Severe Storms/Winter Storms

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

Snow Storms/Winter 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: Storms are listed below
 * 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.

Blizzards
A severe snow storm with winds in excess of 35 mph and visibility of less than a 1/4 mile for more than 3 hours. Once these conditions are expected, the NWS will issue a "blizzard warning." When these conditions are expected to not all happen at the same time, but one or two will, a "Winter Storm Warning" or "Heavy Snow Warning" may be issued. Conditions of a blizzard develop on the northwest side of an intense storm system. The lower pressure in the storm and the higher pressure to the west's difference creates a tight pressure gradient. This means a difference in pressure between two locations resulting in very strong winds. The winds can then pick up snow off the ground or blow falling snow creating low visibilities and a chance for significant drifting of snow. Blizzards most often occur in the upper Midwest and Great Plains of the United States because of the flat land. But, blizzards can occur in any location that has snowfall. Northern Arizona may have blizzard conditions if a strong low pressure system moves across Southern Arizona and high pressure builds into the Great Basin. This does not occur frequently because of the low number of low pressure systems moving through the state. A ground blizzard is a weather condition where snow is not falling but loose snow on the ground is lifted and blown by strong winds. The primary difference between a ground blizzard as opposed to a regular blizzard is that in a ground blizzard no precipitation is produced at the time, but rather all the precipitation is already present in the form of snow or ice at the surface. In the United States, the National Weather Service defines a blizzard as a severe snowstorm characterized by strong winds causing blowing snow that results in low visibilities. The difference between a blizzard and a snowstorm is the strength of the wind, NOT the amount of snow. To be a blizzard, a snow storm must have sustained winds or frequent gusts that are greater than or equal to 56 km/h (35 mph) with blowing or drifting snow which reduces visibility to 400 m or 0.25 mi or less and must last for a prolonged period of time—typically three hours or more. While severe cold and large amounts of drifting snow may accompany blizzards, they are not required. Blizzards can bring whiteout conditions, and can paralyze regions for days at a time, particularly where snowfall is unusual or rare. A severe blizzard has winds over 72 km/h (45 mph), near zero visibility, and temperatures of −12°C (10°F) or lower. In Antarctica, blizzards are associated with winds spilling over the edge of the ice plateau at an average velocity of 160 km/h (99 mph). The Australia Bureau of Meteorology describes a blizzard as, "Violent and very cold wind which is laden with snow, some part, at least, of which has been raised from snow covered ground." Blizzard conditions of cold temperatures and strong winds can cause wind chill values that can result in hypothermia or frostbite. The wind chill factor is the amount of cooling the human body feels due to the combination of wind and temperature. Blizzards can also occur after snowfall when high winds cause whiteouts (fallen snow blowing around) and snowdrifts (huge mountains of snow), which decrease visibility. To avoid hypothermia if caught outdoors during a blizzard, stay hydrated and nourished. Keep blood flowing by moving around. Also build a snow cave to block winds, which reduce your body temperature. The first blizzard to be declared a Federal Emergency was in 1977, affecting upstate New York and Southern Ohio. The storm’s accumulation was only about 12 inches over 5 days, but the winds were deadly. The Storm of the 20th Century took place in March, 1993. It was iconic for its hurricane wind force and massive size. And stretched from Canada to mid-America. The blizzard caused roughly 300 deaths and 10 million power outages. Traveling by car or foot is highly discouraged during blizzard conditions. It increases the chance of hypothermia, accident and death. Many blizzards stem from Nor’Easters (storms traveling up the east coast of America). Moisture gathers from the Atlantic and dumps large accumulations of snow all the way from Delaware to Maine. Rochester, New York is said to be the largest city with the most snow in the United States, accumulating an average of 94 inches of snow every year. When a blizzard is in the forecast, you may receive a “Winter Storm Watch," which means there is a possibility of a storm taking effect. You could also receive a “Winter Storm Warning," which means a storm is on the way or already taking place. As soon as you receive a storm warning, get prepared. You could lose electricity (this includes hot water and heat), so stock up on non-perishable foods, blankets, flashlights, extra batteries, and candles beforehand. Blizzards can create conditions that threaten lives. Moving by automobile becomes difficult/impossible because of the "whiteout" conditions and drifting snow. Whiteout conditions occur the most with major storms producing a dry, powdery snow. In this situation, snow does not need to be falling in order to create whiteout situations because snow on the ground blows around lowering visibility to near zero sometimes. Strong winds/cold temperatures combine to create danger, such as the wind chill factor which can drop to –60F during blizzards in the Midwest. Hypothermia or frostbite can ensue in these situations. Power outages, pipe freezing, and the cutoff of regular fuel sources may also occur. A watch is issued when severe winter weather is probable because of the conditions. A warning is issued when hazardous winter weather is occurring, imminent, or there is a high probability. An advisory is issued when less serious conditions are occurring, imminent, or there is a high probability. For transportation safety, make sure your car is in good working condition, your gas tank must always be half full, good winter tires, high energy snacks and water is stored in your car, dress warmly, use public transportation if you must go outside, listen to the radio/call state highway patrol, travel during daylight with another person, plan long road trips carefully, and always carry a windshield scraper or broom in your car. At home/work you will want to be concerned about a loss of heat, power, telephone service, and supplies. You should have flashlights, extra batteries, weather/portable radio, extra food/water requiring no cooking/refrigeration, first aid supplies, medicine, baby items, heating fuel, emergency heat source, first aid kit, cold medications, allergy medications, sunscreen, and personal medications. Shoveling too much snow can kill people so make sure a very healthy person does it for short amounts of time.

