text: Wind Shear Tutorial 
   
   Wind shear is often the most critical factor controlling hurricane formation and destruction. In general, wind shear refers to any change in wind speed or direction along a straight line. In the case of hurricanes, wind shear is important primarily in the vertical direction--from the surface to the top of the troposphere. The troposphere is the region of the atmosphere that our active weather is confined to, and extends up to about 40,000 feet altitude (a pressure of about 200 mb) in the tropics in summer. Hurricanes fill the entire vertical extent of the troposphere, and are steered by the average wind through this layer. When one hears the phrase, "wind shear is 20 knots over the hurricane", this typically refers to the difference in wind speed between 200 mb (the top of the troposphere, 40,000 feet altitude) and a layer where a pressure of 850 mb is found--about 5,000 feet above the surface. This wind shear is computed over a large area--a circle of 700 miles in diameter centered on the hurricane is one technique used. This 200-850 mb wind shear is a crude measure of the actual wind shear a storm experiences, since only changes in wind speed--not wind direction--are considered. Furthermore, the computed shear does not consider any smaller scale changes that may occur within this large volume of the atmosphere. For example, it is common to find a strong jet of wind at about 600 mb blowing along the edge of the Saharan Air Layer (SAL)--that area of dry, dusty air that frequently lies to the north of developing tropical cyclones in the mid-Atlantic. This jet will create significant wind shear that will not show up on the standard 200-850 mb wind shear plots. Since upper-air measurements are very sparse over the open ocean, wind shear that is invisible on 200-850 mb wind shear analysis charts will often unexpectedly kill or weaken a developing tropical cyclone. 
   
   Tropical cyclones are heat engines powered by the release of latent heat when water vapor condenses into liquid water. Wind shear hurts tropical cyclones by removing the heat and moisture they need from the area near their center. Shear will also distort the shape of a hurricane by shearing it (blowing the top away from the lower portion), so that the vortex is tilted. A tilted vortex is usually a less efficient heat engine--the delicate balance of inflowing low-level winds and outflowing upper-level winds that ventilate the storm gets disrupted. Dr. Bill Gray of Colorado State University was one of the first scientists to study the effect of winds shear on hurricane formation. In his classic 1968 paper, "Global View of the Origin of Tropical Disturbances and Storms", Dr. Gray writes: 
   
   "In the SW Atlantic and central Pacific, where tropical storms do not occur, the observed climatological tropospheric wind shear is large (i.e., 20-40 kt). This is believed to be the major inhibitor to development in these areas. Large vertical wind shears do not allow for area concentration of the tropospheric distributed cumulonimbus condensation. Large shears produce a large ventilation of heat away from the developing disturbance. The condensation heat released by the cumulus to the upper troposphere is adverted in a different direction relative to the released heat at lower levels. Concentration of heat through the entire troposphere becomes more difficult. 
   
   Dr. Gray also discovered that the east-west or "zonal" component of the wind shear was what mattered most to hurricanes. Wind shear in the north-south or "meridional" direction did not significantly affect the storms. This is why one often sees "zonal wind shear" plotted in addition to the total wind shear. (By the way, we now know that tropical storms do occur in the central Pacific, thanks to satellite imagery--and we have also had one tropical cyclone in the SW Atlantic, Hurricane Catarina of 2004)." 
   
   Rules of thumb 
   
   A general rule of thumb is that the shear must be 20 knots or less for intensification to occur. Most instances of rapid intensification of hurricanes occur when the wind shear is 10 knots or less. However, large and powerful hurricanes can be resistant to shear values as high as 40 knots, as demonstrated by Hurricane Wilma (Figure 1). We often see tropical disturbances under 10 knots of wind shear that do not develop. Why? Oftentimes, this is because cold, dry air aloft associated with an upper level trough of low pressure is interfering with development. Tropical cyclones develop most readily when an upper level anticyclone (high pressure system aloft) is present overhead. 
   
   Figure 1. Wind shear plot of Hurricane Wilma at 00GMT October 25, 2005. At time time, Wilma had just intensified to a Category 3 hurricane with 125 mph winds and a 955 mb pressure, despite the presence of 40 knots of wind shear over the storm. Just to the west of Wilma one can see wind shear values of 120 knots, associated with the jet stream. 
   
