Tropical Storm Allison was the first named storm of the 2001 Atlantic hurricane season. It formed in the northwest Gulf of Mexico on June 5th, just five days into the season.
Although the system actually began off of the west coast of Africa May 21, it didn’t become organized enough to be classified as a tropical storm until two weeks later. Hurricane recon aircraft indicated that Tropical Storm Allison was born while located 80 miles south of Galveston on June 5th.
This was not a big wind event as Allison only had winds to 60 mph: about the same as low-end severe thunderstorms. Tides only rose 2 to 3 feet along the Texas coast, and had little impact on the area.
Allison slogged inland that evening less than 12 hours after forming; but during the next five days its slow and erratic wobbles kept it over the same general area, all the while having access to abundant moisture. This made for a rain machine that would not quit.
Here you see downtown Houston surrounded by floodwaters as the event unfolded.
The flood was extreme in some portions of the metro area.The flooding produced was devastating with more than $5 Billion in damage around Houston.
The first round of flooding on June 5th and 6th accompanied the storm’s landfall. Eight to 12 inches of rain in twelve hours caused flash flooding throughout the area, but not as of yet catastrophic.
Allison was caught in an area of weak steering currents and drifted slowly north on the 6th, away from Houston, but it began to lumber back towards the Energy Capital on the 7th.
A band of torrential rain built from the Beaumont area westward towards Houston during the day of the 7th.
It worked into Liberty County, north of Houston proper. This produced 5 to 10 inches of rain. Another area of thunderstorms developed over the Sugarland-Stafford areas of Fort Bend and Harris Counties (southwestern suburbs of Houston) and dumped another 8 to 12 inches of rain. So now all sides of the metro area were thoroughly soaked: and the worst was yet to come.
See radar loops from Allison:
By the afternoon of the 8th the circulation center was between College Station and Huntsville and drifting slowly to the southwest; since the eastern side was the rainy side, extreme rains produced rising floodwaters lasting all day over the northern Houston suburbs.
During the mid to late afternoon hours, thunderstorms began to develop where skies had been sunny and they quickly intensified closer to downtown Houston. The storms began to repeat over the same general area for many hours, called a training effect.
Tremendous rainfall was observed for up to 10 hours in some locations, with rainfall rates of 4 inches or more per hour were observed on and off throughout the night. One located in northeast Houston measured over 26 inches of rain in just 10 hours!
Flash flooding came upon the already waterlogged city. High rainfall amounts forced almost all the major Houston area bayou systems into severe/record flooding. All major freeways in the Houston area were severely flooded at least one time during this event.
The center of Allison's remains gradually made its way toward the coast by the evening on the 9th. Before moving offshore, another round of heavy rain - between 4 and 8 inches - developed across extreme southern Harris and extreme northern Galveston Counties, hammering the south side of the metro area.
The Federal Emergency Management Agency (FEMA) declared a large part of east and southeast Texas and portions of Louisiana disaster areas.
The center finally moved off to the east on Sunday the 10th, but Allison was far from finished.
The remnants of Tropical Storm Allison went on to rip across several Gulf Coast states with tornadoes and heavy rainfall. Tallahassee, FL broke their 24 hour rainfall record with more than 10” of rain, 1 death, and $20 million in damage. The flooding reached all the way to Pennsylvania as the soggy remains of Allison moved northeast before finally dissipating.
Total damage across Southeast Texas came close to $5 billion ($4.88 billion in Harris County alone). Twenty-two deaths were caused by Allison, all in Harris County, which is the county where Houston is located. Another $100 million was done in states from Louisiana to Pennsylvania.
The severe nature of Allison’s flooding earned the storm another mark of record: The National Hurricane Center retired the name “Allison”, something normally only done for intense hurricanes.