Hazards
A blizzard's main dangers are its strong winds, freezing temperatures, and deep snow.
 * Find shelter indoors
 * Stay away from windows and doors
 * If you're stuck in a car with the engine running to stay warm, keep the windows open a little bit. This will let poisonous carbon monoxide escape from the inside of the car.
 * Keep extra food and water, a flashlight, a battery-powered radio, and, if possible, a cell phone with you
 * If trudging through deep snow, keep moving. Do not lie down to rest.
 * If caught outdoors, use clothing to cover your face and as much of skin as possible.

Nor'easters
A macro-scale cyclone. The name derives from the direction of the strongest winds—as an offshore air mass rotates counterclockwise, winds tend to blow northeast-to-southwest over the region covered by the northwest quadrant of the cyclone. Use of the term in North America is associated with several different types of storms, some of which can form in the North Atlantic Ocean and some of which form as far south as the Gulf of Mexico. The term is most often used in the coastal areas of New England and the Mid-Atlantic states. Typically, such storms originate as a low-pressure area that forms within 100 miles (160 km) of the shore between Georgia and New Jersey. The precipitation pattern is similar to that of other extratropical storms. Nor’easters are usually accompanied by very heavy rain or snow, and can cause severe coastal flooding, coastal erosion, hurricane-force winds, or blizzard conditions. Nor'easters are usually most intense during winter in the Mid-Atlantic, New England, and Canadian Maritimes. They thrive on converging air masses—the cold polar air mass and the warmer air over the water—and are more severe in winter when the difference in temperature between these air masses is greater. Nor'easters tend to develop most often and most powerfully between the months of September and April, although they can (less commonly) develop during other parts of the year as well. The Northeast megalopolis is very heavily affected by Nor'easters each winter, as the East Coast of the United States is a prime spot for these storms to develop, where warm air from the Atlantic Ocean collides with the arctic cold to the north and west. Nor'easters are usually formed by an area of vorticity associated with an upper-level disturbance or from a kink in a frontal surface that causes a surface low-pressure area to develop. Such storms are very often formed from the merging of several weaker storms, a "parent storm", and a polar jet stream mixing with the tropical jet stream. Until the nor'easter passes, thick, dark, low-level clouds often block out the sun. Temperatures usually fall significantly due to the presence of the cooler air from winds that typically come from a northeasterly direction. During a single storm, the precipitation can range from a torrential downpour to a fine mist. All precipitation types can occur in a nor'easter. High wind gusts, which can reach hurricane strength, are also associated with a nor'easter. On very rare occasions, such as in the North American blizzard of 2006 and a nor'easter in 1978, the center of the storm can take on the circular shape more typical of a hurricane and have a small eye. A northeaster is a storm that blows from the East Coast of North America, meaning it blows from the northeast. They are the most frequent and violent between September and April. Some hazards of these storms are economic, transportation, and human disruption along with coastal flooding. This damage can exceed a billion dollars. Typically developing between Georgia and New Jersey (100 miles east or west of the East Coast), the storms move northeastward and are at their strongest point around New England and the Maritime Provinces of Canada. They bring precipitation as heavy rain or snow along with strong winds, rough seas, and coastal flooding. The I-95 Corridor or the areas between Washington D.C., Philadelphia, New York and Boston is most often hit by these storms. During Winter, the polar jet stream moves Arctic air to the east area of the U.S. where warm air is moving upwards. The difference in temperatures in the water and air fuels these storms. Winter storm, blizzard, high wind and coastal flood watches may be issued if conditions are favorable for a nor’easter and may be changed to a warning if a storm is imminent. The National Centers for Environmental Prediction near Washington D.C. helps monitor these conditions. Winds circulate counter-clockwise around a low-pressure center. Some famous nor’easters are the Blizzard of 1888, the “Ash Wednesday” storm of March 1962, the New England Blizzard of February 1978, the March 1933 “Superstorm” and the recent Boston snowstorms of January and February 2015.

