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| Outline of tropical cyclones |
A tropical cyclone rainfall climatology is developed to determine rainfall characteristics of past tropical cyclones. A tropical cyclone rainfall climatology can be used to help forecast current or upcoming tropical cyclone impacts. The degree of a tropical cyclone rainfall impact depends upon speed of movement, storm size, and degree of vertical wind shear. One of the most significant threats from tropical cyclones is heavy rainfall. Large, slow moving, and non-sheared tropical cyclones produce the heaviest rains. The intensity of a tropical cyclone appears to have little bearing on its potential for rainfall over land, but satellite measurements over the last several years show that more intense tropical cyclones produce noticeably more rainfall over water. Flooding from tropical cyclones remains a significant cause of fatalities, particularly in low-lying areas.
While inland flooding is common to tropical cyclones, there are factors which lead to excessive rainfall from tropical cyclones. Slow motion, as was seen during Hurricane Danny (1997) and Hurricane Wilma, can lead to high amounts of rainfall. The presence of mountains/hills near the coast, like across much of Mexico, Haiti, the Dominican Republic, Central America, Madagascar, Réunion, China, and Japan acts to magnify rainfall potential due to forced upslope flow into the mountains. Strong upper level forcing from a trough moving through the Westerlies and its associated cold front, as was the case during Hurricane Floyd, can lead to high amounts even from systems moving at an average forward motion. Larger tropical cyclones drop more rainfall as they precipitate upon one spot for a longer time frame than average or small tropical cyclones. A combination of two of these factors could be especially crippling, as was seen during Hurricane Mitch in Central America.During the 2005 season, flooding related to slow-moving Hurricane Stan's broad circulation led to 1,662–2,000 deaths.
|Rainfall Rate per day within radius of the center (Riehl)|
|Radius (mi)||Radius (km)||Amount (in)||Amount (mm)|
Isaac Cline was the first to investigate rainfall distribution around tropical cyclones in the early 1900s. He found that a larger proportion of rainfall falls in advance of the center (or eye) than after the center's passage, with the highest percentage falling in the right front quadrant. Father Viñes of Cuba found that some tropical cyclones have their highest rainfall rates in the rear quadrant within a training (non-moving) inflow band. knots/39 mph/63 km/h) winds. Rainfall is more common near the center of tropical cyclones overnight. Over land, outer bands are more active during the heating of the day, which can act to restrict inflow into the center of the cyclone. Recent studies have shown that half of the rainfall within a tropical cyclone is stratiform in nature. The chart to the right was developed by Riehl in 1954 using meteorological equations that assume a gale radius of about 140 miles (230 km), a fairly symmetric cyclone, and does not consider topographic effects or vertical wind shear. Local amounts can exceed this chart by a factor of two due to topography. Wind shear tends to lessen the amounts below what is shown on the table.Normally, as a tropical cyclone intensifies, its heavier rainfall rates become more concentrated around its center. Rainfall is found to be heaviest in tropical cyclone's inner core, whether it be the eyewall or central dense overcast, within a degree latitude of the center, with lesser amounts farther away from the center. Most of the rainfall in tropical cyclones is concentrated within its radius of gale-force (34
Larger tropical cyclones have larger rain shields, which can lead to higher rainfall amounts farther from the cyclone's center.This is generally due to the longer time frame rainfall falls at any one spot in a larger system, when compared to a smaller system. Some of the difference seen concerning rainfall between larger and small storms could be the increased sampling of rainfall within a larger tropical cyclone when compared to that of a compact cyclone; in other words, the difference could be the result of a statistical problem.
