Tropical Storm Allison

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 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 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 $ in ) 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.

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 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 just a few hundred feet above the surface, towards the east side of the system.

right|thumb|Subtropical Storm Allison with an [[Eye (cyclone)|eye-like feature over Mississippi]]

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 , with tropical storm-force winds extending up to east of the center, and a minimum central pressure of . 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, 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. 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. 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 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. Strong thunderstorms redeveloped over the circulation, and Allison strengthened into a subtropical storm over southeastern Louisiana.

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. 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 . The remnants of Allison briefly reintensified to a subtropical storm through baroclinic processes, though it became extratropical while south of Long Island. 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. 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

{| class="wikitable" style="float:right; margin:0 0 1em 1em;"

|+ Death tolls by State

! Area

! Deaths

| Texas

| 23

| Louisiana

| 1

| Mississippi

| 1

| Florida

| 8

| Virginia

| 1

| Pennsylvania

| 7

! Total

! 41

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 killed 41 people throughout the country. 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. and impact was minimal near the coast. While moving northward through Texas as a minimal tropical depression, Allison produced minor wind gusts. with rainfall amounts across the state peaking at just over in northwestern Jefferson County. In the Port of Houston, a total of was reported.

Houston experienced torrential rainfall in a short amount of time. The six-day rainfall in Houston amounted to . Houston Hobby Airport received of rain from June 5 to 10, 2001, while Bush Intercontinental Airport received . Two-thirds of the bayous and creeks in Harris County experienced 500-year flood events. Houston Intercontinental Airport, which typically receives of rain in a year, experienced 35.7% of its expected total in the first nine days of June. Four of them broke 100-year high-water records, causing excessive surface run-off. The tunnel system, which connects most large office buildings in downtown Houston, was submerged, as were many streets and parking garages adjacent to Buffalo Bayou. In the Houston Theater District, also near Buffalo Bayou in the northern part of downtown, the Houston Ballet, Houston Symphony, Houston Grand Opera, and Alley Theater lost millions of dollars' worth of costumes, musical instruments, sheet music, archives, props, and other artifacts. forcing hundreds of motorists to abandon their vehicles for higher ground. Eighteen-wheeled trucks were filmed floating down major freeways and highways, swept along by floodwaters. Days later, Allison hit the state as a subtropical storm, dropping more heavy rains to the area. Rainfall totals peaked at in Thibodaux, the highest rainfall total in Louisiana from a tropical cyclone since another Tropical Storm Allison in 1989. Most of the southeastern portion of the state experienced over 10 inches of rain (255 mm).

When Allison first made landfall, heavy rainfall flooded numerous houses and businesses. Minor wind gusts caused minor roof damage to 10 houses in Cameron Parish, while its storm surge flooded portions of Louisiana Highway 82. When the system returned, more rainfall occurred, flooding over 1,000 houses in St. Tammany Parish, 80 houses in St. Bernard Parish, and hundreds of houses elsewhere in the state. The flooding also forced 1,800 residents from their homes in East Baton Rouge Parish. The deluge left numerous roads impassable, while runoff resulted in severe river flooding. The Bogue Falaya River in Covington crested past its peak twice to near-record levels. while some areas in the southwestern portion of the state received over . Severe thunderstorms in George County damaged 15 houses, destroyed 10, and injured 5 people. Damage in Mississippi totaled to over $1 million (2001 USD). The storm, combined with a high pressure, produced coastal flooding in southern Alabama. Allison produced an F0 tornado in southwest Mobile County that caused minor roof damage and another F0 tornado in Covington County that caused minor damage to six homes and a church.

The storm, combined with a high pressure system, produced a strong pressure gradient, resulting in strong rip currents off the coast of Florida. The currents prompted sirens, which are normally used for storm warnings, to be activated in Pensacola Beach. The rip currents killed 5 off the coast of Florida. Outer rain bands from the storm dropped heavy rainfall across the Florida panhandle of over in one day. The Tallahassee Regional Airport recorded in 24 hours, breaking the old 24‑hour record set in 1969. Throughout the state, Allison destroyed 10 homes and damaged 599, 196 severely, primarily in Leon County. Including the deaths from rip currents, Allison killed eight people in Florida The storm also spawned two tornadoes. and downed power lines. Allison produced from of rainfall in North Carolina, closing nearly all roads in Martin County and damaging 25 homes. The severe flooding washed out a bridge in eastern Halifax County and flooded numerous cars. Allison also produced one tornado in the state. In Maryland, rainfall from Tropical Depression Allison totaled to in Denton and peak wind gusts of were recorded in Salisbury.

Allison, in combination with an approaching frontal boundary, dropped heavy rainfall across southeastern Pennsylvania, peaking at in Chalfont in Bucks County and over in portions of Philadelphia. The rainfall caused rivers to rise, with the Neshaminy Creek in Langhorne peaking at . Several other rivers and creeks in southeastern Pennsylvania crested at over . The rainfall downed numerous weak trees and power lines, leaving 70,000 without power during the storm. The flooding washed out several roads and bridges, including a few SEPTA rail lines. In addition, the rainfall destroyed 241 homes and damaged 1,386 others. Flooding at a Dodge dealership totaled 150 vehicles. Hundreds of people were forced to be rescued from damaged buildings from flood waters. The flooding dislodged a clothes dryer in the basement of the "A" building of the Village Green Apartment Complex in Upper Moreland Township, breaking a natural gas line. The gas leak resulted in an explosion and an ensuing fire that killed six people. Firefighters were unable to render assistance as the building was completely surrounded by floodwaters. Additionally, one man drowned in his vehicle in a river. Damage in Pennsylvania totaled to $215 million (2001 USD). Gusty winds of up to in Atlantic City downed weak trees and power lines, leaving over 13,000 without power. Several people had to be rescued from high waters, though no fatalities occurred in the state. Overall damage was minimal.