Lake-Effect Snow
Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water, warming the lower layer of air which picks up water vapor from the lake, rises up through the colder air above, freezes and is deposited on the leeward (downwind) shores. The same effect also occurs over bodies of salt water, when it is termed ocean-effect or bay-effect snow. The effect is enhanced when the moving air mass is uplifted by the orographic influence of higher elevations on the downwind shores. This uplifting can produce narrow but very intense bands of precipitation, which deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall. The areas affected by lake-effect snow are called snowbelts. These include areas east of the Great Lakes, the west coasts of northern Japan, the Kamchatka Peninsula in Russia, and areas near the Great Salt Lake, Black Sea, Caspian Sea, Baltic Sea, and parts of the northern Atlantic Ocean. A lake-effect blizzard is the blizzard-like conditions resulting from lake-effect snow. Under certain conditions, strong winds can accompany lake-effect snows creating blizzard-like conditions; however the duration of the event is often slightly less than that required for a blizzard warning in both the US and Canada. If the air temperature is low enough to keep the precipitation frozen, it falls as lake-effect snow. For lake-effect rain or snow to form, the air moving across the lake must be significantly cooler than the surface air (which is likely to be near the temperature of the water surface). Specifically, the air temperature at an altitude where the air pressure is 850 millibars (85 kPa) (roughly 1.5 kilometers or 0.93 miles vertically) should be 13 °C (23 °F) lower than the temperature of the air at the surface. Lake-effect occurring when the air at 850 millibars (85 kPa) is much colder than the water surface can produce thundersnow, snow showers accompanied by lightning and thunder (caused by larger amounts of energy available from the increased instability). There are several key elements that are required to form lake-effect precipitation and which determine its characteristics: instability, fetch, wind shear, upstream moisture, upwind lakes, synoptic (large)-scale forcing, orography/topography, and snow or ice cover. This is a very common occurrence across the Great Lakes during late fall and winter. It occurs when cold air from Canada moves across the waters of the Great Lakes, moisture transfers to the lowest part of the atmosphere. The warm air rises and leads to cloudiness and snow. It usually takes the form of narrow bands characterized by intense snowfall and limited visibility. Sometimes sunny skies can quickly be replaced by thus blinding and wind-driven snowfall in minutes. It is extremely dangerous to motorists. Lake effect snowstorms occur in only three places in the world: The Great Lakes, the east shore of Hudson Bay and along the west coasts of the Japanese islands of Honshu and Hokkaido.

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.

Severe Storms
Severe Storms listed below

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.

