Severe Storms/Winter Storms

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

Snow Storms
Snow is less dense than liquid by a factor of a approximately ten when in temperatures just under freezing. This means that 1 inch of rain would be about 10 inches of snow. This can make snow storms very problematic, especially in areas that are not used to getting heavy snow; however more than 6 inches of snow will be a problem anywhere. Some of the key dangers of snow storms include hypothermia, frostbite, car wrecks, or even avalanches if near or on a mountain.

In order for a snow storm to be classified as a "blizzard" it must have the following characteristics:
 * Visibility reduced to less than 1/4 mile
 * Winds greater than 35 miles per hour
 * Last for a long period of time such as three hours.

Freezing Rain
Freezing rain can be extremely dangerous. It occurs when there is a layer of warm air aloft and freezing cold air near the ground. Rain will fall, become supercooled in the cold air layer, and freeze when it strikes the ground or an object on the ground. The result is a layer of ice instead of snow. Even for places that are accustomed to snow storms, as little as 1 cm can completely paralyze a city. Dangers including driving, telephone and electrical wire damage, and entire crops can be destroyed.

THUNDERSTORMS
A thunderstorm, also known as an electrical storm, lightning storm, or thundershower, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Thunderstorms occur in association with a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds, heavy rain, and sometimes snow, sleet, hail, or, in contrast, no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms, known as supercells, rotate as do cyclones. Life Cycle: Thunderstorms can form and develop in any geographic location but most frequently within the mid-latitude, where warm, moist air from tropical latitudes collides with cooler air from polar latitudes. Thunderstorms are responsible for the development and formation of many severe weather phenomena. Thunderstorms, and the phenomena that occur along with them, pose great hazards. Damage that results from thunderstorms is mainly inflicted by downburst winds, large hailstones, and flash flooding caused by heavy precipitation. Stronger thunderstorm cells are capable of producing tornadoes and waterspouts. Warm air has a lower density than cool air, so warmer air rises upwards and cooler air will settle at the bottom (this effect can be seen with a hot air balloon). Clouds form as relatively warmer air, carrying moisture, rises within cooler air. The moist air rises, and, as it does so, it cools and some of the water vapor in that rising air condenses. Generally, thunderstorms require three conditions to form: moisture, an unstable air mass, a lifting force (heat). All thunderstorms, regardless of type, go through three stages: the developing stage, the mature stage, and the dissipation stage. The average thunderstorm has a 24 km (15 mi) diameter. Depending on the conditions present in the atmosphere, each of these three stages take an average of 30 minutes.

TYPES

 * Dryline thunderstorms: A dryline is a separation line between two different air mass boundaries. The air mass on the west side of the dryline is dry, continental air (usually from the Rockie Mountains or the desert southwest), while the air mass east of the dryline is maritime, tropical air (usually from the Gulf of Mexico).
 * Single-cell thunderstorms- small brief and weak storms that grow and die within an hour; they are typically driven by the heat of the summer
 * Multicell: A multicellular thunderstorm cluster is a thunderstorm that is composed of multiple cells, each being at a different stage in the life cycle of a thunderstorm. Made of multiple single cell thunderstorms, this is a common thunderstorm where new updrafts form along the leading edge of rain-cooled air (the gust front). Individual cells within the system last from 30 to 60 minutes while the whole system may last for many hours. They can produce hail, strong winds, brief tornadoes, and flooding. They can form because of convergence (warm and cold air meeting and being forced up) or orographic lifting where a cold front is forced to move to a higher elevation and moves over terrain like mountains.
 * Supercell: Type of storm where rising air rapidly rotates, descending air rotates more slowly and tornado formation. This is a long-lived (lasts longer than 1 hour) and highly organized storm feeding off an updraft (rising current of air) that is tilted and rotates. The rotating updraft (can be as large as 10 miles in diameter and up to 50,000 feet tall) is present for up to 20 to 60 minutes before a tornado forms. The rotation is called a mesocyclone when detected of Doppler radar. This tornado is a small extension of the larger rotation. The largest and most violent tornadoes come from these supercells. Severe thunderstorms usually form in areas with strong vertical wind shear. Thunderstorms occurring in weak vertical wind shear have an erect appearance. These types of storms don’t last as long and severe weather within the storms will be brief. Organized storms formed in sheared environments are longer-lived and allow for some predictability. Extremely unstable atmospheres with the right wind shear can cause a supercell thunderstorm to form. The wind will rotate counterclockwise as the air rises. The updraft speeds may reach up to 100 mph. These types of thunderstorms make damaging microbursts, large hail, and torrential rains. Rotating wall clouds are a lowered area of rotating clouds that are firmly attached to the base of the thunderstorm; located in a supercell thunderstorm.
 * Air Mass: This type of thunderstorm is common in Florida. It lasts approximately 1 hour with a distinctive life cycle. The cumulus stage has rising air (updraft) which cools and forms the cloud. This happens in an environment favorable for convection. Lifting mechanisms include solar heating or convergence from a sea breeze. No rain is present during this stage. The mature stage is when precipitation particles form and fall from the cloud into the updraft. The falling rain drags down air referred to as downdrafts. Some precipitation will evaporate and make a cooling causing the air to become denser and increases the downdraft. Rain reaches the surface and maybe even some small hail. The dissipating stage occurs when downdrafts encompass the storm and the updrafts are shut off causing the storm to die and rain to cease. After the storm, the temperature may become warm again. Some hazards would be lightning, strong winds, and flooding.

