Hazards: Exam 2

Hydrologic System

The hydrologic cycle begins with the evaporation of water from the surface of the ocean. As moist air is lifted, it cools and water vapor condenses to form clouds. Moisture is transported around the globe until it returns to the surface as precipitation. Once the water reaches the ground, one of two processes may occur; 1) some of the water may evaporate back into the atmosphere or 2) the water may penetrate the surface and become groundwater. Groundwater either seeps its way to into the oceans, rivers, and streams, or is released back into the atmosphere through transpiration. The balance of water that remains on the earth's surface is runoff, which empties into lakes, rivers and streams and is carried back to the oceans, where the cycle begins again.

Cyclone Dissipation

Tropical cyclones dissipate when they can no longer extract sufficient energy from warm ocean water. As mentioned above, a tropical cyclone can contribute to its own demise by stirring up deeper, cooler ocean waters. In addition, a storm that moves over land will abruptly lose its fuel source and quickly lose intensity. A tropical cyclone that remains over the ocean and moves into higher latitudes will change its structure and become extratropical as it encounters cooler water. The transformation from a tropical to an extratropical cyclone is marked by an increase in the storm's diameter and by a change in shape from circular to comma- or v-shaped as its rainbands reorganize. An extratropical cyclone typically has a higher central pressure and consequently has lower wind speeds. Extratropical cyclones, which are fueled by a north-to-south variation of temperature, weaken and dissipate in a few days. A circulation system passes through a sequence of stages as it intensifies into a mature tropical cyclone.

Recurrence Interval

A return period, also known as a recurrence interval (sometimes repeat interval) is an estimate of the likelihood of an event, such as an earthquake, flood or a river discharge flow to occur. It is a statistical measurement typically based on historic data denoting the average recurrence interval over an extended period of time, and is usually used for risk analysis (e.g. to decide whether a project should be allowed to go forward in a zone of a certain risk, or to design structures to withstand an event with a certain return period). The following analysis assumes that the probability of the event occurring does not vary over time and is independent of past events.

Flood Types

A river flood occurs when water levels rise over the top of river banks due to excessive rain from tropical systems making landfall, persistent thunderstorms over the same area for extended periods of time, combined rainfall and snowmelt, or an ice jam. A coastal flood, or the inundation of land areas along the coast, is caused by higher than average high tide and worsened by heavy rainfall and onshore winds (i.e., wind blowing landward from the ocean). Places like Charleston, South Carolina, and Savannah, Georgia, experience impacts from shallow coastal flooding several times a year because of coastal development and lower elevation. Storm surge is an abnormal rise in water level in coastal areas, over and above the regular astronomical tide, caused by forces generated from a severe storm's wind, waves, and low atmospheric pressure. Storm surge is extremely dangerous, because it is capable of flooding large coastal areas. Extreme flooding can occur in coastal areas particularly when storm surge coincides with normal high tide, resulting in storm tides reaching up to 20 feet or more in some cases. Along the coast, storm surge is often the greatest threat to life and property from a hurricane. In the past, large death tolls have resulted from the rise of the ocean associated with many of the major hurricanes that have made landfall. Hurricane Katrina (2005) is a prime example of the damage and devastation that can be caused by surge. At least 1500 persons lost their lives during Katrina and many of those deaths occurred directly, or indirectly, as a result of storm surge. What we do: Coastal and Inland Flooding Observation and Warning (CI-FLOW) is a collaborative prototype real-time system that predicts total water level in North Carolina. CI-FLOW captures the complex interaction between rainfall, river flows, waves, tides, and storm surge, and how they will impact ocean and water levels. CI-FLOW is currently being tested in real-time when coastal storms approach North Carolina. NOAA NWS forecasters have access to CI-FLOW during these events to help them make more accurate flood and flash flood forecasts and warnings. NSSL, with support from the NOAA National Sea Grant leads the unique interdisciplinary team of federal, state and university partners. Inland flooding occurs when moderate precipitation accumulates over several days, intense precipitation falls over a short period, or a river overflows because of an ice or debris jam or dam or levee failure. Hurricane Floyd (1999), aided by Tropical Storm Dennis (1999), caused widespread severe flooding that caused the majority of the $3 to 6 billion in damage reported after those storms. flash flood waters cross a road in a backwoods area A flash flood is caused by heavy or excessive rainfall in a short period of time, generally less than six hours. Flash floods are usually characterized by raging torrents after heavy rains that rip through river beds, urban streets, or mountain canyons sweeping everything before them. They can occur within minutes or a few hours of excessive rainfall. They can also occur even if no rain has fallen, for instance after a levee or dam has failed, or after a sudden release of water by a debris or ice jam. Areas damaged by wildfires are known as burn scars and are particularly susceptible to flash floods and debris flows during rainstorms. Rainfall that is normally absorbed by vegetation can run off almost instantly, causing creeks and drainage areas to flood much earlier and with higher magnitude than normal.

