Tropical waves, easterly waves, or tropical easterly waves, also known as African easterly waves in the Atlantic region, are a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. West-moving waves can also form from the tail end of frontal zones in the subtropics and tropics, and may be referred to as easterly waves, but these waves are not properly called tropical waves; they are a form of inverted trough sharing many characteristics with fully tropical waves. All tropical waves form in the easterly flow along the equatorward side of the subtropical ridge or belt of high pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.
A tropical wave normally follows an area of sinking, intensely dry air, blowing from the northeast. After the passage of the trough line, the wind veers southeast, the humidity abruptly rises, and the atmosphere destabilizes. This yields widespread showers and thunderstorms, sometimes severe. As the wave moves westward, the showers gradually diminish.
An exception to this rain is in the Atlantic. Sometimes, a surge of dry air called the Saharan Air Layer (SAL) follows a tropical wave, leaving cloudless skies, as convection is capped by the dry layer inversion. Also, any dust in the SAL reflects sunlight, cooling the air below it.
Tropical waves in the Atlantic basin develop from disturbances, which develop as far east as Sudan in east Africa, and drift across the continent into the Atlantic Ocean. These are generated or enhanced by the African Easterly Jet. The clockwise circulation of the large transoceanic high-pressure cell or anticyclone centered near the Azores islands (known as the Azores High) impels easterly waves away from the coastal areas of Africa towards North America.
Tropical cyclones can sometimes degenerate back into a tropical wave. This normally occurs if upper-level wind shear is too strong. The storm can redevelop if the upper-level shear abates.
If a tropical wave is moving quickly, or is organized enough, it can have winds of a strength in excess of tropical storm force, but it is not considered a tropical storm unless it has a closed low-level circulation. An example of this was Hurricane Claudette in 2003, where the original wave had winds of 45 mph (72 km/h) before developing a closed low-level circulation.
It has been suggested that some eastern Pacific Ocean tropical cyclones are formed out of tropical easterly waves that originate in North Africa as well. After developing into a tropical cyclone, some of those systems can then reach the Central Pacific Ocean, such as Hurricane Lane in 2018. During the summer months, tropical waves can extend northward as far as the desert southwest of the United States, producing spells of intensified shower activity embedded within the prevailing monsoon regime.
A screaming eagle is a tropical wave whose convective pattern loosely resembles the head of an eagle. This phenomenon is caused by shearing from either westerly winds aloft or strong easterly winds at the surface. These systems are typically located within 25 degrees latitude of the equator. Rain showers and surface winds gusting to 29 mph (47 km/h) are associated with these waves. They move across the ocean at a rate of 15 mph (24 km/h). Strong thunderstorm activity can be associated with the features when located east of a tropical upper tropospheric trough. The term was first publicly seen in an Air Force satellite interpretation handbook written by Hank Brandli in 1976. In 1969, Brandli discovered that a storm of this type threatened the original splashdown site for Apollo 11.
The 1988 Atlantic hurricane season was a near average season that proved costly and deadly, with 15 tropical cyclones directly affecting land. The season officially began on June 1, 1988, and lasted until November 30, 1988, although activity began on May 30 when a tropical depression developed in the Caribbean Sea. The June through November dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. The first cyclone to attain tropical storm status was Alberto on August 8, nearly a month later than usual. The final storm of the year, Tropical Storm Keith, became extratropical on November 24.
The season produced 19 tropical depressions of which 12 attained tropical storm status. One tropical storm was operationally classified as a tropical depression but was reclassified in post-analysis. Five tropical cyclones reached hurricane status of which three became major hurricanes reaching Category 3 on the Saffir-Simpson Hurricane Scale.