   One excellent web site to diagnose current wind shear values is the University of Wisconsin CIMSS site. They compute upper level winds by looking at cloud motion from satellites. A mean low level wind, averaged over a layer between 925 mb and 700 mb (1500 feet to 10,000 feet), is subtracted from a mean upper level wind, averaged over a layer between 300 mb and 150 mb (30,000 to 45,000 feet). If a tropical cyclone is present, the winds due to the circulation around the storm are removed, so that one can just look at the environmental wind field the storm is embedded in. A sample of one of their wind shear analyses is shown in Figure 1. Yellow contour lines mark wind shear values in knots. Pink lines with arrows show the direction of the shear (which is almost always the same as the direction of the upper level winds at 200 mb, since winds aloft are almost always stronger than winds near the surface). The contour lines are overlaid on a water vapor satellite image. Dark areas show where dry air is. The CIMSS plots are updated every three hours, and available about two hours after the time stamped on the image. All times are Greenwich time (UTC), four hours ahead of EDT. 
   
   Shear example: Tropical Storm Gabrielle of 2007 
   
   Figure 2 shows a still image of the disturbance that eventually became Tropical Storm Gabrielle of 2007. At the time, Gabrielle had a well defined circulation from the surface to 850 mb, but could not develop any heavy thunderstorm activity near the center because strong upper level winds from the southwest blew away the thunderstorms as quickly as they formed. A 0.5Mb animation of satellite images shows this process quite clearly. The low level vortex of Gabrielle is stationary, and one can see thunderstorms trying to form at the center get sheared away by strong upper level southwesterly winds at 200 mb. The winds carry high, wispy cirrus clouds from lower left to upper right in the animation. These winds blew at about 20 knots (Figure 3), creating 20 knots of wind shear over the center of Gabrielle (where the winds were calm). Just north of the Gabrielle's center, the low-level flow from the surface to 850 mb (5,000 feet) was out of the northeast at about 10 knots, due to the counter-clockwise flow of air around the vortex. As one can see from Figure 3, the shear was thus 20 knots at 200 mb minus negative 10 knots at 850 mb (since the 850 mb winds were blowing in the opposite direction as the 200 mb winds), for a total wind shear of 30 knots. To the south of the vortex, the surface to 850 mb flow was out of the southwest (same direction as the 200 mb flow), so the shear was 20 knots minus positive 10 knots, for a total of 10 knots of shear. 
   
   Figure 2. Visible satellite image of the tropical disturbance that eventually became Tropical Storm Gabrielle of 2007. The thick band of clouds extending from southwest to northeast is due to the presence of an upper level trough of low pressure. Image credit: University of Wisconsin CIMSS satellite blog. ||||| Irma may become Category 4 hurricane while tracking across Atlantic; Will it affect the US? 
   
   You will be redirected to the latest story on Irma in a moment. 
   
   While the Gulf Coast continues to deal with the devastating impacts of Harvey, emergency managers in the United States have another tropical threat to monitor by the name of Irma. 
   
   Far across the Atlantic, west of the Cabo Verde Islands, Irma is a Category 3 hurricane. 
   
   "There is the potential for Irma to ramp up to an even more powerful hurricane into early week," according to AccuWeather Hurricane Expert Dan Kottlowski. 
   
   
   
   
   
   "While fluctuation in strength is likely, we expect Irma to become a Category 4 well before it reaches waters near the Lesser Antilles," Kottlowski said. 
   
   A Category 4 hurricane has sustained winds of 130-156 mph (209-251 km/h). 
   
   Subtle changes in water temperatures and atmospheric conditions, such as slightly drier air and a small patch of strong winds aloft, can cause significant fluctuations in strength in even the strongest of hurricanes. 
   
   As of early Sunday morning, Irma was located about 945 miles (1,520 km) east of the Leeward Islands with maximum sustained winds of 115 mph (185 km/h). 
   
   Irma will take about a week to make its trek westward across the Atlantic Ocean. Meteorologists will likely be tracking this storm through the middle of September. 
   
   "All interests in the eastern Caribbean will need to monitor the progress of this evolving and dangerous hurricane," Kottlowski said. 
   
   "Surf will begin to build on the east-facing beaches in the Leeward and Windward islands into early week," according to AccuWeather Senior Meteorologist Alex Sosnowski. 
   
   How much seas, surf and winds increase during the first part of the week will depend on the exact track of Irma. 
   
   “Steering winds will guide Irma close to the Leeward Islands and then perhaps Puerto Rico and Hispaniola during the middle days of the week,” according to AccuWeather Meteorologist Brian Thompson. 
   
   
   
   
   
   "While the hurricane is strong, it is relatively small in size at this time," AccuWeather Senior Meteorologist Alex Sosnowski said. "Should this small size continue, severe effects from the storm may be limited to a radius 50 miles (80 km) of the center, while the storm moves through the tropics." 
   
