Tropical Storm Allison was a weak but devastating tropical cyclone that struck southeast Texas in June 2001. It lasted unusually long for a June storm, remaining tropical and subtropical for 16 days, most of which was when the storm was over land dumping torrential rainfall. The storm developed from a tropical wave in the northern Gulf of Mexico on June 5, and struck the upper Texas coast shortly thereafter. It drifted northward through the state, turned back to the south, and re-entered the Gulf of Mexico. The storm continued to the east-northeast, made landfall on Louisiana, then moved across the southeast United States and Mid-Atlantic. Allison was the first storm since Tropical Storm Frances in 1998 to strike the northern Texas coastline.
The storm dropped heavy rainfall along its path, peaking at over 40 inches (1,000 mm) in Texas. The worst flooding occurred in Houston, where most of Allison's damage occurred: 30,000 became homeless after the storm flooded over 70,000 houses and destroyed 2,744 homes. Downtown Houston was inundated with flooding, causing severe damage to hospitals and businesses. Twenty-three people died in Texas. Along its entire path, Allison caused $8.5 billion (2001 USD, equivalent to $14.7 billion in 2025) in damage and 50 deaths. Aside from Texas, the places worst hit were Louisiana and southeastern Pennsylvania.
Following the storm, President George W. Bush designated 75 counties along Allison's path as disaster areas, which enabled the citizens affected to apply for aid. Then the fourth-costliest Atlantic tropical cyclone and still the costliest Atlantic tropical cyclone that was never a major hurricane, Allison was the first Atlantic tropical storm to have its name retired without ever having reached hurricane strength, and the only until Tropical Storm Erika in 2015.
Meteorological history
A tropical wave moved off the coast of Africa on May 21, 2001. It moved westward across the Atlantic Ocean, retaining little convection on its way. After moving across South America and the southwestern Caribbean, the wave entered the eastern North Pacific Ocean on June 1. A low-level circulation developed on June 2, while it was about 230 mi (370 km) south-southeast of Salina Cruz, Mexico. Southerly flow forced the system northward, and the wave moved inland on June 3. The low-level circulation dissipated, though the mid-level circulation persisted. It emerged into the Gulf of Mexico on June 4, and developed deep convection on its eastern side. Early on June 5, satellite imagery suggested that a tropical depression was forming in the northwest Gulf of Mexico, which was furthered by reports of wind gusts as high as 60 mph (95 km/h) just a few hundred feet above the surface, towards the east side of the system.
At 12:00 UTC on June 5, the disturbance developed a broad, low-level circulation, and was classified as Tropical Storm Allison, the first storm of the 2001 Atlantic hurricane season. Some intensification was projected, though it was expected to be hindered by cool offshore sea surface temperatures. Due to the cold-core nature of the center, Allison initially contained subtropical characteristics. Despite this, the storm quickly strengthened to attain peak sustained winds of 60 mph (95 km/h), with tropical storm-force winds extending up to 230 mi (370 km) east of the center, and a minimum central pressure of 1,000 millibars (29.53 inHg). The storm initially moved very little, and the presence of several small vortices from within the deep convection caused difficulty in determining the exact center location. Later in the day, several different track forecasts arose. One scenario had the cyclone tracking westward into Mexico. Another projected the storm moving east towards southern Louisiana. At the time, it was noted that little rain or wind persisted near the center, but rather to the north and east. Under the steering currents of a subtropical ridge that extended in an east–west orientation across the southeast United States, Allison weakened while nearing the Texas coastline, and struck near Freeport, Texas with 50 mph (85 km/h) winds. Inland, the storm rapidly weakened, and the National Hurricane Center discontinued advisories early on June 6. Shortly after being downgraded to a tropical depression, surface observations showed an elongated circulation with a poorly defined center, which had reformed closer to the deep convection.
