Severe Storms/Thunderstorms

This page is to be used for the Severe Storms topic of the Meteorology event.

Air Mass Thunderstorms
In the United States, Air Mass Thunderstorms frequently occur when maritime tropical (mT) air moves northward from the Gulf of Mexico. These warm, humid air masses contain abundant moisture in their lower levels and can be unstable when heated from below or lifted along a front.

Life Cycles of Thunderstorms
For the average thunderstorm there are three stages:

The Cumulus
The Cumulus stage is dominated by rising currents of air (updrafts) and the formation of a towering cumulonimbus cloud. Falling precipitation within the cloud causes drag on the air and initiates a downdraft that is further aided by the influx of cool, dry air surrounding the cloud, a process termed entrainment. This stage then progresses to the mature stage.

Mature Stage
The mature stage is marked by the downdraft leaving the base of the cloud and the release of precipitation. With gusty winds, lightning, heavy precipitation, and sometimes hail, the mature stage is the most active period of a thunderstorm.

Dissipating Stage
Marking the end of the storm, the dissipating stage is dominated by downdrafts and entrainment. Without a supply of moisture from updrafts, the cloud soon evaporates. It should be noted that within a single air-mass thunderstorm there may be several individual cells—that is, zones of adjacent updrafts and downdrafts.

Summary
So in summary here are all the stages together:



Severe Thunderstorms
Severe Thunderstorms are capable of producing heavy downpours and flash flooding as well as strong, gusty straight-line winds, large hail, frequent lightning, and perhaps tornadoes.

For a thunderstorm to be officially classified as severe by the National Weather Service, it must have winds in excess of 93 kilometers (58 miles) per hour or produce hailstones with diameters larger than 1.0 inch or generate a tornado. Of the estimated 100,000 thunderstorms that occur annually in the United States, about 10 percent (10,000 storms) reach severe status.

Regular air-mass thunderstorms are localized, relatively short lived phenomena that dissipate after a brief, well-defined life cycle (above). The key factor for a severe thunderstorm is a strong vertical wind shear. That way the cold downdraft does not cut off the updrafts, which are the thunderstorm's "fuel".

Mesoscale Convective Complexes
A mesoscale convective complex (MCC) is a unique kind of mesoscale convective system which is defined by characteristics observed in infrared satellite imagery. They are long-lived, nocturnal in formation and commonly contain heavy rainfall, wind, hail, lightning and possibly tornadoes.

Single Cell
Single cell thunderstorms, also known as pulse storms, usually last 20 to 30 minutes. Although severe weather is uncommon in single cell storms, heavy rainfall, downbursts, hail, and even weak tornadoes are possible. Single cell thunderstorms form in environments with low wind shear.

Multi-Cell Cluster
A multi-cell cluster storm is a group of cells moving as a single unit. Each constituent cell of a multi-cell cluster is at a different stage of thunderstorm development. New cells develop on the upwind side of the cluster, mature cells can be found in the center, and dissipating cells are on the downwind side. Alike single cells, multi-cell clusters can still produce heavy rainfall, downbursts, and hail, but the risk of flash flooding can be significant, and tornadoes are less likely.

Multi-Cell Line
A squall line is a line of severe thunderstorms that can form along and/or ahead of a cold front. It contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts. Squall lines typically form in unstable atmospheric environments where low-level air can rise unaided after being initially lifted (e.g., by a front) to the point where condensation of water vapor occurs. Heat is released during condensation, resulting in the rising air becoming lighter than nearby air at the same height. This leads to an increase in the speed of the rising air which sometimes reaches speeds above 30 mph. In models this initial lifting is specified through an idealization of the flow associated with the front or other lifting mechanism or through the use of observational flow information. The gust front is located along the line where these winds meet -- which extends from the surface well up into the the storm.



Supercells
A supercell is a large rotating thunderstorm with a mesocyclone. They can last longer than normal thunderstorms and can produce tornadoes and baseball size hail.

Mesocyclones

A mesocyclone is a large rotating vortex of air. They rotate in the same direction as a low air pressure system would in the same hemisphere as the mesocyclone. They are formed when wind shear starts a portion of air in the lower atmosphere spinning in a tube like formation around a horizontal axis. The updraft found in a supercell can cause the "tube" to angle upwards until the air is rotating around a vertical axis.