LIFE CYCLE

 * Genesis Stage-Tropical disturbances form in regions where there is a net inflow of air at the surface, known as convergence. When convergence occurs at the surface, it must ascend to balance this accumulation of air. As air rises, it will saturate and form the base of a cloud. Once the air is saturated, ascent may be enhanced where the atmosphere is in a state of static instability. In a statically unstable atmosphere, saturated air forced upward by convergence is less dense than surrounding unsaturated air. As a result, it accelerates upward because the air is buoyant relative to its environment, forming towering puffy clouds. While convergence in a statically unstable atmosphere is a first criterion, thunderstorm formation is common in the tropics. Much of the warm, humid tropics are already in a state of static instability. Several conditions must simultaneously exist for a tropical disturbance to develop complete rotation and become a tropical depression. First, the disturbance must be in a trough, defined as an elongated area of relatively low atmospheric pressure. Atmospheric pressure is the weight of a column of air on a given area on Earth, typically 1 meter or 1 centimeter squared. For genesis to occur, troughs must contain a weak, cyclonic rotation. All troughs at least 5 degrees from the equator will obtain a partial cyclonic spin due to the Coriolis force. The Coriolis force results from an apparent twisting of the north-south-east-west coordinate system as the Earth spins. The vast majority of genesis cases are associated with the monsoon though. However, the dynamics can be complex and are still not well understood. Some disturbances undergo the transition to tropical depression directly inside monsoon drought. Others experience this transition as tropical waves (called easterly waves for the US) that form when a monsoon trough breaks down into a cyclonic wavelike pattern in the wind field and travels westward away from this trough. One prerequisite for genesis is a trough at least 5 degrees from the equator where the Coriolis force can induce a partial cyclonic rotation, and these troughs fall into 3 categories: a monsoon trough, frontal trough, and surface trough. Tropical (easterly) waves sometimes break off from monsoon troughs due to dynamic instability and are also a source of genesis. The genesis time frame of both disturbance and depression lasts for several days or longer. However, under ideal conditions, a disturbance or depression can evolve much quicker. When the cyclonic sustained winds increase up to 39 mph somewhere in the depression, the system is upgraded to a tropical storm. At this point, the intensification stage begins.
 * Intensification Stage- For a tropical storm to intensify into a hurricane, the same conditions that allowed its initial development (warm water, moist air, and weak wind shear) must continue. When favorable environmental factors persists, the rate of development increases compared to the genesis stage. This is because as the wind increases, more moisture is transferred from the ocean to the air, and when this moisture changes from gas to liquid stage during cloud formation, latent heat associated with this phase change is released into the vortex. Furthermore, because the system has complete rotation, a larger percentage of this latent heat is retained in the storm (unlike in a non-rotating thunderstorm, in which all the latent heat released by the clouds just propagates away). The column of air begins to warm, which lowers surface pressure. More air will flow toward the lower surface pressure, trying to redistribute the atmosphere’s weight, resulting in faster winds. The faster the cyclonic winds also enhance convergence. Both factors increase thunderstorm production and low-level inflow. A feed-back  mechanism now occurs in which faster cyclonic winds breed more potent thunderstorms, dropping central surface pressure more and creating stronger inflow, which breeds faster cyclonic winds, et cetera. Under favorable conditions, a tropical storm can “spin up” rather quickly, with winds increasing as much as 50 mph or more in a day. When sustained winds reach 74 mph, the storm is a hurricane. Water temperature is linked to development of these storms. Hurricanes rarely form over water colder than 80 degrees F. They also weaken dramatically if a mature system moves over water colder than 80 degrees F or if they make landfall, since their heat and moisture source has been removed. For a hurricane to maintain thunderstorms through static instability, warm, moist surface air is required near the low-pressure center. This warmth is provided by sensible heat transfer from warm ocean water, because otherwise air flowing toward lower pressure would expand and cool. The warmer the water, the greater are the chances for genesis, the faster is the rate of development, and the stronger these storms can become. Under conditions of prolonged weak wind shear and water temperature greater than 85 degrees F, sustained winds may reach almost 200 mph.
 * Weakening Stage and Dissipation- Few hurricanes reach their maximum potential. Conditions that stop intensification include wind shear, landfall, dry air intrusion, storm induced ocean cooling by mixing or upwelling colder water beneath the warm surface water, and movement over colder water. Temporary occurrence of (any or a combination)  of these influences will stall development or cause weakening. When hurricanes significantly weaken over warm water, there are two possible culprits. The primary cause is usually vertical wind shear. Wind shear is the result of environmental wind direction changing with height, the environmental wind increasing by 20 mph or more with height, or a combination of both. Wind shear disrupts the vertical structure of the hurricane. Large hurricanes withstand wind shear better than small hurricanes. Even when no inhibiting factors are evident over warm water, hurricanes that reach their potential intensity rarely maintain their intensity for any appreciable period. Apparently, the Internal physics of a hurricane preclude a steady-state storm. Instead, strong wind conditions promote interior adjustments near the storm’s center. Persistent occurrence of any or several inhibiting factors will cause disintegration of the hurricane. Of these possibilities, most dissipating cases occur due to landfall or movement over colder water. When a hurricane moves over colder water, expansional cooling dominates that stabilizing the atmosphere, disintegrates the thunderstorm and weakens the hurricane. While warm water is significant, it’s just as important that the warm water is at least 100 feet deep. The upper ocean layer under a hurricane can cool due to enhanced air-sea exchanges, mixing of the layer by wind forcing, and mixing by ocean currents. A thick layer of warm water is required to reduce or offset these effects. In general, mixing results in 1-3 degrees F cooling the ocean under the hurricane. Also, a sluggish process occurs in slow-moving hurricanes where ocean water is transported away from the storm’s center; this is known as upwelling. Water from below is required to replenish the lost surface water. If the warm ocean layer is too thin, cold water from below the layer will be upwelled to the surface, cutting off the storm’s warm-water energy supply and weakening the storm. The hurricane can commit suicide by being stationary for an extended period of time. Should a hurricane stop moving for several days, it can mix and upwell the ocean significantly, replacing all the warm water with cold water, and the hurricane dissipates.

