Tropical Storm Allison suddenly formed 80 miles off the coast of Galveston, Texas, on Tuesday, June 5, 2001, no one expected that, five days later, it would go on record as one of the most devastating rain events in the history of the United States. Neither historical data nor weather forecasts could adequately predict this extraordinary storm that, before leaving the area, would dump as much as 80 percent of the area's average annual rainfall over much of Harris County, simultaneously affecting more than 2 million people. When the rains finally eased, Allison had left Harris County, Texas, with 22 fatalities, 95,000 damaged automobiles and trucks, 73,000 damaged residences, 30,000 stranded residents in shelters, and over $5 billion in property damage in its wake. Leaving 31 counties with declared disasters in Texas, Allison went on to spread disaster declarations to Louisiana (25 parishes), Florida (nine counties), Mississippi (5 counties) and Pennsylvania (2 counties). Allison was the costliest tropical storm in the history of the United States.