Parts


 * The overshooting top is a dome shaped formation on the top of a supercell caused by a very strong updraft lifting a portion of clouds above the anvil.
 * The anvil is the overshooting portion at the top of the supercell. It is very cold and has almost no moisture in it.
 * The precipitation free base is a portion of the supercell from which no precipitation is falling. Hail may be present, however.
 * The wall cloud is the portion of the supercell between the precipitation free base and precipitating areas. It forms when cool air is pulled into the updraft. The air from this area quickly becomes completely saturated, and becomes visible as a cloud. The area of saturated air moves downward, so the wall cloud appears as a descending column. Very few of these turn into tornadoes.
 * The rear flank downdraft is the downdraft on the back side of a supercell. It is dry air that wraps around the mesocyclone and is important to tornado development.  After it descends to the ground, it moves east into the updraft.
 * The forward flank downdraft is the downdraft on the leading side of a supercell. It is the main downdraft and the area with the heaviest precipitation.
 * The flanking line is a line of cumulus clouds on the southwest side of the supercell. It forms along the rear flank downdraft.
 * The gust front is the boundary between the updraft and downdraft of a thunderstorm. In supercells it is located between the updraft and the rear flank downdraft.
 * The mesocyclone is the rotating updraft of a supercell.

Tornado Characteristics
Tornadoes are large clouds mostly characterized by extremely high winds. They are usually found in the most intense supercells and are caused by winds traveling in different directions, or wind shear. They usually look like large funnels touching down from the main cloud. Note that although most tornadoes look like funnel clouds, they do not necessarily need to have one, as long as the winds touch both the ground and the cloud. Consequently, a funnel cloud may occur but not a tornado if the funnel does not touch down.

Geographical and Seasonal distribution
The United States are home to the largest amount of tornadoes. Most of them occur in a central region known as Tornado Alley, which contains the states of Texas, Oklahoma, Kansas, Nebraska, and the edges of other states, depending on the definition. However, tornadoes have been observed on every continent excluding Antarctica, and every state in the United States.



Meteorologist also now reference a 2nd "alley" referred to as "Dixie Alley." Dixie Alley includes states of the SE United States such as Missouri, Tennessee, Arkansas, Alabama and Mississippi. There is also a pattern with the time of year and the frequency of tornadoes. The majority of tornadoes form between April and mid-June.

Tornado Hazards
Much of the damage caused by a tornado can be related to the high winds, as this is the essence of a tornado. However, a lot of damage is also caused by the flying debris resulting from the destruction of some structures. Their impact can destroy other buildings more easily. Other hazards include downed power lines, broken gas lines and pumps, and fires.

The Fujita Scale
Two major scales measure tornadoes: the Fujita scale and the Enhanced Fujita Scale. Both measure from 0 to 5, but the characteristics of both are different.

The Fujita scale, or Fujita-Pearson Scale, is as follows.

An F6 category was also thought of, but it is purely hypothetical and no F6 tornado has actually existed.

The Enhanced Fujita Scale was intended to improve the Fujita scale.

Life Cycles of Tornadoes
Three stages usually categorize a tornado's life. Please note that although these are the most common, not all tornadoes follow this exact pattern and it is merely a model. Nevertheless, it is something you should know.

Formation

If conditions are right, the rotation of winds within a mesocyclone allows a vortex to form underneath it, and a funnel cloud usually forms with this. It gains energy as it descends and it becomes a tornado once it touches down.



Maturity

Once the funnel cloud becomes a tornado, it enters its mature stage. This is where all the destruction comes in.

Dissipation

When the mesocyclone loses its rotation and/or conditions are no longer right for a tornado, it begins to dissipate. The shape of the tornado can be altered into a rope-like form or some other shape, depending on the characteristics of the storm it is in.

Waterspouts
Waterspouts are similar vortexes that occur over water. They are usually less violent than regular tornadoes, although they can be rather powerful given a strong storm.



Causes of Lightning
A storm is only classified a a thunderstorm when there is lightning. Thus, its important to discuss the causes of lightning. Some cloud physicists believe that charge separation occurs during the formation of ice pellets. Experimentation shows that as droplets begin to freeze, positively charged ions are concentrated in the colder regions of the droplets, whereas negatively charged ions are concentrated in the warmer regions. Thus, as the droplets freeze from the outside in, they develop a positively charged ice shell and a negatively charged interior. As the interior begins to freeze, it expands and shatters the outside shell. The small positively charged ice fragments are carried upward by turbulence, and the relatively heavy droplets eventually carry their negative charge toward the cloud base. As a result, the upper part of the cloud is left with a positive charge, and the lower portion of the cloud maintains an overall negative charge with small positively charged pockets. As the cloud moves, the negatively charged cloud base alters the charge at the surface directly below by repelling negatively charged particles. Thus, the surface beneath the cloud acquires a net positive charge. These charge differences build to millions and even hundreds of millions of bolts before a lightning stroke acts to discharge the negative region of the cloud by striking the positive area of the ground below, or, more frequently, the positively charged portion of that cloud, or a nearby cloud.