HAZARDS

 * Storm Surge: An abnormal rise of water generated by a storm's winds. Storm surge can reach heights well over 20 feet and can span hundreds of miles of coastline.
 * Storm Tide: Water level rise during a storm due to the combination of storm surge and the astronomical tide. The destructive power of storm surge and large battering waves can result in loss of life, buildings destroyed, beach and dune erosion and road and bridge damage along the coast. Storm surge can travel several miles inland. In estuaries and bayous, salt water intrusion endangers public health and the environment. Tropical cyclones often produce widespread, torrential rains in excess of 6 inches, which may result in deadly and destructive floods. In fact, flooding is the major threat from tropical cyclones for people living inland.
 * Flash flooding: Defined as a rapid rise in water levels, can occur quickly due to intense rainfall. Longer term flooding on rivers and streams can persist for several days after the storm. When approaching water on a roadway, always remember Turn Around Don't Drown. Rainfall amounts are not directly related to the strength of tropical cyclones but rather to the speed and size of the storm, as well as the geography of the area. Slower moving and larger storms produce more rainfall. In addition, mountainous terrain enhances rainfall from a tropical cyclone. Tropical storm-force winds are strong enough to be dangerous to those caught in them.
 * Winds: For this reason, emergency managers plan on having their evacuations complete and their personnel sheltered before the onset of tropical storm-force winds, not hurricane-force winds. Hurricane‐force winds, 74 mph or more, can destroy buildings and mobile homes. Winds can stay above hurricane strength well inland. In 2004, Hurricane Charley made landfall at Punta Gorda on the southwest Florida coast and produced major damage well inland across central Florida with gusts of more than 100 mph. Atlantic and Eastern Pacific hurricanes are classified into five categories according to the Saffir-Simpson Hurricane Wind Scale, which estimates potential property damage according to the hurricane's sustained wind speed.
 * Debris: Signs, roofing material, siding and small items left outside become flying missiles during hurricanes.
 * Rip Currents: Channeled currents of water flowing away from shore, usually extending past the line of breaking waves, that can pull even the strongest swimmers away from shore. In 2008, despite the fact that Hurricane Bertha was more than a 1,000 miles offshore, the storm resulted in rip currents that killed three people along the New Jersey coast and required 1,500 lifeguard rescues in Ocean City, Maryland, over a 1 week period. In 2009, all six deaths in the United States directly attributable to tropical cyclones occurred as the result of drowning from large waves or strong rip currents.
 * Tornadoes: Most often occur in thunderstorms embedded in rain bands well away from the center of the hurricane; however, they can also occur near the eyewall. Usually, tornadoes produced by tropical cyclones are relatively weak and short-lived, but they still pose a significant threat.