Urban Heat Island

An urban heat island (UHI) is an urban area or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. UHI is most noticeable during the summer and winter. The main cause of the urban heat island effect is from the modification of land surfaces.[1][2] Waste heat generated by energy usage is a secondary contributor.[3] As a population center grows, it tends to expand its area and increase its average temperature. The less-used term heat island refers to any area, populated or not, which is consistently hotter than the surrounding area.[4]

Coriolis Effect

Coriolis effect is an inertial force described by the 19th-century French engineer-mathematician Gustave-Gaspard Coriolis in 1835. Coriolis showed that, if the ordinary Newtonian laws of motion of bodies are to be used in a rotating frame of reference, an inertial force--acting to the right of the direction of body motion for counterclockwise rotation of the reference frame or to the left for clockwise rotation--must be included in the equations of motion. The effect of the Coriolis force is an apparent deflection of the path of an object that moves within a rotating coordinate system. The object does not actually deviate from its path, but it appears to do so because of the motion of the coordinate system.

Cyclone Category

Cyclones are divided into categories depending on the strength of the winds produced. There are many different classification scales but one you may be familiar with is the Saffir-Simpson Hurricane Scale. This scale is used to desribe storms in Hollywood movies, e.g. "Twister" and "The Day After Tomorrow". • The classifications (1-5) are intended primarily for use in measuring the potential damage and flooding (storm surge) a cyclone will cause upon landfall.

Global Pressure Patterns

Global pressure systems play a direct role in the geographic distribution of precipitation. This influence is especially apparent in the relationship between pressure patterns and the distribution of precipitation and climate in Africa. For the most part, there is a horizontal pattern of climates stretching from the equator to poleward of 30o N related to the location of the Intertropical Convergence Zone (ITCZ), the semi-permanent Subtropical High (STH), and cyclones of the midlatitudes. Starting at the equator are the rainiest climates on earth, the tropical rain forest (Af) and tropical monsoon (Am). The tropical rain forest is noted to for its high, year-round rain fall that often exceeds 100 inches in a year. The copious rainfall is a product of the warm moist air masses that converge into the low pressure belt that straddles the equator and uplifted by convection. The monsoon climate in Africa, like the Asian monsoon, has a notable dry period during the low sun season. The dry period is due to the presence of the subsiding air of the subtropical high, while the wet season is due to the presence of the ITCZ and moisture-bearing trade winds along the coast.