The most notable cyclone of the season was Hurricane Gilbert, which at the time was the strongest Atlantic hurricane on record. The hurricane tracked through the Caribbean Sea and the Gulf of Mexico and caused devastation in Mexico and many island nations, particularly Jamaica. Its passage caused $2.98 billion in damage (1988 USD) and more than 300 deaths, mostly in Mexico. Hurricane Joan, striking Nicaragua as a Category 4 hurricane, caused about US$1.87 billion in damage and more than 200 deaths. The hurricane crossed into the eastern Pacific Ocean and was reclassified as Tropical Storm Miriam.2003 Atlantic hurricane season
The 2003 Atlantic hurricane season was a very active Atlantic hurricane season with tropical activity before and after the official bounds of the season—the first such occurrence since the 1964 season. The season produced 21 tropical cyclones, of which 16 developed into named storms; seven cyclones attained hurricane status, of which three reached major hurricane status. With sixteen storms, the season was tied for the sixth-most active Atlantic hurricane season on record. The strongest hurricane of the season was Hurricane Isabel, which reached Category 5 status on the Saffir–Simpson hurricane scale northeast of the Lesser Antilles; Isabel later struck North Carolina as a Category 2 hurricane, causing $5.5 billion in damage (2003 USD) and a total of 51 deaths across the Mid-Atlantic region of the United States.
The season began with Subtropical Storm Ana on April 20, prior to the official start of the season; the bounds of the season are from June 1 to November 30, which conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. In early September, Hurricane Fabian struck Bermuda as a Category 3 hurricane, where it was the worst hurricane since 1926; on the island it caused four deaths and $300 million in damage (2003 USD). Hurricane Juan caused considerable destruction to Nova Scotia, particularly Halifax, as a Category 2 hurricane, the first hurricane of significant strength to hit the province since 1893. Additionally, Hurricanes Claudette and Erika struck Texas and Mexico, respectively, as minimal hurricanes.2003 Pacific hurricane season
The 2003 Pacific hurricane season was the first season to feature no major hurricanes – storms of Category 3 intensity or higher – since 1977. It produced an unusually large number of tropical cyclones which affected Mexico. The most notable cyclones during the year were Hurricanes Ignacio and Marty, which killed 2 and 12 people in Mexico, respectively, and were collectively responsible for about US$1 billion (2003 USD) in damage. Three other Pacific storms, two of which were hurricanes, and three Atlantic storms also had a direct impact on Mexico. The only other significant storm of the season was Hurricane Jimena, which passed just to the south of Hawaii, the first storm to directly threaten Hawaii for several years.
The season officially started on May 15, 2003, in the eastern Pacific Ocean, and on June 1, 2003, in the central Pacific, and lasted until November 30, 2003. These dates conventionally delimit the period of each year when most tropical cyclones form in the northeastern Pacific Ocean. The season saw 16 tropical storms form, of which 7 became hurricanes, which is about average.2007 Pacific hurricane season
The 2007 Pacific hurricane season was a below-average Pacific hurricane season, featuring one major hurricane. The season officially started on May 15 in the eastern Pacific and on June 1 in the central Pacific, and ended on November 30; these dates conventionally delimit the period during which most tropical cyclones form in the region. The first tropical cyclone of the season, Alvin, developed on May 27, while the final system of the year, Kiko, dissipated on October 23. Due to unusually strong wind shear, activity fell short of the long-term average, with a total of 11 named storms, 4 hurricanes, and 1 major hurricane. At the time, 2007 featured the second-lowest value of the Accumulated cyclone energy (ACE) index since reliable records began in 1971. Two tropical cyclones – Cosme and Flossie – crossed into the central Pacific basin during the year, activity below the average of 4 to 5 systems.