   Locally heavy showers and gusty thunderstorms may extend beyond 100 miles (160 km) from the eye and could bring torrential rain and flooding to the islands. 
   
   Beyond early this week, there is a wide range as to where Irma may go and the storm may grow in size. 
   
   Plenty of atmospheric factors highlighted by non-tropical systems will come into play to determine its path. These non-tropical systems will be moving around. 
   
   "At this early stage, it is unclear whether a non-tropical storm will draw Irma toward the U.S., push it away or miss affecting it entirely prior to the middle of the month," Kottlowski said. 
   
   Possibilities range from a landfall from the northern Caribbean islands to Florida, the Carolinas or Bermuda. There is also a chance the system curves northward and avoids North America entirely. 
   
   "At some point this week, Irma will begin to take a more west-northwest path," Kottlowski said. "Whether Irma then continues to curve northwestward and then northward or remains on a steady west-northwest path is uncertain." 
   
   RELATED: 
   
   AccuWeather hurricane center 
   
   Downpours may hinder cleanup and recovery efforts in Texas, Louisiana early this week 
   
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   AccuWeather predicts Hurricane Harvey to be more costly than Katrina, Sandy combined 
   
   Reports: Water supply cut off for entire city of Beaumont, Texas, amid Harvey’s horrific flooding 
   
   More details on the path of Irma will follow in the coming days. 
   
   Residents who live along the East Coast or have interests in the Caribbean Sea should take time to review emergency procedures and make sure they are prepared for extreme tropical conditions. 
   
   Cruise and shipping interests may need to adjust their route as Irma approaches. 
   
   Report a Typo ||||| (CNN) While much attention remains on Texas and the destruction left by Hurricane Harvey and its historic rainfall , powerful Hurricane Irma is rapidly intensifying in the open Atlantic and poses a major threat to the Caribbean and potentially to the United States next week. 
   
   Satellite image taken at 8 a.m. ET Friday shows Hurricane Irma in the eastern Atlantic 
   
   With the storm still five days away from the outermost Caribbean islands and at least a week away from any potential US impacts, there is still a lot of uncertainty about where it will go. 
   
   The range of possibilities presented by the forecast models more than a week out literally spreads from Mexico to Canada -- and everywhere in between. 
   
   Good bet at a casino is an intense Hurricane Irma at Cat 4 or 5. But it's still red or black on U.S. impacts. Hope it's a "fish storm" pic.twitter.com/w6rWbbGVts 
   
   Irma was designated a tropical storm Wednesday morning, and by Thursday afternoon, it had strengthened into a large Category 3 hurricane, with winds of 115 mph. 
   
   Such explosive strengthening is known as "rapid intensification," defined by the National Hurricane Center as having its wind speed increase at least 30 knots (35 mph) in 24 hours. 
   
   "Irma has become an impressive hurricane," the National Hurricane Center said Thursday, noting the rapid intensification. "This is a remarkable 50 knot (58 mph) increase from yesterday at this time." 
   
   Hurricane Harvey underwent rapid intensification just before it made landfall late last Friday, strengthening quickly from a tropical storm into a Category 4 hurricane when it moved onshore near Corpus Christi, Texas. 
   
   Hurricane kryptonite not in the mix 
   
   Irma is a classic " Cape Verde hurricane ," a type of hurricane that forms in the far eastern Atlantic, near the Cape Verde Islands (now known as the Cabo Verde Islands), then tracks all the way across the Atlantic. Cape Verde storms frequently become some of the largest and most intense hurricanes. Examples are Hurricane Hugo, Hurricane Floyd, and Hurricane Ivan 
   
   Hurricane Irma is forecast to continue to strengthen as it moves westward over the next five days, and the official forecast from the National Hurricane Center puts a dangerous Category 4 Hurricane Irma on the doorstep of the Caribbean by the end of the five-day forecast on Wednesday. 
   
   #Irma has continued to strengthen over the eastern Atlantic and is now a major hurricane. Details at https://t.co/tW4KeGdBFb pic.twitter.com/gRAl1hzRnN — NHC Atlantic Ops (@NHC_Atlantic) August 31, 2017 
   
   A strong high-pressure ridge to the north of Irma, over the Atlantic, is steering the storm to the west and limiting the wind shear in the upper levels of the atmosphere, which has allowed the storm to grow so quickly. Wind shear is like hurricane kryptonite, and prevents storms from forming or gaining strength. 
   
   Unfortunately, Irma will remain in a low-shear environment for the next several days, so there isn't much hope that Irma will weaken any time soon. 
   
   There is considerable confidence that Hurricane Irma will track to the west through the weekend and then take a slight jog to the southwest early next week in response "to a building ridge (of high pressure) over the central Atlantic." 
   
