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.

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. enerally, 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. 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.

Types of Thunderstorms

 * 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.