Impact during the season was relatively minimal. In early June, Tropical Storm Barbara moved ashore just northwest of the Mexico–Guatemala border, causing $55 million (2007 USD) in damage and 4 deaths. In late July, Cosme passed south of the island of Hawaii as a weakening tropical depression; light rain and increased surf resulted. A few days later, Dalila passed offshore the coastline of southwestern Mexico, killing 11 and causing minimal damage. Hurricane Flossie followed a similar track to Cosme in mid-August, producing gusty winds and light precipitation in Hawaii. Hurricane Henriette in early September produced torrential rainfall in southwestern Mexico, killing 6 and causing $25 million in damage. Baja California received moderate rains from Hurricane Ivo in mid-September, though no damage nor fatalities were reported. In mid-October, Tropical Storm Kiko passed just offshore the coastline of southwestern Mexico. Though no deaths were reported on the Mexico mainland, the storm capsized a ship with 30 people on board, 15 of whom were recovered dead, and 9 of whom were reported missing. Overall, the season ended with $80 million in damage and 49 deaths.2008 Pacific hurricane season
The 2008 Pacific hurricane season was a near average hurricane season. It officially started May 15, 2008 in the eastern Pacific, started on June 1, 2008 in the central Pacific, and lasted until November 30, 2008. This season is the first since 1996 to have no cyclones cross into the central Pacific. Activity this year was near average, with 16 storms forming in the Eastern Pacific proper and an additional 1 in the Central Pacific. There were 7 hurricanes, a low number compared to the typical 9, and only 2 major hurricanes, unlike the typical 5. There were only a few notable storms this year. Tropical Storm Alma made landfall along the Pacific coast of Nicaragua, becoming the first known storm to do so. It killed 9 and did US$35 million in damage (value in 2008). It also became the first tropical storm to be retired in the Eastern Pacific basin. Hurricane Norbert became the strongest hurricane to hit the western side of the Baja Peninsula on record, killing 25.Hurricane Daniel (2006)
Hurricane Daniel was the second strongest hurricane of the 2006 Pacific hurricane season. The fourth named storm of the season, Daniel originated on July 16 from a tropical wave off the coast of Mexico. It tracked westward, intensifying steadily to reach peak winds of 150 mph (240 km/h) on July 22. At the time, the characteristics of the cyclone resembled those of an annular hurricane. Daniel gradually weakened as it entered an area of cooler water temperatures and increased wind shear, and after crossing into the Central Pacific Ocean, it quickly degenerated into a remnant low-pressure area on July 26, before dissipating two days later.
Initial predictions suggested that the cyclone would pass through the Hawaiian Islands as a tropical storm; however, Daniel's remnants dissipated southeast of Hawaii. The storm brought light to moderate precipitation to the Island of Hawaii and Maui, causing minor flooding, although no major damage or fatalities were reported.Hurricane Gordon (2000)
Hurricane Gordon caused minor damage in the Eastern United States. The seventh named storm and fourth hurricane of the 2000 Atlantic hurricane season, Gordon developed in the extreme western Caribbean Sea from a tropical wave on September 14. Shortly thereafter, the depression moved inland over the Yucatán Peninsula and later emerged into the Gulf of Mexico on September 15. The depression began to quickly organize, and by early on September 16, it was upgraded to Tropical Storm Gordon. After becoming a tropical storm, Gordon continued to intensify and was reclassified as a hurricane about 24 hours later; eventually, the storm peaked as an 80 mph (130 km/h) Category 1 hurricane. However, southwesterly upper-level winds caused Gordon to weaken as it approached land, and it was downgraded to a tropical storm by late on September 17. At 0300 UTC on September 18, Gordon made landfall near Cedar Key, Florida as a strong tropical storm. After moving inland, Gordon rapidly weakened and had deteriorated to tropical depression status by nine hours later. Later that day, Gordon merged with a frontal boundary while centered over Georgia.
Prior to becoming a tropical cyclone, the precursor tropical wave caused severe flooding in Guatemala, killing 23 people. While crossing the Yucatán Peninsula, the storm dropped heavy rainfall, with a few areas experiencing more than 10 inches (250 mm) of precipitation. Similarly, portions of western Cuba reported rainfall totals reaching 10 inches (250 mm). Gordon brought moderate storm surge to the west coast of Florida; one person drowned due to rough seas. Numerous trees and power lines sustained damage, which left 120,000 people without electricity. In the Tampa Bay area and Cedar Key, minor roof damage to houses and street flooding occurred. In addition, two tornadoes caused some damage in Cape Coral and Ponce Inlet. Elsewhere, affects were minimal, though two indirect fatalities occurred in North Carolina, and minor flooding was reported in South Carolina, Virginia, West Virginia, Maryland, Delaware, Pennsylvania, New Jersey, and New York. Overall, Gordon caused $10.8 million (2000 USD) in damage and 26 fatalities.Hurricane Joyce (2000)
Hurricane Joyce caused minor impact in the ABC and Windward islands during late September and early October 2000. The fourteenth tropical cyclone, tenth named storm, and sixth hurricane of the 2000 Atlantic hurricane season, Joyce developed from a tropical wave located southwest of Cape Verde on September 25. Only twelve hours after becoming a cyclone, the depression was upgraded to Tropical Storm Joyce early on September 26. Joyce continued to strengthen and became a hurricane by September 27. On the following day, Joyce peaked with winds of 90 mph (140 km/h) on September 28. After peak intensity on September 28, unfavorable conditions caused Joyce to become disorganized and weaken.