Tropical Storm Allison caused flash flooding in New York, dropping up to of rain in one hour in several locations and peaking at in Granite Springs. The rains also caused river flooding, including the Mahwah River which crested at . Allison's rainfall damaged 24 houses and several stores, while the flooding closed several major highways in the New York City area. Overall damage was light, and no fatalities occurred in New York due to Allison. Similarly, rainfall in Connecticut peaked at in Pomfret, closing several roads and causing minor damage to numerous houses. The Yantic River at Yantic crested at , while a state road was closed when a private dam in Hampton failed from the rainfall. In Rhode Island, Allison produced up to of rainfall in North Smithfield, washing out several roads and houses, and destroying a log house in Foster.

An isolated severe thunderstorm in the outer bands of Allison produced an F1 tornado in Worcester and Middlesex Counties in Massachusetts, impacting over 100 trees and damaging one house and one small camper. A microburst in Leominster and another in Shirley damaged several trees. Lightning from the storm hit two houses, causing significant damage there but little elsewhere. Allison also produced moderate rainfall in the state, mainly ranging from . The rainfall caused drainage and traffic problems. Damage in Massachusetts totaled to $400,000 (2001 USD).

Aftermath

Within weeks of the disaster, President George W. Bush declared 75 counties in Texas, southern Louisiana, southern Mississippi, northwestern Florida, and southeastern Pennsylvania as disaster areas. The declarations allowed affected citizens to receive aid for temporary housing, emergency home repairs, and other serious disaster-related expenses. The Federal Emergency Management Agency (FEMA) also provided 75% for the cost of debris removal, emergency services related to the disaster, and repairing or replacing damaged public facilities, such as roads, bridges and utilities. The American Red Cross and the Salvation Army opened 48 shelters at the peak of need for people driven from their homes, which served nearly 300,000 meals. After the extensive damage to Houston hospitals, the U. S. Public Health Service team set up a temporary hospital at the Houston Police Academy. The National Disaster Medical System deployed a temporary hospital to Houston with 88 professionals, aiding nearly 500 people. The team of 87 medical personnel served over 1,000 patients within two weeks of Allison's passage. Thirty-five volunteer services provided aid for the flood victims in Texas, including food, clothing, and volunteers to help repair the houses. After nearly 50,000 cars were flooded and ruined, many people attempted to sell the cars across the country without telling of the car's history. Many people took advantage of the flood victims, including fraudulent contractors and suppliers raising the price of goods more than is actually justified. Following the extreme flooding, a mosquito outbreak occurred, though FEMA provided aid to control the problem. By six months after the storm, around 120,000 Texas citizens applied for federal disaster aid, totaling to $1.05 billion (2001 USD).

FEMA, in conjunction with the Harris County Flood Control District, began a $100 million buyout program later in 2001 to move residents' homes to less flood prone areas. The district identified approximately 2,400 at-risk dwellings and purchased about 1,800 by June 2006, five years after Allison. The district had spent approximately $650 million on other flood control measures by that time, including increasing the size of bayous and raising the height of bridges. A study published shortly before Hurricane Harvey in 2017 argued that the program did not go far enough and that at least 3,300 additional homes should have been eligible for buyout. However, The Texas Tribune noted that even then, thousands more residences would remain within 100-year floodplains. The city of Houston attempted to prevent additional building within floodplains. However, this plan backfired when property values plummeted due to residents being prohibited from modifying their homes. Consequently, several lawsuits ensued and the city council was convinced to remove many of these restrictions around 2008. Furthermore, improvements to the bayous took years longer than expected and some flood control experts even argued that those upgrades would provide little additional protection. Overall, mistakes and setbacks with these programs left Houston vulnerable to another catastrophic flood event, such as Harvey in 2017.

Like in Texas, a mosquito outbreak occurred in Louisiana. Only pesticides acceptable to the US Environmental Protection Agency and the US Fish and Wildlife Service were allowed to be used. FEMA officials warned homeowners of the dangers of floodwaters, including mold, mildew, and bacteria. By three months after the storm, just under 100,000 Louisiana citizens applied for federal aid, totaling to over $110 million (2001 USD). $25 million (2001 USD) of the total was for business loans, while an additional $8 million was for public assistance for communities and state agencies. More than 750 flood victims in Florida applied for governmental aid, totaling to $1.29 million (2001 USD). In Pennsylvania, 1,670 flood victims applied for federal aid, totaling to $11.5 million (2001 USD). $3.4 million (2001 USD) of the total was to replace a SEPTA rail bridge over Sandy Run in Fort Washington.

Retirement

Due to the severe damage and deaths caused by the storm, the name Allison was retired from future use in the Atlantic basin in the spring of 2002 by the World Meteorological Organization. The name was replaced with Andrea for the 2007 season. Tropical Storm Allison was the first Atlantic tropical storm to have its name retired without reaching hurricane strength.

Notes

References

External links

National Hurricane Center Tropical Cyclone Report on Allison
US National Hurricane Center Tropical Storm Allison advisory archive
Affects Of Tropical Storm Allison In Houston
wpc Rainfall Summary for Allison
NWS Service Assessment (PDF)
Tropical Storm Allison Recovery Project Community resources and continuing research of the continued flood threat to Houston