The depression drifted northward until reaching Lufkin, Texas, where it stalled due to a high pressure system to its north. While stalling over Texas, the storm dropped excessive rainfall, peaking at just over 40 inches (1,000 mm) in northwestern Jefferson County. On June 7, the subtropical ridge off Florida weakened, while the ridge west of Texas intensified. This steered Tropical Depression Allison to make a clockwise loop, and the storm began drifting to the southwest. As the center reached Huntsville, Texas, a heavy rain band began to back build from Louisiana westward into Liberty County, Texas, which had caused additional flooding. At the time, the system had a minimum central pressure of about 1,004 millibars (29.65 inHg) and maximum sustained winds of about 10 mph (16 km/h). Late on June 9 and early on June 10, Allison's remnants reentered reached the Gulf of Mexico and emerged over open waters. The low once again became nearly stationary about 60 mi (97 km) south of Galveston, Texas, and despite more favorable upper-level winds, it showed no signs of redevelopment. Due to dry air and moderate westerly wind shear, the storm transitioned into a subtropical cyclone. While the subtropical depression moved eastward, a new low level circulation redeveloped to the east, and Allison quickly made landfall on Morgan City, Louisiana on June 11. At around the same time, the surface center reformed to the east-northeast of its previous location, aligning with the mid-level circulation. Strong thunderstorms redeveloped over the circulation, and Allison strengthened into a subtropical storm over southeastern Louisiana. The storm intensified further to attain sustained winds of 45 mph (75 km/h) and a minimum barometric pressure of about 1,000 millibars (29.53 inHg) near Mclain, Mississippi, accompanied by a well-defined eye-like feature.
The storm was officially downgraded to a subtropical depression at 0000 UTC on June 12. Somewhat accelerating, the depression tracked to the east-northeast through Mississippi, Alabama, Georgia, and South Carolina before becoming nearly stationary near Wilmington, North Carolina. The depression drifted through North Carolina and sped to the northeast for a time in response to an approaching cold front. Though satellite and radar imagery show the system was well-organized, the system slowed and moved erratically for a period of time, executing what appeared to be a small counterclockwise loop. The storm began tracking in a generally northeasterly direction, and crossed into the southern Delmarva Peninsula on June 16. The subtropical remnants reached the Atlantic on June 17, and while located east of Atlantic City, New Jersey, winds began to restrengthen, and heavy rains formed to the north of the circulation. The low was interacting with a frontal boundary, and started merging with it, as it accelerated to the northeast at 13 mph (21 km/h). The remnants of Allison briefly reintensified to a subtropical storm through baroclinic processes, though it became extratropical while south of Long Island. By later on June 17, the low was situated off the coast of Rhode Island, spreading a swath of precipitation over New England. The remnants of the tropical storm were then absorbed by the frontal boundary by June 18, and eventually passed south of Cape Race, Newfoundland on June 20, where the extratropical cyclone dissipated.
Preparations
Shortly after the storm formed, officials in Galveston County, Texas issued a voluntary evacuation for the western end of Galveston Island, as the area was not protected by the Galveston Seawall. The ferry from the island to the Bolivar Peninsula was closed, while voluntary evacuations were issued in Surfside in Brazoria County. When the National Hurricane Center issued the first advisory on Allison, officials issued Tropical Storm Warnings from Sargent, Texas to Morgan City, Louisiana. After the storm made landfall, flash flood watches and warnings were issued for numerous areas in eastern Texas. During the flood event, the National Weather Service in Houston issued 99 flash flood warnings with an average lead time of 40 minutes. With an average lead time of 24 minutes, the National Weather Service in Lake Charles, Louisiana issued 47 flash flood warnings. With an average lead time of 39 minutes, the National Weather Service in New Orleans/Baton Rouge issued 87 flash flood warnings, of which 30 were not followed by a flash flood.
In Tallahassee, Florida, a shelter opened the day before Allison's movement northward through the area, seven staff members housing 12 people. Two other shelters were on standby. Teams informed citizens in the Florida panhandle of flood dangers.
