can be reflected back to the radar by things like rain and hail. The amount of energy that is reflected back can tell us how heavy the rain might be or give us an indication of hail. Doppler radar can also show us how the wind is blowing near and inside the storm. This is helpful in understanding what kinds of hazards the thunderstorm might have (tornado, microburst, gust fronts, etc) associated with it. It also helps us understand how the thunderstorm is feeding itself.

On double rainbows the color of the second rainbow is reversed. According to National Geographic: “When sunlight strikes a raindrop, light rays go in bounce of the back of the drop, and come back out. Then passing in and out , the rays bend — as in a prism — and the colors are separated so that we see the hues of the rainbow, with red on the outer arc. A smaller number of rays reflect twice inside the drop before they exit. The second reflection inverts the image, resulting in a paler secondary rainbow with red on the inner arc. Sometimes a third bow, with red again on the outer rim, is visible.”

Winds from Thunderstorms

Damage from severe thunderstorm winds account for half of all severe wind reports in the United States and is more common than damage from tornadoes. Wind speeds can reach up to 100mph and can produce a damage path extending for hundreds of miles. These winds are often called "straight-line" winds to differentiate the damage they cause from tornado damage. Strong thunderstorm winds can come from a number of different processes. Damaging winds are classified as those exceeding 50-60mph.

Since most thunderstorms produce some straight-line winds as a result of outflow generated by the thunderstorm downdraft, anyone living in thunderstorm-prone areas of the world is at risk for experiencing this phenomenon.

Types of damaging winds: 1) Straight-line winds — a term used to define any thunderstorm wind that is not associated with rotation, and is used mainly to differentiate from tornadic winds. 2) Downdrafts — A small-scale column of air that rapidly sinks toward the ground. A downburst is a result of a strong downdraft.

3) Downbursts — A strong downdraft with horizontal dimensions larger than 4 km (2.5 mi) resulting in an outward burst or damaging winds on or near the ground. (Imagine the way water comes out of a faucet and hits the bottom of the sink.) Downburst winds may begin as a microburst and spread out over a wider area, sometimes producing damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder.

4) Microbursts — A small concentrated downburst that produces an outward burst of damaging winds at the surface. Microbursts are generally small (less than 4km across) and short-lived, lasting only 5-10 minutes, with maximum windspeeds up to 168 mph. There are two kinds of microbursts: wet and dry. A wet microburst is accompanied by heavy precipitation at the surface. Dry microbursts, common in places like the high plains and the intermountain west, occur with little or no precipitation reaching the ground.

5) Gust front — A gust front is the leading edge of rain-cooled air that clashes with warmer thunderstorm inflow. Gust fronts are characterized by a wind shift, temperature drop, and gusty winds out ahead of a thunderstorm. Sometimes the winds push up air above them, forming a shelf cloud or detached roll cloud.

6) Derecho — A derecho is a widespread thunderstorm wind event caused when new thunderstorms form along the leading edge of an outflow boundary (a surface boundary formed by the horizontal spreading of thunderstorm-cooled air). The thunderstorms feed on this boundary and continue to reproduce themselves. Derechos typically occur in the summer months when complexes of thunderstorms form over the plains and northern plains states. Usually these thunderstorms produce heavy rain and severe wind reports as they rumble across several states during the night. The word "derecho" is of Spanish origin and means "straight ahead". They are particularly dangerous because the damaging winds can last a long time and can cover such a large area.

Bow Echo — A radar echo which is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Bow echoes can be over 300km in length, last for several hours, and produce extensive swaths of wind damage at the ground.

Image Sources: Wikimedia Commons

Text Sources: World Meteorological Organization; National Oceanic and Atmospheric Administration (NOAA) New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated January 2012

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