Storms which have moved slowly, or loop, over a succession of days lead to the highest rainfall amounts for several countries. Riehl calculated that 33.97 inches (863 mm) of rainfall per day can be expected within one-half degree, or 35 miles (56 km), of the center of a mature tropical cyclone. Many tropical cyclones progress at a forward motion of 10 knots, which would limit the duration of this excessive rainfall to around one-quarter of a day, which would yield about 8.50 inches (216 mm) of rainfall. This would be true over water, within 100 miles (160 km) of the coastline, and outside topographic features. As a cyclone moves farther inland and is cut off from its supply of warmth and moisture (the ocean), rainfall amounts from tropical cyclones and their remains decrease quickly.
Vertical wind shear forces the rainfall pattern around a tropical cyclone to become highly asymmetric, with most of the precipitation falling to the left and downwind of the shear vector, or downshear left. In other words, southwesterly shear forces the bulk of the rainfall north-northeast of the center.If the wind shear is strong enough, the bulk of the rainfall will move away from the center leading to what is known as an exposed circulation center. When this occurs, the potential magnitude of rainfall with the tropical cyclone will be significantly reduced.
As a tropical cyclone interacts with an upper-level trough and the related surface front, a distinct northern area of precipitation is seen along the front ahead of the axis of the upper level trough. This type of interaction can lead to the appearance of the heaviest rainfall falling along and to the left of the tropical cyclone track, with the precipitation streaking hundreds of miles or kilometers downwind from the tropical cyclone.The stronger the upper trough picking up the tropical cyclone, the more significant the left of track shift in the rainfall distribution tends to be.
Moist air forced up the slopes of coastal hills and mountain chains can lead to much heavier rainfall than in the coastal plain. This heavy rainfall can lead to landslides, which still cause significant loss of life such as seen during Hurricane Mitch in Central America.
Globally, tropical cyclone rainfall is more common across the northern hemisphere than across the southern hemisphere. This is mainly due to the normal annual tropical cyclone distribution, as between half and two-thirds of all tropical cyclones form north of the equator. Rainfall is concentrated near the 15th parallel in both hemispheres, with a less steep dropoff seen with latitude across the northern hemisphere, due to the stronger warm water currents seen in that hemisphere which allow tropical cyclones to remain tropical in nature at higher latitudes than south of the equator.In the southern hemisphere, rainfall impacts will be most common between January and March, while north of the equator, tropical cyclone rainfall impacts are more common between June and November. Japan receives over half of its rainfall from typhoons.
Between 1970–2004, inland flooding caused a majority of the tropical cyclone-related fatalities in the United States. inches (406 mm), with 70–75 percent of the storm total falling within a 24-hour period. The highest point total was seen during Hurricane Harvey in 2017, when 60.58 inches (1,538.7 mm) fell in southeast Texas.This statistic changed in 2005, when Hurricane Katrina's impact alone shifted the most deadly aspect of tropical cyclones back to storm surge, which has historically been the most deadly aspect of strong tropical cyclones. On average, five tropical cyclones of at least tropical depression strength lead to rainfall across the contiguous United States annually, contributing around a quarter of the annual rainfall to the southeast United States. While many of these storms form in the Atlantic basin, some systems or their remnants move through Mexico from the Eastern Pacific basin. The average storm total rainfall for a tropical cyclone impacting the lower 48 from the Atlantic basin is about 16
The 2003 Atlantic hurricane season was a hyperactive Atlantic hurricane season with tropical activity before and after the official bounds of the season—the first such occurrence since the 1954 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 and a total of 51 deaths across the Mid-Atlantic region of the United States.
The 2002 Atlantic hurricane season was a near-average Atlantic hurricane season. It officially started on June 1, 2002 and ended on November 30, dates which conventionally limit the period of each year when most tropical cyclones develop in the Atlantic Ocean. The season produced fourteen tropical cyclones, of which twelve developed into named storms; four became hurricanes, and two attained major hurricane status. While the season's first cyclone did not develop until July 14, activity quickly picked up; the 2002 season tied with 2010 in which a record number of tropical storms, eight, developed in the month of September. It ended early however, with no tropical storms forming after October 6—a rare occurrence caused partly by El Niño conditions. The most intense hurricane of the season was Hurricane Isidore with a minimum central pressure of 934 mbar, although Hurricane Lili attained higher winds and peaked at Category 4 whereas Isidore only reached Category 3. The season's low activity is reflected in the low cumulative accumulated cyclone energy (ACE) rating of 67. ACE is, broadly speaking, a measure of the power of the hurricane multiplied by the length of time it existed, so low number reflects the small number of strong storms and preponderance of tropical storms.