How Lightning Strikes
Pop quiz: Does lightning start from the cloud and move down, or does it start from the ground and move up? The answer: Neither. This is because a lightning strike is not a single brilliant bolt, but actually several strokes. First, there is a stream of electrons that moves downwards from the cloud. This is called the initial leader(or Step leader). As it nears the ground, electrons are pulled from the surrounding air, resulting in a ionized path from the cloud to ground. Then, electrons pour from this channel of charge. This is the main stroke (or Return Streamer) and is what we think of "lightning".

Cloud lightning
Lightning does not always strike the ground. It can either occur between two separate clouds, or within the same cloud, which is the most common. When it occurs with in the same cloud, it will usually start in the lower portion of the anvil, and move downward.

Heat lightning
Heat lightning appears to produce no thunder. In fact, it does, but it happens so far away that the observer does not hear it, because the sound dissipates through the air.

Positive Lightning
Positive lightning occurs when there are little to no clouds. These lightning bolts originate from the top of a cloud, usually the anvil, and travels horizontally for several miles before turning and moving downward to meet the initial leader.

Ball Lightning
The entire existence of ball lightning can be disputed because of it's lack of observation. Ball lightning has been spotted hundreds of times around the world, but very rarely by meteorologists. Observers say that ball lightning appears as a sphere, differing in size from between a few inches in diameter to several meters, and varies in color between red, orange, yellow, even green or white. It can appear after a large thunderstorm. It travels mostly horizontally, from about waist high to several meters off the ground. Usually ball lightning comes with a bad smell. It can come in through open doors or windows, including closed screens, and sometimes chimneys. No ball lightning stays for more than a few seconds, and it moves at a brisk pace- several meters per second. Sometimes observers report that it will "bounce" between puddles.

Because even the existence of ball lightning can't be proven, not very much is known about it other than its appearance. As of right now, no theories have been suggested that can explain the strange movement, appearance, and how it can produce a constant stream of light and energy.

It is thought that UFO sightings after a large storm can actually be ball lightning. So the next time you see a ball of light high in the sky after a large storm, you may not be seeing a UFO, but instead a rare example of ball lightning.

Effects
According to the National Weather Service, only 10% of people that are struck by lightning are killed, leaving the remaining 90% with various injuries If you get hit by lightning, it usually damages the nervous system. When the brain is affected, the person may have difficulty with short-term memory, coding new information and accessing old information, multitasking, and being easily distracted. Lightning victims may also suffer personality changes because of frontal lobe damage and become irritable and easy to anger. In addition, some survivors complain of becoming more easily exhausted than before being struck.

Haboobs
Haboob-a dust storm created when a thunderstorm's downdraft reaches the ground and moves outwards from the thunderstorm, creating the outflow boundary (also known as the gust front). As the downdraft hits the ground and spreads outwards, intense winds pick up loose sediments, creating what appears to be a wall of dust.

Dangers of Haboobs
The most eminent danger of haboobs is low visibility, dropping to virtually zero, which is very dangerous for motorists. Also, the sediments may trigger respiratory complications, such as asthma. Another potent threat are fungi spores, which cause fungal infections. The best way to stay safe in a haboob is prior to its arrival, get off roads and indoors. If caught outside, make a makeshift mask and cover your mouth and nose.