Effects of Cyclones

Horizontal wind High winds cause some of the most dramatic and damaging effects associated with tropical cyclones. In the most intense tropical cyclones, sustained winds may be as high as 240 km (150 miles) per hour, and gusts can exceed 320 km (200 miles) per hour. The length of time that a given location is exposed to extreme winds depends on the size of the storm and the speed at which it is moving. During a direct hit from a tropical cyclone, an area may endure high winds for several hours. In that time even the most solidly constructed buildings may begin to suffer damage. The force of the wind increases rapidly with its speed. Sustained winds of 100 km (62 miles) per hour exert a pressure of 718 pascals (15 pounds per square foot), while an approximate doubling of wind speed to 200 km (124 miles) per hour increases the pressure almost fivefold to 3,734 pascals. A building with a large surface area facing the wind may be subjected to immense forces. Some of the local variability in damage that is often observed during tropical cyclones is due to the direction that buildings face relative to the prevailing wind. Horizontal winds associated with a tropical cyclone vary in strength depending on the area of the storm in which they occur. The strongest winds are located in the right-forward quadrant of the storm, as measured along the line that the storm is moving. The intensification of winds in this quadrant is due to the additive effect of winds from the atmospheric flow in which the storm is embedded. For example, in a hurricane approaching the East Coast of the United States, the highest and most damaging winds are located to the northeast of the storm centre. Tornadoes The intense sustained winds present near the centre of tropical cyclones are responsible for inflicting heavy damage, but there is another wind hazard associated with these storms—tornadoes. Most tropical disturbances that reach storm intensity have tornadoes associated with them when they make landfall, though the tornadoes tend to be weaker than those observed in the Midwestern United States. The number of tornadoes varies, but about 75 percent of tropical cyclones generate fewer than 10. The largest number of tornadoes associated with a tropical cyclone was 141, reported in 1967 as Hurricane Beulah struck the Texas Gulf Coast in the United States. Tornadoes can occur in any location near the centre of the storm. At distances greater than 50 km (30 miles) from the centre, they are confined to the northeast quadrant of Northern Hemisphere storms and to the southwest quadrant of Southern Hemisphere storms. How the tornadoes are generated is not clear, but surface friction probably plays a role by causing the wind to slow as the tropical cyclone makes landfall. Wind speeds near the surface decrease while those at higher levels are less affected, setting up a low-level horizontal rotation that becomes tilted into the vertical by updrafts, thus providing the concentrated spin required for a tornado. Gusts, downbursts, and swirls In addition to tornadoes, tropical cyclones generate other localized damaging winds. When a tropical cyclone makes landfall, surface friction decreases wind speed but increases turbulence; this allows fast-moving air aloft to be transported down to the surface, thereby increasing the strength of wind gusts. There is also evidence of tropical cyclone downbursts, driven by evaporative cooling of air. These downbursts are similar to microbursts that may occur during severe thunderstorms. The winds associated with them typically flow in a different direction than those of the cyclone, allowing them to be identified. Other small-scale wind features associated with tropical cyclones are swirls. These are very small, intense, and short-lived vortices that occur under convective towers embedded in the eyewall. They are not classified as tornadoes because their peak winds last only a few seconds. Swirls may rotate in either a counterclockwise or a clockwise direction, and their peak winds are estimated to approach 320 km (200 miles) per hour. The storm surge In coastal regions an elevation of sea level—the storm surge—is often the deadliest phenomenon associated with tropical cyclones. A storm surge accompanying an intense tropical cyclone can be as high as 6 metres (20 feet). Most of the surge is caused by friction between the strong winds in the storm's eyewall and the ocean surface, which piles water up in the direction that the wind is blowing. For tropical cyclones in the Northern Hemisphere this effect is largest in the right-forward quadrant of the storm because the winds are strongest there. In the Southern Hemisphere the left-forward quadrant has the largest storm surge. Devastation in Pardip, Odisha, India, after a tropical cyclone in October 1999. A fertilizer plant at the port of Paradip, India, inundated by a storm surge after the Orissa ... AP A small part of the total storm surge is due to the change in atmospheric pressure across the tropical cyclone. The higher atmospheric pressure at the edges of the storm causes the ocean surface to bulge under the eye, where the pressure is lowest. However, the magnitude of this pressure-induced surge is minimal because the density of water is large compared with that of air. A pressure drop of 100 millibars across the diameter of the storm causes the sea surface under the eye to rise about 1 metre (3 feet). Flooding caused by the storm surge is responsible for most of the deaths associated with tropical cyclone landfalls. Extreme examples of storm surge fatalities include 6,000 deaths in Galveston, Texas, in 1900 and the loss of more than 300,000 lives in East Pakistan (now Bangladesh) in 1970 from a storm surge that was estimated to be 9 metres (30 feet) high. Improvements in forecasting the expected height of storm surges and the issuing of warnings are necessary as the population of coastal areas continues to increase. Rainfall Tropical cyclones typically bring large amounts of water into the areas they affect. Much of the water is due to rainfall associated with the deep convective clouds of the eyewall and with the rainbands of the outer edges of the storm. Rainfall rates are typically on the order of several centimetres per hour with shorter bursts of much higher rates. It is not uncommon for totals of 500 to 1,000 mm (20 to 40 inches) of rain to be reported over some regions. Rainfall rates such as these may overwhelm the capacity of storm drains, resulting in local flooding. Flooding may be particularly severe in low-lying regions such as in Bangladesh and the Gulf Coast of the United States. It is also a problem in areas where mountains and canyons concentrate the rainfall, as occurred in 1998 when floods caused by rains from Hurricane Mitch washed away entire towns in Honduras. Another source of high precipitation may be provided by the migration of moist air from the clouds of the mature tropical cyclone. When this moisture moves into areas of low pressure at higher latitudes, significant precipitation may result. An example of this occurred in 1983, when the remnants of the eastern Pacific Hurricane Octave moved into a Pacific cold front that had stalled over the southwestern United States, drenching the Arizona desert with 200 mm (8 inches) of rain in a three-day period. On average, that region receives 280 mm (11 inches) of rain in an entire year.