The 2000 Atlantic hurricane season was a fairly active hurricane season, but featured the latest first named storm in a hurricane season since 1992. The hurricane season officially began on June 1, and ended on November 30. It was slightly above average due to a La Niña weather pattern although most of the storms were weak. The first cyclone, Tropical Depression One, developed in the southern Gulf of Mexico on June 7 and dissipated after an uneventful duration. However, it would be almost two months before the first named storm, Alberto, formed near Cape Verde; Alberto also dissipated with no effects on land. Several other tropical cyclones—Tropical Depression Two, Tropical Depression Four, Chris, Ernesto, Nadine, and an unnamed subtropical storm—did not impact land. Five additional storms—Tropical Depression Nine, Florence, Isaac, Joyce, and Leslie—minimally affected land areas.
A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and Mesoscale Convective Complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.
Hurricane Kathleen was a tropical cyclone that had a destructive impact in California. On September 7, 1976, a tropical depression formed; two days later it accelerated north towards the Baja California Peninsula. Kathleen brushed the Pacific coast of the peninsula as a hurricane on September 9 and made landfall as a fast-moving tropical storm the next day. With its circulation intact and still a tropical storm, Kathleen headed north into the United States and affected California and Arizona. Kathleen finally dissipated late on September 11.
Hurricane Kyle was the fifth-longest-lived Atlantic tropical or subtropical cyclone on record. The eleventh named storm and third hurricane of the 2002 Atlantic hurricane season, Kyle developed as a subtropical cyclone on September 20 to the east-southeast of Bermuda. Looping westward, it transitioned into a tropical cyclone and became a hurricane on September 25. For the next two weeks, Kyle tracked generally westward, oscillating in strength several times because of fluctuations in environmental conditions. On October 11, the cyclone turned northeastward and made landfalls near Charleston, South Carolina, and Long Beach, North Carolina, at tropical storm status. After lasting as a cyclone for 22 days, Kyle dissipated on October 12 as it was absorbed by an approaching cold front.
Tropical cyclone rainfall forecasting involves using scientific models and other tools to predict the precipitation expected in tropical cyclones such as hurricanes and typhoons. Knowledge of tropical cyclone rainfall climatology is helpful in the determination of a tropical cyclone rainfall forecast. More rainfall falls in advance of the center of the cyclone than in its wake. The heaviest rainfall falls within its central dense overcast and eyewall. Slow moving tropical cyclones, like Hurricane Danny and Hurricane Wilma, can lead to the highest rainfall amounts due to prolonged heavy rains over a specific location. However, vertical wind shear leads to decreased rainfall amounts, as rainfall is favored downshear and slightly left of the center and the upshear side is left devoid of rainfall. The presence of hills or mountains near the coast, as is the case across much of Mexico, Haiti, the Dominican Republic, much of Central America, Madagascar, Réunion, China, and Japan act to magnify amounts on their windward side due to forced ascent causing heavy rainfall in the mountains. A strong system moving through the mid latitudes, such as a cold front, can lead to high amounts from tropical systems, occurring well in advance of its center. Movement of a tropical cyclone over cool water will also limit its rainfall potential. A combination of factors can lead to exceptionally high rainfall amounts, as was seen during Hurricane Mitch in Central America.