Special Topics for 2017

 * Colorado Floods 2013: Through the first week of September 2013, Colorado was exceptionally warm and dry. By September 12, everything had changed. Flood conditions stretched about 150 miles, from Colorado Springs north to Ft. Collins. Saturated soils left water with no place to go, and puddles turned to ponds throughout the densely populated Colorado Front Range. Rainwater swelled rivers and creeks, overtopped dams, flooded basements, and washed out roads. By September 16, authorities had confirmed six deaths, and more than 1,000 people remained missing. Among the hardest-hit communities was Boulder, located on the northwestern end of the Denver metropolitan area. Meteorologist Jeff Masters noted on his blog that the three-day rainfall recorded by the evening of September 12 exceeded the monthly total for any month since rainfall records began in 1897. Similarly high rainfall totals occurred in other spots along the Front Range. Masters exclaimed, "These are the sort of rains one expects on the coast in a tropical storm, not in the interior of North America!" In an interview with KDVRDenver, Russ Schumacher of Colorado State University concluded that the precipitation in Boulder County and other parts of the state qualified as a 1,000-year event, meaning that any one year has just a 1-in-1,000 chance of experiencing such heavy precipitation. Ranking the actual flooding is more challenging than quantifying the precipitation because over time, people use the land differently. On September 12, the Boulder Creek, which flows roughly eastward through town, crested in downtown Boulder at 7.78 feet—the highest water level observed at that location since 1894. The main highway running through Boulder was partially closed southeast of town, and partially destroyed northwest of town, isolating the nearby mountain community of Lyons. Thousands of residents faced power outages and evacuation orders in the Denver-Boulder area as officials called in the National Guard to assist rescue efforts. Schools, businesses, and government offices closed. Many roads remained closed and impassable, so multiple mountain communities remained isolated. And the rain kept falling. As the rain continued, heavy humidity hung in the air. Precipitable water—the height of liquid water that would result if all the water vapor in the atmospheric column were condensed—showed record-high measurements for this time of year, as recorded by balloon soundings from the National Weather Service Storm Prediction Center. Masters noted that the region's highest September precipitable water measurements, dating back to 1948, occurred on September 12-13, 2013. Boulder is no stranger to floods. In fact, it is one of Colorado's most flood-vulnerable communities. The city is situated at the mouth of a canyon, and the Boulder Creek flows through the middle of town. According to weather historian Christopher C. Burt, floods pummeled Boulder in 1894, 1896, 1906, 1909, 1916, 1921, 1938, and 1969. The worst floods struck in 1894 and 1969. But those floods occurred in the springtime: May 31-June 2, 1894, and May 7, 1969. Boulder's 2013 flood not only brought unusual rainfall, it came at an unusual time. On average, April and May are Boulder's wettest months, with precipitation totals of 2.45 and 3.04 inches, respectively, between 1948 and 2005, according to the Desert Research Institute. Precipitation amounts drop slightly in the summer months, but remain relatively high as the North American Monsoon takes hold sometime between June and August. Monsoon winds carry moisture from the Gulf of Mexico northward over the American Southwest. Monsoon storms can bring strong afternoon and evening thunderstorms to the state. Although not the driest month of the year, September is usually much more arid, with average total precipitation of 1.61 inches. Like summertime monsoon storms, the deluge that struck in the second week of September 2013 involved moist air from the Gulf of Mexico. The storm resulted from the interaction of low pressure, warm air loaded with moisture, and Colorado's mountains. After the 90+-degree weather in the first week of September, cool, wet weather moved into the state, leaving the ground saturated by September 11. Meanwhile a low-pressure system centered over Utah and Nevada started air moving in a counter-clockwise direction over the Southwest. This pulled warm, moisture-rich air from the Gulf of Mexico into Colorado from the southeast. As the air traveled up the eastern face of the Rockies, it formed clouds and then rain, and remained in place for days. As for whether a warming climate played a part in this historic storm, Henson described the event as an "excellent candidate for an attribution and detection study." Computerized climate models enable scientists to test how frequently similar storms might occur with or without fossil fuel use. Although it stands to reason that a warming climate could worsen storm intensity since a warmer atmosphere can hold more moisture, Henson cautioned, "Even when researchers find that a given type of disaster has become more likely, a rare event is still going to be rare—and it can occur without any help from greenhouse gases." By September 13, the rain was starting to ease around along the Front Range. But a multitude of damaged or destroyed buildings, waterlogged vehicles, and washed-out roads and bridges meant that Colorado residents would be mopping up for weeks or months after the skies cleared.
 * Tornado Outbreak: On February 23, the Storm Prediction Center issued a moderate risk for severe weather across parts of Louisiana, Mississippi, Alabama, Georgia, and the Florida Panhandle, including a 15% risk area for tornadoes. The first significant tornadoes of the outbreak moved across southeastern Louisiana and southern Mississippi that evening, leaving significant damage and three deaths. The towns of Livingston and Laplace, Louisiana sustained heavy damage from strong EF2 tornadoes, and another EF2 near Purvis, Mississippi killed one person in a mobile home. An EF3 tornado also caused major structural damage in Paincourtville, Louisiana before destroying an RV park in Convent, killing two people at that location. Three simultaneous waterspouts were observed over Lake Pontchartrain during the event as well. Later that night, a large supercell thunderstorm developed over the Gulf of Mexico and moved ashore, producing a destructive EF3 tornado in Pensacola, Florida. The tornado injured three people and destroyed homes, townhouses, apartments, and a GE warehouse. The outbreak continued the following day as the Storm Prediction center issued another moderate risk across parts of the East Coast, again including a 15% risk area for tornadoes. Strong tornadoes impacted the East Coast states of Virginia, Pennsylvania, and North Carolina on February 24, killing four people. An EF1 tornado struck the town of Waverly, Virginia, killing three people in a mobile home, including a two-year old child. An EF3 tornado struck the town of Evergreen, Virginia, causing severe damage and killing one person at that location. An EF2 tornado caused major damage to homes near Oxford, North Carolina, and another EF2 tornado touched down near White Horse, Lancaster County, Pennsylvania, damaging up to 50 structures in the area. Another EF3 tornado occurred later that night near the Virginia town of Tappahannock, destroying multiple homes along its path. About 35,000 people in Virginia, 4,000 in Washington, D.C., and 47,000 in the Carolinas lost power due to the storms. Seven people in total were killed by tornadoes during the outbreak, and a total of 61 tornadoes were confirmed.