Classifying Cyclones

Intensity scales A wide range of wind speeds is possible between tropical cyclones of minimal strength and the most intense ones on record, and tropical cyclones can cause damage ranging from the breaking of tree limbs to the destruction of mobile homes and small buildings. To aid in issuing warnings to areas that may be affected by a storm, and to indicate the severity of the potential threat, numerical rating systems have been developed based on a storm's maximum wind speed and potential storm surge. For tropical systems in the Atlantic and eastern Pacific, the Saffir-Simpson hurricane scale is used. This scale ranks storms that already have reached hurricane strength. A similar scale used to categorize storms near Australia includes both tropical storms and tropical cyclones. Though these two scales have different starting points, the most intense rating in each—category 5—is similar. Numerical ranking scales are not utilized in any of the other ocean basins.

Remote Sensing Relationship

Remote sensing is the art and science of making measurements of the earth using sensors on airplanes or satellites. These sensors collect data in the form of images and provide specialized capabilities for manipulating, analyzing, and visualizing those images. Remote sensed imagery is integrated within a GIS.

Velocity

Storm relative velocity is simply base velocity with the average storm motion subtracted out. When storms are moving quickly, this makes it easier to spot green/red velocity couplets that are indicative of rotation and which might be masked out by the motion of the storm. As with base velocity, green is motion towards the radar and red indicates motion away.

Hydrograph

A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs). It can also refer to a graph showing the volume of water reaching a particular outfall, or location in a sewerage network. Graphs are commonly used in the design of sewerage, more specifically, the design of surface water sewerage systems and combined sewers.

Cyclones & Anticyclones

A cyclone is a storm or system of winds that rotates around a center of low atmospheric pressure. An anticyclone is a system of winds that rotates around a center of high atmospheric pressure. Distinctive weather patterns tend to be associated with both cyclones and anticyclones. Cyclones (commonly known as lows) generally are indicators of rain, clouds, and other forms of bad weather. Anticyclones (commonly known as highs) are predictors of fair weather. Winds in a cyclone blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Winds in an anticyclone blow just the opposite. Vertical air movements are associated with both cyclones and anticyclones. In cyclones, air close to the ground is forced inward toward the center of the cyclone, where pressure is lowest. It then begins to rise upward, expanding and cooling in the process. This cooling increases the humidity of the rising air, which results in cloudiness and high humidity in the cyclone. In anticyclones, the situation is reversed. Air at the center of an anticyclone is forced away from the high pressure that occurs there. That air is replaced in the center by a downward draft of air from higher altitudes. As this air moves downward, it is compressed and warmed. This warming reduces the humidity of the descending air, which results in few clouds and low humidity in the anticyclone.

Cyclone Naming

It is not uncommon for more than one tropical cyclonic system to be present in a given ocean basin at any given time. To aid forecasters in identifying the systems and issuing warnings, tropical disturbances are given numbers. When a system intensifies to tropical storm strength, it is given a name. In the United States, names given to hurricanes during World War II corresponded to radio code names for the letters of the alphabet (such as Able, Baker, and Charlie). In 1953 the U.S. National Weather Service began to identify hurricanes by female names, and in 1978 a series of alternating male and female names came into use. The lists of names are recycled every six years—that is, the 2003 list is used again in 2009, the 2004 list in 2010, and so on. Names of very intense, damaging, or otherwise newsworthy storms are retired. Names that will not be used again include Gilbert, a 1988 category 5 hurricane that had the lowest central atmospheric pressure (888 millibars) ever recorded in the Atlantic. Also retired is Mitch, the name of a category 5 hurricane that stalled off the coast of Honduras for two days in 1998 before slowly moving inland, inundating Central America with heavy rain and causing mudslides and floods that took nearly 10,000 lives. Another notable storm whose name has been retired was Hurricane Ivan, which reached category 5 on three separate occasions during its long life cycle in September 2004. Ivan almost completely destroyed all agricultural infrastructure in Grenada, wrecked much of that year's crops in Jamaica, leveled 1.1 million hectares (2.7 million acres) of timber in Alabama, and caused almost 100 deaths along its path. Pacific and Indian basin storms are named according to systems established by regional committees under the auspices of the World Meteorological Organization. Each region maintains its own list of names, and changes to the list (such as retiring a name) are ratified at formal meetings. Two or more lists of names are alternated each year for several regions, including the central North Pacific (i.e., the Hawaii region), the western North Pacific and South China Sea, the southern Indian Ocean west of 90° E, the western South Pacific Ocean, and Australia's eastern, central, and northern ocean regions. In some areas, such as the northern Indian Ocean, tropical cyclones are given numbers instead of names.