CHARACTERISTICS

 * Eye-a circular region of mostly calm weather at the center of tropical cyclones. It is usually 30-65 kilometers in diameter. It is also home to the lowest barometric pressure of the cyclone. Eyewall- the area of intense rain and wind (the most intense in a tropical cyclone) directly adjacent to the eye. Multiple eyewalls may be present during the eyewall replacement cycle, in which rainbands near the original eye strengthen and organize to form a new eyewall (generally occurs in major hurricanes). Rainbands -lines of clouds (and therefore precipitation) pointing toward the eye and extend outwards. Hurricane season is from June 1st to November 30th in the Atlantic and May 15th to November 30th in the Pacific. The Atlantic hurricane season lasts from June 1 to November 30, and the regions included are Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. The Eastern Pacific hurricane season lasts from May 15 to November 30, and the regions included are The Eastern Pacific Basin which extends to 140 degrees West.
 * ORIGIN/DISTRIBUTION: Hurricanes can form almost anywhere in the Tropical Atlantic Basin. Some more probable places for hurricanes to form are the Gulf of Mexico, Western Caribbean, and the Cape Verde Islands. Hurricanes that form between 5 and 30 degrees North latitude often move toward the West, and might be pulled North or Northwest. Once they reach 30 degrees North latitude, they move northeast.

DIFFERENCE BETWEEN TYPHOONS, HURRICANES, AND CYCLONES
Hurricanes, cyclones, and typhoons are all the same weather phenomenon; we just use different names for these storms in different places. In the Atlantic and Northeast Pacific, the term “hurricane” is used. The same type of disturbance in the Northwest Pacific is called a “typhoon” and “cyclones” occur in the South Pacific and Indian Ocean. The ingredients for these storms include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods we associate with this phenomenon. In the Atlantic, hurricane season officially runs June 1 to November 30. However, while 97 percent of tropical activity occurs during this time period, there is nothing magical in these dates, and hurricanes have occurred outside of these six months. The names rotate every six years.

Arctic Hurricane
A polar low is a small-scale, short-lived atmospheric low pressure system (depression) that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. The systems usually have a horizontal length scale of less than 1,000 kilometers (620 mi) and exist for no more than a couple of days. They are part of the larger class of meso-scale weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms. Such winter storms can cause bitter cold and crop freezes. Polar lows have been referred to by many other terms, such as polar meso-scale vortex, Arctic hurricane, Arctic low, and cold air depression. Today the term is usually reserved for the more vigorous systems that have near-surface winds of at least 17 m/s (38 mph). These are small, but intense cyclones that form in cold polar/arctic air found over the relatively warmer water. They range from being 100 to 500 km long and usually last from 12 to 36 hours. Over satellite images they appear as a comma cloud. They have an eye (the low) with spiral bands surrounding the eye. The heaviest snowfall and strongest winds are found close to the center. Thunderstorms and waterspouts are possible. They are fueled by convection above the warmer ocean water and typically become less strong over land, however some can produce blizzards over land. In the Northern Hemisphere they are most common in Winter. These storms are frequent in ice-free waters in the Polar region, like the Nordic seas, The Labrador Sea, the Gulf of Alaska, the Sea of Japan, and the polar waters in the Southern Hemisphere. Their life cycle consists of severe weather, followed by heavy precipitation (usually snow), and then strong surface winds.

Hurricanes
GALVESTON 1900, ATLANTIC-GULF 1919, MIAMI 1926, SAN FELIPE-OKEECHOBEE 1928, FLORIDA KEYS LABOR DAY 1935, NEW ENGLAND 1938, GREAT ATLANTIC 1944, CAROL AND EDNA 1954, HAZEL 1954, CONNIE AND DIANE 1955, AUDREY 1957, DONNA 1960, CAMILLE 1969, AGNES 1972, TROPICAL STORM CLAUDETTE 1979, ALICIA 1983, GILBERT 1988, HUGO 1989, ANDREW 1992, TROPICAL STORM ALBERTO 1994, OPAL 1995, MITCH 1998, FLOYD 1999, KEITH 2000, TROPICAL STORM ALLISON 2001, IRIS 2001, ISABEL 2003, CHARLEY 2004, FRANCES 2004, IVAN 2004, JEANNE 2004, DENNIS 2005, KATRINA 2005, RITA 2005, WILMA 2005, IKE 2008

Florida Hurricanes
Labor Day Hurricane 1935, 1926 Miami Hurricane, 1928 Okeechobee Hurricane. 1950-1974: Hurricane Donna, Easy, King, Cleo, Isbell, and Betsy. 1975-1999: 1985 Hurricane Andrew, Eloise, David, and Opal. 2000-Present: Hurricane Charley, Matthew Jeanne, Dennis, Wilma, and Hurricane Ivan. Hurricane names rotate every 6 years.