Late on September 29, Joyce was downgraded to a tropical storm while centered well east of the Lesser Antilles. In contrast with predictions, Joyce continued to weaken and became a tropical depression on October 1 while crossing through the southern Windward Islands. Early on the following day, Joyce degenerated into a tropical wave over the southeast Caribbean Sea. The remnants were monitored for regeneration, but never developed back into a tropical cyclone. Overall, impact from Joyce was minimal, limited to mainly rainfall and near-tropical storm force winds in Trinidad and Tobago and Barbados. The remnants of Joyce also caused similar effects on the ABC islands and Dominican Republic, which resulted in minor damage.Hurricane Karen (2007)
Hurricane Karen was the eleventh named storm and fourth hurricane of the 2007 Atlantic hurricane season. Karen was a Cape Verde-type hurricane that developed in the eastern tropical Atlantic out of a large tropical wave. The storm briefly reached Category 1 hurricane intensity before slowly weakening due to increased wind shear. As the storm remained away from land, no damages or fatalities were reported in association with Karen.Post-tropical cyclone
A post-tropical cyclone is a former tropical cyclone. Two classes of post-tropical cyclones are:
Extratropical cyclone, which is frontal, sometimes still retains winds of hurricane or tropical storm force.
Remnant low, which is non-frontal, has maximum sustained winds of less than 34 knots.Not all systems fall into the above two classes. According to the guideline, a system without frontal characteristics but with maximum winds above 34 knots may not be designated as a remnant low. It should be merely described as post-tropical. A few examples falling into this grey area are listed below.
Michael (2018)However, there has been an occasion that the United States National Hurricane Center violated the definition and designated Calvin (2011) as a 35-knot remnant low.Also, if a tropical cyclone degenerates into a tropical wave or trough, then it does not qualify as a post-tropical cyclone. It would be referred as the "remnants of (tropical cyclone name)".
Météo-France classifies systems in the South-West Indian Ocean undergoing an extratropical transition or losing tropical characteristics as “post-tropical depressions”, since the 2012–13 cyclone season. They would be re-classified as extratropical depressions after completing the process.Tropical Depression Nine (2003)
Tropical Depression Nine was a weak tropical depression that developed and dissipated in the eastern Caribbean Sea in August 2003. It formed from a tropical wave on August 21 to the south of Puerto Rico, and was initially forecast to strengthen to tropical storm status, due to the favorable conditions for development in the area. However, wind shear developed over the system, and the depression degenerated to a tropical wave on August 22. The system produced moderate to heavy rainfall throughout its path. In Puerto Rico, the rains flooded ten houses, while in the Dominican Republic the precipitation led to overflown rivers and two injuries.Tropical Depression Ten (2005)
Tropical Depression Ten was a short-lived and weak tropical cyclone that was the tenth system of the record-breaking 2005 Atlantic hurricane season. It formed on August 13 from a tropical wave that emerged from the west coast of Africa on August 8. As a result of strong wind shear, the depression remained weak and did not strengthen beyond tropical depression status. It degenerated on August 14, but its remnants partially contributed to the formation of Hurricane Katrina. Thus, it is often referred to as "the precursor to Katrina". The cyclone had no effect on land and did not directly result in any fatalities or damage.Tropical Storm Barry (2001)
Tropical Storm Barry was a strong tropical storm that made landfall on the Florida Panhandle during August 2001. The third tropical cyclone and second named storm of the 2001 Atlantic hurricane season, Barry developed from a tropical wave that moved off the coast of Africa on July 24. The wave entered the Caribbean on July 29 and spawned a low-pressure area, which organized into Tropical Storm Barry on August 3. After fluctuations in intensity and track, the storm attained peak winds of 70 mph (110 km/h) over the Gulf of Mexico. Barry headed northward and moved ashore along the Gulf Coast before dissipating on August 7.