STRUCTURE
A. Anvil- The Anvil is one of the most impressive features of a severe storm due to its areal coverage and icy texture. Within a severe storm, moisture is transported from the lower troposphere to deep into the upper troposphere. Not all moisture that is ingested into a storm is precipitated out of the storm. Some of the moisture in a strong updraft is lofted so high into the troposphere that it is not able to drop back down immediately. Strong upper level winds move and fan the moisture out over great distances. The temperature of the anvil is frigid cold. The light density of the moisture allows the wind to move it at will. A forecaster can note the direction and speed of the upper level winds by noting the anvil's orientation. The moisture within the anvil will be blown downstream. B. Overshooting Top- The core of the updraft has the strongest convective upward vertical velocity. This core of rapidly rising air will only slow down and stop when it encounters a very stable layer in the atmosphere. This very stable layer is the tropopause. Air will rise as long as it is less dense and therefore more buoyant than surrounding air. The faster air rises the longer it takes generally to slow down and stop once it encounters a very stable layer. This occurs because a moving object has momentum. That part of the updraft that has the greatest momentum will form the overshooting top on a severe thunderstorm. C. Mammatus- Mammatus are pouched shaped clouds that protrude downward from the thunderstorm's anvil. They form as negatively buoyant moisture laden air sinks. The cloud remains visible until the air sinks enough that the relative humidity falls below 100%. The portion that has a relative humidity of 100% remains visible. Theories to how they form include: 1) turbulent eddies mixing down moisture, 2) evaporative cooling with surrounding air causes pockets of sinking air, 3) Pockets of precipitation falling out of the anvil that produce virga. Mammatus tend to be most prominent in extremely severe storms but can occur when storms are not severe also. D. Flanking Line- The flanking line is produced by convergence along an outflow boundary extending from the storm. This outflow is often air from aloft that is converged into warm and moist air near the surface. It can be seen as a line of developing cumulus clouds extending from the storm. The cumulus closer to the storm tend to be more mature and eventually merge into the parent storm. The flanking line often feeds into the updraft of the storm. E. Rain Core / Hail Core- The core refers to the heaviest precipitation. The most violent rain and hail in a supercell tend to be on the outer edge of the updraft on the downdraft side of the storm. Extreme turbulence on the edge of the updraft can contribute to significant hail growth. As hail falls into above freezing air it sheds its moisture as rain. F. Wall Cloud- The wall cloud is located in the updraft region of a supercell. Rising air cools and condenses out moisture once it is saturated. Due to the rapidly rising air and the verticality of the rising air, the cloud base is close to the ground within the wall cloud. The wall cloud will often be witnessed as rotating since directional wind shear acts on the updraft as it rises. Tornadoes can occur under the wall cloud. G. Rain-Free Base- The updraft region in supercells will often lack precipitation. This is most true for developing supercells and for classic/LP supercells. As a supercell matures or has a high moisture content, often precipitation will wrap around the updraft region and eventually fall into the updraft region. The updraft region of a supercell will be tilted with height. This will deposit the precipitation away from the updraft and thus this also results in less precipitation in the updraft region. Being in the rain free base region offers an awe-striking view of the storm. H. Forward Flank / Rear Flank Downdraft- The forward flank downdraft is the outflow from the rain-cooled air of the storm's downdraft. The rear flank downdraft is air from aloft that is transported down to the surface from colliding with the storm. The rear flank downdraft air tends to be dry and warm since the air warms by adiabatic compression as it sinks to the surface. Adiabatically warmed air will also decrease in relative humidity if no precipitation falls into the air. The rear flank downdraft tends to be warmer than the forward flank downdraft also since rain the evaporational cooling is not as common in the rear flank. Shear is enhanced along these flanking downdraft boundaries and the shear can be magnified along where the two flanks merge. The right balance of shear and instability release can lead to tornadogenesis.