Latent and Sensible Heat

Latent and sensible heat are types of energy released or absorbed in the atmosphere. Latent heat is related to changes in phase between liquids, gases, and solids. Sensible heat is related to changes in temperature of a gas or object with no change in phase. Why do I care? Latent heat released during any kind of storm increases instability in the atmosphere potentially causing severe weather. Sensible heat causes change in temperature due to contact with colder or warmer air of surfaces.

Cyclone Formation

Tropical cyclones form only over warm ocean waters near the equator. • To form a cyclone, warm, moist air over the ocean rises upward from near the surface. As this air moves up and away from the ocean surface, it leaves is less air near the surface. So basically as the warm air rises, it causes an area of lower air pressure below. • Air from surrounding areas with higher air pressure pushes in to the low pressure area. Then this new "cool" air becomes warm and moist and rises, too. And the cycle continues... • As the warmed, moist air rises and cools the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean's heat and water evaporating from the ocean surface. • As the storm system rotates faster and faster, an eye forms in the centre. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye. When the winds in the rotating storm reach 39 mph (63 kmph), the storm is called a "tropical storm". And when the wind speeds reach 74 mph (119 kmph), the storm is officially a "tropical cyclone" or hurricane. Tropical cyclones usually weaken when they hit land, because they are no longer being "fed" by the energy from the warm ocean waters. However, they often move far inland, dumping many centimeters of rain and causing lots of wind damage before they die out completely. The fuel for a tropical cyclone is provided by a transfer of water vapour and heat from the warm ocean to the overlying air, primarily by evaporation from the sea surface. As the warm, moist air rises, it expands and cools, quickly becoming saturated and releasing latent heat through the condensation of water vapour. The column of air in the core of the developing disturbance is warmed and moistened by this process. The temperature difference between the warm, rising air and the cooler environment causes the rising air to become buoyant, further enhancing its upward movement. If the sea surface is too cool, there will not be enough heat available, and the evaporation rates will be too low to provide the tropical cyclone enough fuel. Energy supplies will also be cut off if the warm surface water layer is not deep enough, because the developing tropical system will modify the underlying ocean. Rain falling from the deep convective clouds will cool the sea surface, and the strong winds in the centre of the storm will create turbulence. If the resulting mixing brings cool water from below the surface layer to the surface, the fuel supply for the tropical system will be removed. The vertical motion of warm air is by itself inadequate to initiate the formation of a tropical system. However, if the warm, moist air flows into a preexisting atmospheric disturbance, further development will occur. As the rising air warms the core of the disturbance by both release of latent heat and direct heat transfer from the sea surface, the atmospheric pressure in the centre of the disturbance becomes lower. The decreasing pressure causes the surface winds to increase, which in turn increases the vapour and heat transfer and contributes to further rising of air. The warming of the core and the increased surface winds thus reinforce each other in a positive feedback mechanism.