   From there, the forecast becomes a lot less clear, with some major differences among some of the key models meteorologists use to forecast hurricanes. The differences are so drastic that one prediction has Irma sliding harmlessly back out to sea, while in another, it makes multiple disastrous landfalls in the Caribbean and likely hits the United States after that. 
   
   Dueling prediction models 
   
   The European model, or ECMWF, and the American GFS model have had some notable showdowns before, most notably with Hurricane Sandy. 
   
   With Sandy, the ECMWF correctly predicted a landfall in the Northeast nearly a week ahead, while the GFS continually kept the storm offshore in what became a major black eye for the US weather-modeling industry . There have been other examples in which the GFS model has outperformed its European counterpart, such as with a few major snowstorms in the Northeast. 
   
   Right now, the GFS has Irma taking a more northerly track that curves to the north before it reaches the Caribbean, thus making a US landfall much less likely. 
   
   The European model keeps the storm tracking further west, nearing the Bahamas by the end of next week. 
   
   European vs American weather models 
   
   "The ECMWF sees a much stronger ridge or Bermuda High (than the GFS) which forces Irma west, whereas the GFS has a weaker ridge and a more rightward, parabolic track," said Ryan Maue, a meteorologist with WeatherBell Analytics. 
   
   "The prospects for major impacts anywhere from Cuba to Carolinas is concerning for this very reliable model," Maue said. 
   
   Irma is still more than 1,700 miles east of the Leeward Islands, and any impacts from the storm wouldn't be felt until Tuesday or Wednesday for the Leeward Islands and Puerto Rico. 
   
   The forecast picture should become clearer after the weekend. 
   
   Bottom line: Hurricane Irma is already a powerful hurricane and looks to only become more so. Those with interests in the Caribbean and southeast US coast should pay close attention to the forecast. ||||| CHICAGO (Reuters) - Hurricane Harvey has left major devastation in Texas, but there is another hurricane called Irma out in the Atlantic that could take aim at the United States within the next week or so. 
   
   Irma became a hurricane early on Thursday and by the evening it was upgraded to a major hurricane, which requires sustained winds greater than 110 miles (177 km) per hour (reut.rs/2wqRtvW). 
   
   Given the early stages, weather models are not in total agreement over Irma’s likely path or potential power, but all hurricane-prone areas - from the Gulf of Mexico clear up the Eastern Seaboard - could be in its target. 
   
   Concerns are heightened for residents in coastal regions of the Southern and Eastern United States in the wake of Harvey, which dumped a record amount of rainfall for the continental United States over the Houston area within the past few days, leaving tens of thousands homeless. 
   
   The damage is so devastating that Texas could require as much as $125 billion in aid from the U.S. government, larger than the record $110.2 billion aid package assembled in 2005 following Hurricane Katrina. 
   
   Recent runs of the U.S. GFS model suggest Irma could gain significant strength and impact the Eastern Seaboard the weekend of Sept. 9. Whether that means a direct hit on the Carolinas or the Northeast, or just an offshore encounter is still uncertain. 
   
   The midday Thursday European model shows some more alarming possibilities, with the hurricane gaining strength, clipping the southern end of Florida, and heading into the Gulf of Mexico – perhaps the worst-case scenario for those still reeling from Harvey. 
   
   Although the Atlantic hurricane season is near its peak and storms in close succession are common, the prospect of another hurricane is stomach-churning. In addition to the human impact, Harvey has sent shockwaves through the transportation, energy, and retail industries, causing supply shortages and billions in losses. 
   
   COMMODITY MARKETS JOLTED 
   
   With Houston at the heart of the U.S. oil and gas industry, Harvey’s hit has been felt most directly in the energy markets. The catastrophic flooding has shut down nearly a quarter of U.S. oil-refining capacity, threatening a national supply crunch. 
   
   As such, benchmark U.S. gasoline prices hit two-year highs on Monday and tested nearly three-year highs on Thursday, and gasoline margins have rallied to the strongest levels in five years for this time of year. 
   
   Transportation woes are plaguing the agriculture market. Grain elevators in coastal Texas have not been significantly damaged, but loading for export has halted as they are unable to receive supplies due to closure of flooded rail lines in the Houston area. 
   
   Cattle and cotton have also been affected, as Texas is the No. 1 producer of each. Some 27 percent of the state’s 4.46 million-head beef cow herd resides in the disaster area and cotton crop losses are estimated around $150 million, though the vast majority of cotton production takes place in the Panhandle, out of the storm’s path. 
   