Hazards
Thunderstorms are common occurrences in the Midwest and Central United States. Each year, an estimated 100,000 thunderstorms occur in the United States. Of those, about 10 percent are classified as severe thunderstorms - those that produce hail at least three-quarters of an inch in diameter, have winds of 58 miles per hour or higher, or produce a tornado. All thunderstorms are dangerous and can be associated with a number of hazards. Heavy rains can lead to flash flooding events – one of the primary causes of death associated with thunderstorms. Lightning, which is produced by every thunderstorm, causes an average of 80 fatalities and 300 injuries each year. Lightning can also start building fires, damage electrical equipment, electrocute humans and livestock, and is the leading cause of farm fires. High winds generated by thunderstorm can cause damage to homes, overturn vehicles, uproot or damage trees, or blow down utility poles causing wide spread power outages. Hail causes billions of dollars in damage to crops and property each year and can injure people or animals left outdoors.

Squall Lines
This is a group of storms arranged in a line, coming with “squalls” of high wind and heavy rain. These lines tend to pass quickly and are less prone to make tornadoes than supercells. They can be hundreds of miles long but are usually 10 or 20 miles wide. It’s any line of convective complexes. They are typically made of ordinary cells spread along/behind the leading edge of the system. They can, however, be made of multiple supercells. Behind the leading edge of the squall line is an extensive region of stratiform precipitation. The line of thunderstorms has a common lifting mechanism that tend to occur in bands. Some examples of these banded lifting mechanisms are fronts, large outflow boundaries, gravity waves, and isentropic lifting associated with CSI. The typical squall line will form ahead and parallel to a cold front or dry line boundary. The storms will first form where the best combo of moisture, instability, and lift is. The storms will evolve and new cells will form usually towards the south and east. The squall line will be sustained by its own production of lift caused by outflow boundaries. The squall line will persist if instability and moisture are present in front of the squall line.

Mesoscale Convective System (MCS)
This is a collection of thunderstorms acting as a system. These ca spread across an entire state and last more than 12 hours. These may appear as a solid line, broken line, or cluster of cells on radar.

Mesoscale Convective Complex (MCC)
This is a type of MCS that is large, circular, and long-lived. It is a cluster of showers and thunderstorms identifiable by satellite. It can emerge out of other storm types during the late night and early morning hours. These can cover an entire state.

Mesoscale Convective Vortex (MCV)
This is a low-pressure center within an MCS. It pulls the winds into a circling pattern or vortex. The core is 30 to 60 miles wide and 1 to 3 miles deep making this often overlooked in weather analyses. These can take on a life of their own and persist up to 12 hours after the MCS has dissipated. The abandoned MCV might become the seed of the next thunderstorm outbreak. If the MCV moves into tropical waters (like the Gulf of Mexico) it can be the nucleus of a tropical storm or hurricane.