The United States tropical cyclone rainfall climatology concerns the amount of precipitation, primarily in the form of rain, which occurs during tropical cyclones and their extratropical cyclone remnants across the United States. Typically, five tropical cyclones and their remnants impact the country each year, contributing between a tenth and a quarter of the annual rainfall across the southern tier of the country. The highest rainfall amounts appear close to the coast, with lesser amounts falling farther inland. Obstructions to the precipitation pattern, such as the Appalachian mountains, focus higher amounts from northern Georgia through New England. While most impacts occur with systems moving in from the Atlantic ocean or Gulf of Mexico, some emanate from the eastern Pacific ocean, with a few crossing Mexico before impacting the Southwest. Those making landfall within the Southeast portion of the country tend to have the greatest potential for heavy rains.
Tropical Storm Beryl was an unusual Atlantic tropical cyclone that formed over southeastern Louisiana in August 1988. The second tropical storm of the 1988 Atlantic hurricane season, Beryl developed from a slow-moving trough of low pressure on August 8. It tracked southeastward into the coastal waters of eastern Louisiana, and Beryl reached peak winds of 50 mph (85 km/h) while located about 75 miles (120 km) southeast of New Orleans. The storm turned to the northwest over Louisiana and Texas, and slowly dissipated. The remnants of Beryl continued northward into the central United States, dropping some rainfall and providing relief to a severe heat wave.
Tropical cyclone forecasting is the science of forecasting where a tropical cyclone's center, and its effects, are expected to be at some point in the future. There are several elements to tropical cyclone forecasting: track forecasting, intensity forecasting, rainfall forecasting, storm surge, tornado, and seasonal forecasting. While skill is increasing in regard to track forecasting, intensity forecasting skill remains nearly unchanged over the past several years. Seasonal forecasting began in the 1980s in the Atlantic basin and has spread into other basins in the years since.
Tropical Storm Barry was a rapidly forming tropical cyclone that made landfall on Florida, United States, in early June 2007. The second named storm of the 2007 Atlantic hurricane season, Barry developed from a trough of low pressure in the southeastern Gulf of Mexico on June 1. It tracked rapidly northeastward, reaching peak winds of 60 mph (95 km/h) before weakening and making landfall near Tampa Bay as a tropical depression. Barry quickly lost tropical characteristics after wind shear removed much of the convection, and early on June 3 it completed the transition into an extratropical cyclone. The extratropical remnants tracked up the East Coast of the United States, and were absorbed by a larger extratropical cyclone on June 5.
Texas' weather varies widely, from arid in the west to humid in the east. The huge expanse of Texas encompasses several regions with distinctly different climates: Northern Plains, Trans-Pecos Region, Texas Hill Country, Piney Woods, and South Texas. Generally speaking, the part of Texas that lies to the east of Interstate 35 is subtropical, while the portion that lies to the west of Interstate 35 is arid desert.
The 2011 Atlantic hurricane season was the second consecutive season to feature the third highest count of tropical storms, but most of the storms were weak. The season was above-average, mostly due to a La Niña that persisted during the previous year. Therefore, the season was tied with 1887, 1995, 2010, and the following 2012 season for the third highest number of tropical storms since record-keeping began in 1851. Although the season featured 19 tropical storms, only 7 of them intensified into hurricanes and 4 of those became major hurricanes: Irene, Katia, Ophelia, and Rina. The season officially began on June 1 and ended on November 30, dates which conventionally delimit the period during each year in which most tropical cyclones develop in the Atlantic Ocean. However, the first tropical storm of the season, Arlene, did not develop until nearly a month later. The final system, Tropical Storm Sean, dissipated over the open Atlantic on November 11.
Tropical Storm Danny was a weak and disorganized tropical cyclone that formed in August 2009. The fourth tropical system and third named storm of the 2009 Atlantic hurricane season, Danny developed on August 26 from the interaction between a westward-moving tropical wave and an upper-level trough while situated east of the Bahamas. The storm never fully matured, and resembled a subtropical cyclone. It meandered generally northwestward before being absorbed into another weather system on August 29.
