Tropical Storm Allison was a tropical cyclone that devastated southeast Texas in June 2001. An arguable example of the "brown ocean effect", Allison 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 4, 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 $9 billion (2001 USD, equivalent to $15.2 billion in 2024) in damage and 41 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.

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 Sea, 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 mbar (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 mbar (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 transformed 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 mbar (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.

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.