   Harvey’s remnants are now moving into the U.S. Delta states, where 20 percent of the nation’s cotton and 15 percent of soybeans are grown. Soybean harvest has just begun in the far south areas, but parts of the region could receive up to 8 inches (0.2 m) over the next five days. 
   
   ACTIVE SEASON PREDICTED 
   
   The hurricane threat in the North Atlantic Basin is far from over, though. 
   
   Back in May, forecasters at the National Hurricane Center gave the 2017 Atlantic hurricane season – which began on June 1 – a 45 percent chance of featuring above-normal activity. In its Aug. 9 update, NHC bumped these chances to 60 percent and noted “the possibility that the season could be extremely active.” 
   
   Forecasters point to a few primary reasons. One is that the large-scale climate indicator El Niño-Southern Oscillation – or ENSO – has much higher chances of sustaining neutral conditions through the end of the year rather than trend toward the warm phase El Niño as earlier forecasts suggested. 
   
   El Niño often suppresses tropical storm development in the North Atlantic since it features greater atmospheric stability and stronger winds that make it difficult for big, organized storms to form. ENSO’s cool phase, La Niña, usually promotes a favorable environment for tropical development in the Atlantic. 
   
   Additionally, sea surface temperatures in the key formation region of the ocean are warmer than models had previously predicted and the West African monsoon is stronger. Tropical cyclone formation in the North Atlantic often begins with an atmospheric wave off the African coast and most generally, warmer waters provide more fuel to the storm. 
   
   As of Aug. 31, there have been nine named storms in the North Atlantic Basin – above the long-term average of five for the date – so the season has been more active than normal so far, including a rare April-forming tropical storm. 
   
   NHC forecasters also mention that the enhanced early-season activity back in June and July in the tropical Atlantic usually suggests an above-average season ahead. 
   
   The climatological peak of the Atlantic hurricane season is Sept. 10 and the season officially ends on Nov. 30. Activity can persist into December, but these storms usually never make it to hurricane status and have occurred only a handful of times in the last couple decades. 
   
   During August and September, all coastal areas along the Gulf of Mexico and Atlantic Ocean are historically susceptible to tropical cyclones that could make landfall. Into October and beyond, this threat tends to focus more on Florida and the Eastern Seaboard rather than the Gulf (reut.rs/2wlDF7D). 
   
   But to have another storm as severe as Harvey anytime soon may be unlikely, as Harvey was statistically rare. It was only the fifth hurricane in the last 50 years to make landfall in the United States with Category 4 strength or greater, with the last one being Charley in 2004. 
   
   (The opinions expressed here are those of the author, a market analyst for Reuters.) ||||| 
   
   Back to Basic Definitions Page | Back to Main FAQ Page 
   
   Subject: A2) What is a "Cape Verde" hurricane? Contributed by Chris Landsea (NHC) Cape Verde-type hurricanes are those Atlantic basin tropical cyclones that develop into tropical storms fairly close (<1000 km [600 mi] or so) of the Cape Verde Islands and then become hurricanes before reaching the Caribbean. (That would be my definition, there may be others.) Typically, this may occur in August and September, but in rare years (like 1995) there may be some in late July and/or early October. The numbers range from none up to around five per year - with an average of around 2. 
   
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target: – As Texas still deals with the devastation from Harvey, another weather system churning in the Atlantic, and moving west toward the US, has some worried. CNN reports on Irma, currently a Category 3 hurricane that experienced "rapid intensification" from Wednesday into Thursday, meaning its wind speed ramped up at least 35mph in that period—though the National Hurricane Center on Thursday noted the "remarkable" increase measured 58mph. A National Hurricane Center alert issued early Friday pegs maximum sustained winds at 115mph, with higher gusts. The storm is expected to approach the Caribbean by Tuesday or so as a Category 4 storm, then possibly the US—though meteorological models with wildly different scenarios show it's too early to make any definitive predictions on where the storm will land. Irma is what's called a "Cape Verde" hurricane, meaning one that forms off the coast of Africa then shoots across the Atlantic; CNN notes these storm systems can turn into particularly powerful ones (Hurricane Hugo is one example). What's emboldening Irma is a lack of the "hurricane kryptonite" known as wind shear, which often acts to tamp down hurricanes. A powerful high-pressure ridge is pushing Irma westward and keeping wind shear low. What one of the models is showing that has some concerned, per Reuters: Irma could head straight into the Gulf of Mexico, which could be bad news for areas still struggling after Harvey. "All interests in the eastern Caribbean will need to monitor the progress of this evolving and dangerous hurricane," an AccuWeather hurricane expert notes, reiterating he can't say "with certainty" how the US will be affected.