Tropical Storm Candy produced minor impact in the state of Texas during the 1968 Atlantic hurricane season. The third tropical cyclone of the annual season, it developed from a tropical disturbance in the southwestern Gulf of Mexico on June 22. Gradual strengthening occurred, with the depression becoming Tropical Storm Candy on the following day. The storm reached its peak intensity of 70 mph (110 km/h) later that day and made landfall Port Aransas, Texas on June 23. Candy weakened into a tropical depression only hours after moving inland. However, the system remained a designated cyclone until June 26, at which time it completed extratropical transition over the state of Michigan.
The 2010 Atlantic hurricane season was the first in a group of three very active Atlantic hurricane seasons. It is tied alongside 1887, 1995, 2011, and 2012 for the third-most active Atlantic hurricane season on record, with 19 tropical storms, only behind the 1933 and the 2005 seasons. The hyperactive season featured 12 hurricanes, tied with 1969 for the second highest total. Only the quintessential 2005 season saw more activity. Despite the high number of hurricanes, not one hurricane hit the United States making the season the only season with 10 or more hurricanes without a United States landfall. The overall tropical cyclone count in the Atlantic exceeded that in the West Pacific for only the second time on record. The season officially began on June 1 and ended on November 30, dates that conventionally delimit the period during each year when tropical cyclone formation is most likely. The first cyclone, Alex intensified into the first June hurricane since Allison in 1995. The month of September featured eight named storms, tying 2002 and 2007 for the record. October featured five hurricanes, just short of the record set in 1870. Finally, Hurricane Tomas became the latest hurricane on record to move through the Windward Islands in late October. Activity was represented with an accumulated cyclone energy (ACE) value of 165 units, which was the eleventh highest value on record at the time. The very active activity in 2010 was due to a very strong La Niña, which led to a very quiet Pacific hurricane season.
The 2012 Atlantic hurricane season was the final year in a consecutive string of three very active seasons, although many of the storms were weak and short-lived. It is tied with 1887, 1995, 2010, and 2011 for having the third-most named storms on record. It was also the third-costliest season, behind 2005 and 2017. The season officially began on June 1 and ended on November 30, dates that conventionally delimit the period during each year in which most tropical cyclones form in the Atlantic Ocean. However, Alberto, the first system of the year, developed on May 19 – the earliest date of formation since Tropical Storm Ana in 2003. A second tropical cyclone, Beryl, developed later that month. This was the first occurrence of two pre-season named storms in the Atlantic basin since 1951. It moved ashore in North Florida on May 29 with winds of 65 mph (100 km/h), making it the strongest pre-season storm to make landfall in the Atlantic basin. This season marked the first time since 2009 where no tropical cyclones formed in July. Another record was set by Hurricane Nadine later in the season; the system became the fourth-longest-lived tropical cyclone ever recorded in the Atlantic, with a total duration of 22.25 days. The final storm to form, Tony, dissipated on October 25 – however, Hurricane Sandy, which formed before Tony, became extratropical on October 29.
The meteorological history of Hurricane Florence spanned 22 days from its inception on August 28, 2018, to its dissipation on September 18. Originating from a tropical wave over West Africa, Florence quickly organized upon its emergence over the Atlantic Ocean. Favorable atmospheric conditions enabled it to develop into a tropical depression on August 31 just south of the Cape Verde islands. Intensifying to a tropical storm the following day, Florence embarked on a west-northwest to northwest trajectory over open ocean. Initially being inhibited by increased wind shear and dry air, the small cyclone took advantage of a small area of low shear and warm waters. After achieving hurricane strength early on September 4, Florence underwent an unexpected period of rapid deepening through September 5, culminating with it becoming a Category 4 hurricane on the Saffir-Simpson scale. Thereafter, conditions again became unfavorable and the hurricane quickly diminished to a tropical storm on September 7.