Though heavy rain is a common element that can occur during the average hurricane or tropical cyclone, Tropical Storm Allison was anything but ordinary. Lasting almost a month from the time it first began to form, Allison generated extreme amounts of rainfall over Houston, Texas and other areas along the Gulf Coast, causing widespread flooding and devastating damage to lives and property across several states.

Before the official start of the 2001 Atlantic Hurricane season (June 1st), Allison had started to form from a tropical wave off the coast of Africa around May 21, soon crossing the ocean and reaching Mexico by the first of June. By June 5th, Allison had evolved into a tropical storm less than 150 miles from the Texas coast. However, as the storm moved inland, it quickly weakened and became a tropical depression. Keeping this in mind, if the storm had weakened by the time it made landfall, why did such intense rainfall occur?  

The rainfall potential of tropical cyclones doesn’t depend on the strength of the storm system. Many other factors contribute to the amount of rain to be expected from a tropical storm or depression: topography of the landscape, wind shear, the time of day that a storm strikes, storm size, and more. In the case of Tropical Storm Allison, the flat topography of southeast Texas and the lack of wind shear present during the storm certainly played a part. Ultimately, however, Allison was a slow-moving storm caught between two high pressure centers, trapping it in place until the pressure changed. From June 5 to June 9, Allison spun slowly over the Houston area, generating intense amounts of rain over the same confined region and causing catastrophic flooding to the Texas and Louisiana coasts.

Although widespread flooding occurred quite quickly due to the amount of rain that fell in such a short period of time, no flood-related deaths were reported in Texas until June 8. However, during the last two days of Allison’s rains, 22 flood-related deaths were reported in Texas alone. The highest rainfall amounts were measured at around 38 inches, and continuous, heavy rainfall was reported as lasting up to 10 hours in certain areas. Flooding became widespread and major freeways experienced severe floodwaters, making them impassable. An estimate by the Federal Emergency Management Agency (FEMA) and state emergency agencies reported the property damage near $5 billion, with public facilities, residential properties, and businesses being affected. About 14,000 homes were destroyed or incurred major damage, while 34,000 more received minor damage. 

All told, by the time Tropical Storm Allison dissipated on June 19, 41 deaths were reported across several states. Half of those deaths occurred in Texas, with others in Florida, Louisiana, Missouri, Pennsylvania, and Virginia. 27 of those deaths occurred due to individuals attempting to walk or drive through floodwaters. More than 2 million people were affected by the flooding and power outages in the Houston area alone. 

Tropical Storm Allison, and Hurricane Harvey more recently, are reminders that freshwater flooding from tropical systems pose major hreats to lives and property across the United States. At the NWS, it has been our mission since our founding over 150 years ago to provide warnings, forecasting, and safety precautions so that those in danger can be prepared for the worst.

Additional Reading: 

• https://www.noaa.gov/education/stories/remembering-tropical-storm-allison

• https://www.weather.gov/media/publications/assessments/allison.pdf

• https://www.weather.gov/hgx/wxhistory?mm=06&dd=05

• https://www.nhc.noaa.gov/data/tcr/AL012001_Allison.pdf