Life Cycle
Some visual clues of a tornado formation is: a large and round rain-free base (suggests mesocyclone is present), an increasing spin in wall cloud and cloud base around wall cloud (suggests low-level rotation is increasing), clear slot formation meaning a bright cloud free notch in the rain free base (suggests rear-flank downdraft, a possible mechanism for tornado formation), rapid vertical motions (scud rising into wall, sinking motion around wall cloud from rear flank downdraft), and a local burst of heavy rain or hail that is just west or southwest of the wall cloud (another formation mechanism). A tornado would form within a few minutes of these clues unless a gust front or outflow spreads out from the storm and cuts off the process. The first stage of the tornado life cycle is the developing stage. In this stage, tornado circulation will begin in the mid-level and have developments toward the ground. The rear-flank downdraft or clear slot and rain burst southwest of the wall might help to get the circulation established on the ground. In some cases, circulation may start in the low levels near the cloud base. The first sign of a tornado might be a dust whirl with evidence of a connection to the cloud base. A funnel or tight rotation should be visible in the wall cloud/cloud base. Next is the mature stage which is the most dangerous part of the tornado’s life cycle. The funnel will be near vertical, but the visible funnel might not always reach the ground. The RFD or clear slot will wrap around the south and east side of the wall cloud and cut off the original inflow air. The rain free base might appear like a horseshoe with the tornado/wall cloud at the north end of said horseshoe. RFD air can be warm and moist, but will not harm the tornado. The final stage of the tornado is the dissipating stage where the RFD wraps around the tornado and cuts off original inflow. The tornado will take in RFD and cold outflow air from the precipitation. The funnel will shrink, tilt, and contort into a snake shape (rope stage). This stage is still dangerous, but not as large or strong as the mature stage. Large tornadoes may not go through this rope stage. Inflow may refocus a few miles east of the original tornado and a new wall cloud should be looked for. A multi-vortex tornado has two cyclones (vortices) circulating around each other and a central point. Landspouts occur over land from a cumulus cloud and are relatively weak. Waterspouts are the same thing, but over water.

Characteristics/Basics
A narrow, violently rotating column of air that extends from the base of a thunderstorm to the ground. Since wind is invisible, a tornado can be difficult to spot unless it forms around a condensation funnel made of water droplets, dust and debris. These storms are the most violent of all atmospheric storms. Tornadoes occur everywhere in the world, with Argentina and Bangladesh being the highest place of concentrations other than the U.S. About 1,200 tornadoes hit the U.S. each year. Texas, Kansas, and Oklahoma are the places with the most tornadoes (1st, 2nd, and 3rd). Tornado Alley is in Central Texas, goes north through Oklahoma, central Kansas/Nebraska, eastern South Dakota, and sometimes dog-legs east through Iowa, Missouri, Illinois, and Indiana to western Ohio. It is an area of relatively high tornado occurrence. Tornado season for the Southern Plains is from May into early June. For the gulf coast, it is during spring. For the northern plains and upper Midwest, tornado season is June and July. Tornadoes mainly occur between 4-9 p.m. They typically move from 10-20 mph. Supercell tornadoes are tornadoes that occur from a thunderstorm while non-supercell tornadoes are tornadoes that occur from already spinning air near to the ground. Some examples of these are gustnadoes, waterspouts, and landspouts.

Hazards
Tornado damage comes from the strong winds (can sometimes go up to 300 mph). These wind speeds can cause automobiles to go airborne, rip houses to shreds, and turn debris into missiles. The biggest threat to living beings is from flying debris and being tossed into the air. To prepare for these hazards, people should have a NOAA radio. People should know their safe places, have a plan, and have a disaster supply kit. In that kit, should be food, water, medicines, prescription copies, personal hygiene items, first aid supplies, important documents, personal identifications, insurance copies, cash/travelers check, flashlights, batteries, clothing, blankets, battery operated/crank radio, and cell phone chargers. Tornadoes can be invisible; wind speeds in a tornado may exceed 300mph.