Allison (2001)

Tropical Storm Allison formed from a tropical wave and became a tropical storm on 5 June 2001 in the northwestern Gulf of Mexico about 120 nautical miles south of Galveston, Texas. It moved north-northwest and made its first landfall on the upper Texas coast during the night of 5–6 June, then weakened to a depression and lingered over eastern Texas. The system moved back over the Gulf on 10 June, acquired subtropical characteristics, redeveloped a new center near Morgan City, Louisiana (11 June), and tracked east-northeast across the Deep South and up the U.S. East Coast before becoming extratropical on 18 June and dissipating southeast of Nova Scotia on 19 June.

Allison made landfall near Freeport, Texas (just southwest of Galveston) late on 5 June as a tropical storm with estimated sustained winds near 45 kt (about 50 mph). A second landfall occurred near Morgan City, Louisiana, around 0200 UTC on 11 June when the system had become a subtropical storm with sustained winds near 35 kt (about 40 mph). The storm’s center moved across southern Mississippi, Alabama, Georgia, and the Carolinas while mainly producing heavy rain.

The storm’s maximum analyzed intensity was 50 kt (about 58 mph) with a minimum central pressure near 1000 mb on 11 June; the operational peak intensity earlier on 5 June was estimated at 50 kt (the best-track peak). At its peak it was classified at tropical-storm strength (it later alternated between tropical, subtropical, and extratropical stages).

Storm surge and rainfall caused the worst impacts. Coastal overwash and surge of 2–3 feet with waves up to 8 feet occurred on western Galveston Island; portions of southeast Louisiana saw 1–2 feet of surge on 11 June. Extremely heavy rainfall fell over eastern and southeastern Texas, especially the Houston metropolitan area — several locations east of Houston received more than 30 inches, with many individual station totals: Furr High School (Harris County) recorded 35.14 inches, Pearland 21.41 inches, Brays Bayou locations recorded near 19–21 inches, and many bayous and creeks recorded totals in the teens to twenties of inches. Morgan City, LA reported about 20.96 inches; Lafayette, LA recorded 15.58 inches. Other notable totals along the track included nearly a foot or more across parts of Louisiana, Mississippi, Alabama, and the Southeast.

Allison caused widespread flooding and damage and was deadly. The post‑storm direct death total attributed to Allison was 41 people (Texas 23, Florida 8, Pennsylvania 7, Louisiana 1, Mississippi 1, Virginia 1), with 27 drownings in freshwater flooding; there were also 9 indirect deaths in North Carolina from traffic accidents on wet roads. Property damage estimates were large and were revised upward after the event: insured losses and standard multipliers gave about $2.4 billion, FEMA and state estimates approached $5 billion, and a later update (11 August 2011) placed total damages near $9 billion. In the Houston area alone, damage was concentrated on public facilities (notably the Texas Medical Center), residences, and businesses; over 14,000 homes were destroyed or had major damage and tens of thousands more were damaged.

Noteworthy aspects: Allison intensified to tropical-storm strength quickly with little lead time for warnings, so operational warning lead was short. The system was unusually long-lived and complex, switching between tropical, subtropical, and extratropical phases while producing catastrophic inland flooding rather than extreme coastal winds. A total of 23 tornadoes were confirmed from Mississippi to South Carolina during 11–16 June. Forecasting responsibility shifted between centers as the system moved inland and changed character, and some observations (e.g., ship and sailboat reports) were later judged inconsistent with official measurements.