Unlike the devastating Tropical Storm Allison earlier in the season, Barry's effects were moderate. Nine deaths occurred: six in Cuba and three in Florida. As a tropical cyclone, Barry produced heavy rainfall that peaked at 8.9 in (230 mm) at Tallahassee. Gusts in the area reached 79 mph (127 km/h), which was the highest wind speed recording for the storm. The precursor tropical wave to Barry dropped large amounts of rain on southern Florida, leading to significant flooding and structural damage. Moderate flooding and wind damage occurred throughout the Florida Panhandle. As the storm's remnants tracked inland, parts of the Mississippi Valley received light precipitation. Barry caused an estimated $30 million (2001 USD) in damage.Tropical Storm Bonnie (2004)
Tropical Storm Bonnie was a tropical storm that made landfall on Florida in August 2004. The second storm of the 2004 Atlantic hurricane season, Bonnie developed from a tropical wave on August 3 to the east of the Lesser Antilles. After moving through the islands, its fast forward motion caused it to dissipate. However, it later regenerated into a tropical storm near Yucatán Peninsula. Bonnie attained peak winds of 65 miles per hour (105 km/h) over the Gulf of Mexico, turned to the northeast, and hit Florida as a 45 miles per hour (72 km/h) tropical storm. The storm accelerated to the northeast and became an extratropical cyclone to the east of New Jersey. Bonnie was the first of five tropical systems to make landfall on Florida in the 2004 Atlantic hurricane season, and the second of a record eight disturbances to reach tropical storm strength during the month of August.
Bonnie's impact was minimal. Throughout the Caribbean Sea, the storm's effects consisted primarily of light rainfall, and in Florida, the precipitation caused flooding and minor damage. The tropical storm caused a tornado outbreak across the Southeastern United States which killed three people and inflicted over $1 million (2004 USD) in damage. Bonnie made landfall in Florida the day before Hurricane Charley struck.Tropical Storm Bret (2005)
Tropical Storm Bret was a short-lived tropical storm of the 2005 Atlantic hurricane season that made landfall in the Mexican state of Veracruz, the first of four during the season. The second named storm of the season, Bret developed along a tropical wave on June 28 in the Bay of Campeche, and quickly intensified. Tracking to the west-northwest, Bret moved ashore within 24 hours of forming, and dissipated shortly thereafter.
Bret was the first of six tropical cyclones (three hurricanes, two of them major, and three tropical storms) to make landfall in Mexico during the season. With the formation of the tropical storm on June 28, the 2005 season became the first since 1986 with two storms in the month of June. The storm dropped heavy rainfall along its path, peaking at 266 mm (10.67 inches), which caused flooding and one drowning death. About 7,500 people were affected, and damage totaled over 100 million pesos (2005 MXN, $9.3 million 2005 USD, $10.3 million 2008 USD).Tropical Storm Chantal (2001)
Tropical Storm Chantal was a North Atlantic tropical cyclone that moved across the Caribbean Sea in August 2001. Chantal developed from a tropical wave on August 14 in the tropical Atlantic Ocean. It tracked rapidly westward for much of its duration, and after degenerating into a tropical wave, it passed through the Windward Islands. Chantal reached a peak intensity of 70 mph (110 km/h) twice in the Caribbean Sea, and each time it was anticipated to attain hurricane status; however, wind shear and later land interaction prevented strengthening to hurricane status. On August 21 Chantal, moved ashore near the border of Mexico and Belize, before dissipating on the next day.