Impact
Tropical Storm Allison impacted much of the Gulf and Atlantic coast of the United States, from Texas to New England. Most of the impacts were in the form of rainfall, although Allison also spawned at least 23 tornadoes. Nationwide, Allison caused approximately $8.5 billion in damage (2001 USD), making it the costliest American tropical storm. Allison also caused 41 direct deaths throughout the country, while an additional 9 people died due to indirect causes, all by traffic accidents related to the storm in North Carolina.
Texas
Throughout Texas, Allison left at least $5.16 billion (2001 USD) in damage. The storm killed 23 people, including 12 deaths from driving, 6 from walking, 3 from electrocution, 1 in an elevator, and 1 drowning death in a ditch. However, some survivors disputed the claim of the fatality inside an elevator, suggesting that the victim may have instead been swept away by floodwaters while attempting to reach the elevator.
Allison made landfall with a storm surge of 2 to 3 ft (0.61 to 0.91 m). Combined with waves on top, areas of Galveston Island experienced a wall of water 8 feet (2.4 m) in height, creating overwash along the coastline. The storm caused winds of up to 43 mph (69 km/h) at the Galveston Pier. While Allison was stalling over Texas, it dropped heavy rainfall across the state, including 9.77 inches (248 mm) in Galveston, 12.13 inches (308 mm) in Jamaica Beach, Texas, and other similar totals along the coast. Minimal beach erosion was reported, and impact was minimal near the coast. While moving northward through Texas as a minimal tropical depression, Allison produced minor wind gusts. Shortly after making landfall, the storm spawned a tornado in Manvel of Brazoria County, causing damage to one home. Within hours of making landfall, rainfall totals of 8 to 12 inches (200 to 300 millimetres) were common in Galveston and Harris County. Flash flooding continued for days, with rainfall amounts across the state peaking at just over 40 inches (1,000 mm) in northwestern Jefferson County. In the Port of Houston, a total of 36.99 inches (940 mm) was reported.
Houston experienced torrential rainfall in a short amount of time. The six-day rainfall in Houston amounted to 38.6 inches (980 mm). Houston Hobby Airport received 20.84 inches (529 mm) of rain from June 5 to 10, 2001, while Bush Intercontinental Airport received 16.48 inches (419 mm). Two-thirds of the bayous and creeks in Harris County experienced 500-year flood events. Houston Intercontinental Airport, which typically receives 46.07 inches (1,170 mm) of rain in a year, experienced 35.7% of its expected total in the first nine days of June. The deluge flooded 95,000 automobiles and 73,000 houses throughout Harris County. Tropical Storm Allison destroyed 2,744 homes, leaving 30,000 homeless with residential damages totaling to $1.76 billion (2001 USD). Residential neighborhoods inside and to the north of Interstate 610 were hardest hit. Additionally, five of the six bayou systems in downtown Houston were severely flooded. Four of them broke 100-year high-water records, causing excessive surface run-off.
Several hospitals in the Texas Medical Center, the largest medical complex in the world, experienced severe damage from the storm, which hit quickly and with unexpected fury on a Friday evening. Hospital personnel were forced to evacuate thousands of patients in a major effort that included Coast Guard and Army helicopters. Many of the hospitals had lost all power, including back-up generators, meaning that patients had to be carried down dark stairwells by the staff in temperatures over 100 °F (38 °C). Patients who could not breathe on their own had to be continuously manually ventilated during the evacuation, which lasted hours. Most hospitals lost power and backup power when basements, the area where power and research data were kept, flooded. The Baylor College of Medicine experienced major damage, totaling $495 million (2001 USD). The medical school lost 90,000 research animals, 60,000 tumor samples, and 25 years of research data. The University of Texas Health Science Center at Houston, across the street, lost thousands of laboratory animals, including expensive genetic-specific mice. Decades of research was lost, including, for many scientists, their life's work. The UT-Houston gross anatomy lab, cyclotron, and other important facilities were completely destroyed. Throughout the Medical Center, damage totaled to over $2 billion (2001 USD). Most were reopened after a month, though it took much longer to become fully operational.