Tracking Cyclones

Use of satellites and aircraft An array of geostationary satellites (those that remain over a fixed position on Earth) is operated by a number of countries. Each of these satellites provides continuous displays of Earth's surface in visible light and in infrared wavelengths. It is the latter that are most important in tracking the stages of tropical cyclone development. Infrared images show the temperatures of cloud tops, thus allowing the loosely organized convection associated with easterly waves to be detected by the presence of cold, high clouds. They also show the deep, organized convection characteristic of a tropical cyclone. Satellite images not only show a storm's location but also can be used to estimate its intensity because certain cloud patterns are characteristic of particular wind speeds. Although satellite images provide general information on the location and intensity of tropical cyclones, detailed information on a storm's strength and structure must be obtained directly, using aircraft. This information is essential in providing the most accurate warnings possible. Operational reconnaissance is done only by the United States for storms that may affect its continental landmass. No other country does this type of reconnaissance. Tropical cyclones in other ocean basins occur over a larger region, and most countries do not have the financial resources to maintain research aircraft. When evidence of a developing circulation is detected in the Atlantic or Caribbean, a U.S. Air Force C-130 aircraft is dispatched to determine if a closed circulation is present. The centre of circulation is noted, and an instrument called a dropsonde is released through the bottom of the aircraft to measure the temperature, humidity, atmospheric pressure, and wind speed. In many cases, the naming of a tropical storm, or its upgrade from tropical storm to tropical cyclone, is based on aircraft observations. Landfall forecasts Tropical storms developing in the world's ocean basins are tracked by various national weather services that have been designated Regional Specialized Meteorological Centres (RSMCs) by the World Meteorological Organization (WMO). The RSMCs are located at Miami, Florida, and Honolulu, Hawaii, U.S.; Tokyo, Japan; Nadi, Fiji; Darwin, Northern Territory, Australia; New Delhi, India; and Saint-Denis, Réunion. Warnings are also issued for more limited regions by Tropical Cyclone Warning Centres in a number of locations, including Port Moresby, Papua New Guinea; Wellington, New Zealand; and Perth, Western Australia, and Brisbane, Queensland, Australia. In addition, the Joint Typhoon Warning Centers in Hawaii are responsible for U.S. military forecasts in the western Pacific and Indian Oceans, which overlap a number of WMO regions of responsibility. Forecasting hurricane landfall and providing warnings for storms that will effect the United States is done by the National Hurricane Center in Miami. Forecasters use a variety of observational information from satellites and aircraft to determine the current location and intensity of the storm. This information is used along with computer forecast models to predict the future path and intensity of the storm. There are three basic types of computer models. The simplest ones use statistical relations based on the typical paths of hurricanes in a region, along with the assumption that the current observed motion of the storm will persist. A second type of model, called a statistical-dynamical model, forecasts the large-scale circulation by solving equations that describe changes in atmospheric pressure, wind, and moisture. Statistical relations that predict the track of the storm based on the large-scale conditions are then used to forecast the storm's future position. A third type of model is a purely dynamic forecast model. In this model, equations are solved that describe changes in both the large-scale circulation and the tropical cyclone itself. Dynamic forecast models show the interaction of the tropical cyclone with its environment, but they require the use of large and powerful computers as well as very complete descriptions of the structure of the tropical cyclone and that of the surrounding environment. Computer models currently do well in forecasting the path of tropical cyclones, but they are not as reliable in forecasting changes in intensity more than 24 hours in advance. Once forecasters have determined that a tropical cyclone is likely to make landfall, warnings are issued for the areas that may be affected. The forecasters provide a "best-track" forecast, which is an estimate of the track and maximum wind speed over a period of 72 hours based on all available observations and computer model results. Strike probability forecasts are issued that indicate probabilities (in percentages) that the tropical cyclone will affect a given area over a given time interval. These forecasts allow local authorities to begin warning and evacuation plans. As the storm approaches, a tropical cyclone watch is issued for areas that may be threatened. In especially vulnerable areas, evacuation may be initiated based on the watch. If tropical cyclone conditions are expected in an area within 24 hours, a tropical cyclone warning is issued. Once a warning is issued, evacuation is recommended for areas prone to storm surges and areas that may be isolated by high water. Long-term forecasts Forecasts of expected numbers of Atlantic tropical cyclones are now being made well in advance of the start of each year's tropical cyclone season. The forecast model takes into account seasonal trends in factors related to tropical cyclone formation such as the presence of El Niño or La Niña oceanic conditions (see the section below), amount of rainfall over Africa, winds in the lower stratosphere, and atmospheric pressure and wind tendencies over the Caribbean. Based on these factors, forecasts are issued concerning the expected numbers of tropical storms, tropical cyclones, and intense tropical cyclones for the Atlantic. These forecasts are issued in December, and they are revised in June and again in August of each year for the current Atlantic tropical cyclone season. The forecast model has displayed reasonable skill in predicting the total number of storms each season. Tropical cyclone track forecasting involves predicting where a tropical cyclone is going to track over the next five days, every 6 to 12 hours. The history of tropical cyclone track forecasting has evolved from a single-station approach to a comprehensive approach which uses a variety of meteorological tools and methods to make predictions. The weather of a particular location can show signs of the approaching tropical cyclone, such as increasing swell, increasing cloudiness, falling barometric pressure, increasing tides, squalls, and heavy rainfall. The forces that affect tropical cyclone steering are the higher-latitude westerlies, the subtropical ridge, and the beta effect caused by changes of the coriolis force within fluids such as the atmosphere. Accurate track predictions depend on determining the position and strength of high- and low-pressure areas, and predicting how those areas will migrate during the life of a tropical system. Computer forecast models are used to help determine this motion as far out as five to seven days in the future.