In the Windward Islands, lightning caused two indirect deaths in Trinidad. Chantal dropped light to moderate rainfall across its path, most significantly in Quintana Roo in Mexico where it caused widespread mudslides. Damage in Belize totaled $4 million (2001 USD; $5.66 million 2019 USD), due to the combined impact of high waves, moderate winds, and rainfall. Overall damage was minor.Tropical Storm Danielle (1980)
Tropical Storm Danielle caused considerable flooding in the state of Texas during September 1980. The eighth tropical cyclone and fourth named storm of the 1980 Atlantic hurricane season, Danielle developed from a tropical wave that emerged into the Atlantic Ocean from the west coast of Africa on August 22. Three days later, the tropical wave developed into a tropical depression. Four days later, the depression degenerated into a tropical wave. After tracking westward and entering the Gulf of Mexico, the system gradually developed, and became a tropical depression on September 4. The depression gradually strengthened and became Tropical Storm Danielle only hours before landfall in eastern Texas on September 5. Danielle steadily weakened inland and dissipated two days later.
Danielle produced widespread rainfall in Louisiana, though few areas reported more than 5 inches (130 mm) of precipitation. Rainfall was heavier in Texas, peaking at 18.29 inches (465 mm) in Nederland. Much of the damage caused by the storm was as a resulting of flooding. In Port Arthur, twelve homes were damaged, while Interstate 10 was inundated by flood waters. Danielle also spawned several tornadoes in Texas, though none effects from them are unknown. Outside of Texas and Louisiana, the storm also dropped light rainfall in Oklahoma and Mississippi. Overall, Danielle caused three fatalities and at least $25 million (1980 USD) in damage.Tropical Storm Earl (2004)
Tropical Storm Earl caused minor damage in the Windward Islands in mid-August 2004. The sixth tropical cyclone and named storm of the annual hurricane season, Earl developed on August 13 from a tropical wave centered well east of the Lesser Antilles. The depression gradually organized as it tracked west-northwestward and was upgraded to Tropical Storm Earl roughly a day after genesis. As the system approached the Windward Islands it continued to slowly strengthen, peaking as a 50 mph (85 km/h) tropical storm early on August 15. However, the system unexpectedly degenerated into a tropical wave that afternoon, likely due to its fast forward motion. The remnants of Earl continued across the Caribbean Sea and eventually re-developed into Hurricane Frank in the eastern Pacific Ocean on August 23.
Gusty winds in Grenada damaged at least 34 roofs and knocked down twelve trees and six electrical poles. Additionally, a nursing home on the island was evacuated due to significant structural damage. Two houses lost their roofs in Saint Vincent and the Grenadines, while moderate crop damage was reported on the island. Downed trees and power lines in Tobago left 90% of the island without electricity. Overall, Earl was responsible for one fatality, nineteen missing, and an unknown amount of damage.Tropical Storm Helene (2000)
Tropical Storm Helene was a long-lived tropical cyclone that oscillated for ten days between a tropical wave and a 70 mph (110 km/h) tropical storm. It was the twelfth tropical cyclone and eighth tropical storm of the 2000 Atlantic hurricane season, forming on September 15 east of the Windward Islands. After degenerating into a tropical wave, the system produced flooding and mudslides in Puerto Rico. It reformed into a tropical depression on September 19 south of Cuba, and crossed the western portion of the island the next day while on the verge of dissipation. However, it intensified into a tropical storm in the Gulf of Mexico, reaching its peak intensity while approaching the northern Gulf Coast.
The storm rapidly weakened before moving ashore near Fort Walton Beach, Florida on September 22. It produced heavy rainfall along the Florida Panhandle that reached 9.56 in (243 mm). The rains flooded hundreds of houses and caused the Sopchoppy River to reach a record crest. Gusty winds left about 5,000 people without power, though the rains alleviated drought conditions. In South Carolina, Helene spawned a tornado that killed one person and injured six others; heavy rainfall in the state also led to a death when a driver hydroplaned into a tree. The rainfall extended northeastward into Delaware. Overall damage in the United States was estimated at $16 million. Helene emerged from North Carolina as a tropical storm, and re-intensified to near-hurricane strength before being absorbed by a cold front on September 25.