Wind Shear

Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with change in altitude. Horizontal wind shear is a change in wind speed with change in lateral position for a given altitude.[1] Wind shear is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts. It is commonly observed near microbursts and downbursts caused by thunderstorms, fronts, areas of locally higher low-level winds referred to as low level jets, near mountains, radiation inversions that occur due to clear skies and calm winds, buildings, wind turbines, and sailboats. Wind shear has a significant effect during take-off and landing of aircraft due to its effects on control of the aircraft, and it has been a sole or contributing cause of many aircraft accidents. As wind blows across the many different levels of the atmosphere, there are changes in wind speed and direction. These changes in speed and direction over a relatively short distance in the atmosphere are known as wind shear. Wind shear is most prevalent in the high latitudes as well as close to the jet stream; however, this shear plays a crucial role in the tropics as well. One critical factor in determining tropical cyclone development is calculating the shear of the environment over which the tropical cyclone will be above. Through research and observation, two of the primary reasons that ordinary disturbances in the tropics can acquire tropical characteristics are due to the presence of warm sea surface temperatures as well as low values of wind shear. As a tropical cyclone is developing, heavy thunderstorms will build up near the center. Given a favorable environment, the whole system from sea level up to around 50,000 feet in the atmosphere will eventually begin to turn counter-clockwise (or cyclonically). Without wind shear, the turning within the tropical system will be uniform, or vertically aligned, which helps to keep the storm intact and likely strengthening. If, however, strong wind shear is present, then a system's structure will be vertically tilted toward the direction that the wind shear is blowing. A vertically tilted system is inefficient at drawing in warm and moist air from the ocean necessary for intensification; therefore, the storm never fully develops tropical characteristics. If a well-defined hurricane moves from a region of low wind shear into an area of high wind shear, the vertically aligned center of the storm will become tilted in the direction of the wind shear and likely cause weakening or "tearing apart" of the center of the storm. It is essential for meteorologists to determine and monitor wind shear across any tropical basin in order to forecast a tropical system's intensity as accurately as possible.

Watch vs. Warning

With the severe weather season underway, now is a good time to go over the difference between tornado and severe thunderstorm watches and warnings. A WATCH means that the potential exists for the development of severe thunderstorms or tornadoes, depending upon the specific type of watch issued. In the case of a tornado watch, this DOES NOT mean that a tornado has been seen or even indicated on radar...it just means that conditions are favorable for the formation of tornadoes in thunderstorms. Similarly, a severe thunderstorm watch means that conditions are just conducive for the development of severe weather, and DOES NOT indicate that severe weather has been reported. While no immediate action on the part of the general public is required for the issuance of a watch, citizens should keep up to date on the current weather situation and be prepared to seek shelter if necessary. A WARNING, on the other hand, requires more immediate action and should be taken seriously. A severe thunderstorm warning indicates that severe weather is imminent in your area or is already occurring (based on either human observation or doppler radar). The term severe refers to hail greater than or equal to 1.00" in diameter and/or wind gusts that meet or exceed 58 mph. Although these storms can also be associated with dangerous cloud to ground lightning or heavy rainfall that is capable of causing flash flooding, neither of these two items serve as criteria for a severe thunderstorm warning being issued. A tornado warning can be thought of as a very specific severe thunderstorm warning, and means that a tornado has been either spotted by a human observer or indicated by doppler radar. Similar to a severe thunderstorm warning, once a tornado warning is issued for your area, you should take cover immediately...as the likelihood of you being directly impacted by a tornado is at a heightened risk. With this knowledge of watches and warnings, you will be better prepared the next time severe weather threatens your area.