Ten years ago, Tropical Storm Allison made landfall on June 5, 2001. No one realized how bad it would be a few days later for the Houston area.
Allison formed from a tropical wave that left the African coast on May 21. It traveled across the Atlantic Ocean and moved over northern part of South America. It crosses Central America and into the Pacific Ocean. A low level circulation develops as it is moving towards Mexico on June 3. The disturbance traverses over Mexico and on June 4 enters the Gulf of Mexico. This disturbance was not forecasted to develop into anything tropical. Despite not so favorable conditions, the disturbance developed into Tropical Storm Allison on June 5th, the first named storm for 2001. It was an elongated and strong tropical storm. Had Allison stayed over the water longer, it would have been a hurricane.
Allison makes landfall around Freeport, Texas that same day. As it moved inland, Allison dumped heavy rain. Some areas saw up to 12 inches of rain, mainly in Friendswood and Greens Bayou. As Allison moved to the north, it weakened to a tropical depression. Allison that stalled when it came around Lufkin, Texas. As it stalled, Allison continued dumping heavy rain over Texas and Louisiana. In the early morning hours of June 7th, feeder bands formed over Beaumont and Sugarland area. Heavy rain begin to fall, which leads to street flooding. Once the rain ended, Sugarland saw up to 12 inches of rain, while Beaumont saw up to 15 inches of rain.
Allison is still lingering over Texas due to a high pressure system to the north. Allison is a weak area of warm core low pressure with 10 mph winds. This weak tropical depression had plenty of moisture, which allowed heavy rain to fall. Allison was viewed as an annoyance at this point. Even in the early morning hours of June 8th, more feeder bands formed along coastal Texas, where 6 to 10 inches of rain fell. Once the rain ended, it was a sunny and clear day. One can see the blue sky. However, it was very humid and one would realize that heavy rain can develop again.
As the heat of the Sun beats down, thunderstorms begin to form north of Houston over Huntsville area in the afternoon hours. The area of storms is dumping heavy rain. Meanwhile, west of Houston, intense thunderstorms are forming and move into Houston. The two area of thunderstorms merge together over Houston setting the stage for more heavy rain. This time the heavy rain is near the center of Allison and is a core rain event. Core rain events happen when the atmosphere cools down for the night and thunderstorms form near the center of a warm core low pressure system. It can be tropical or non-tropical in nature, it is just the low pressure is warm. The warm and cool air clashes leading to thunderstorm development. The storm is confined to a county or couple counties and the rain falls at night. Core rains can lead to extremely heavy rain as it is happening on the night of June 8th.
Heavy rain falls during the rush hour as people are heading home for the weekend. However, the commute is proving to be a dangerous one as roadways and even freeways start to flood. The heavy rain gets worse as the night progresses and storms get more intense. Storms keep forming and back building which leads to even heavier rain. People are wondering when the rain will stop since it had been raining since afternoon. The heavy rain goes on endlessly and that all Houston television stations have to cut into regular programming to cover the unfolding disaster. Many people are now stranded since they cannot get home due to the ever worsening floods. The heavy rain just keeps on pounding hard with plenty of lightning and thunder booming overhead. Bayous are overflowing from the heavy rain and it is not even over yet. If things were bad, it got worse when midnight came. The large area of thunderstorms get more intense, which means heavier rain. This also leads to more misery for the stranded people who want to get home. They are wondering if their houses are flooded due to Allison.
As the bayous continue to overflow from all the rain that had fallen in the past few days, they are getting overwhelmed by the extremely heavy rain. The bayous are going out of their banks and streets are flooding on top of super saturated grounds. The water had nowhere to go, which means flooding is happening again and in places that were hit hard by it from the landfall of Allison. To make matters worse, up to 6 inches of rain fell in one hour!
The Sun rises on the morning of June 9th and morning revealed massive flooding throughout Houston. Streets and freeways were heavily flooded. Houston became a lake from all the heavy rain that fell. Freeways became lakes and many cars and even big rig trucks were flooded. The Texas Medical Center, the largest medical area in the world was flooded, especially at the basements. Many research projects that were going on for years were lost in the flood. Research projects that improved the quality of life for people with health problems. The Texas Medical Center flooded before in the June 15, 1976 flood, when heavy rain up to 15 inches fell in a short time and history repeated itself 25 years later. Some people could not leave their own home and the Coast Guard had to rescue them.
Once it all ended, up to 28.3 inches of rain fell in 12 hours in the Greens Bayou area. It is likely up to 34 inches of rain fell in that same area. Many areas saw 10 to 20 inches of rain from the massive storm over Houston. The five day total were up to 40 inches of rain! It is certainly possible, some areas saw a total equivalent to an average annual rainfall total in five days. Allison dumped some of the heaviest rain in Texas and American history. The 40 inch rainfall total ranks up there with Tropical Storm Amelia (1978) and Claudette (1979) and a flood event triggered by a hurricane remnant in Central Texas in 1921. Amelia dumped up to 52 inches of rain from August 1-4, 1978 . Claudette dumped 43 inches of rain in 24 hours near Alvin on July 25-26, 1979 and remains the 24 hour record to this very day. Thrall, Texas saw 38 inches of rain in 18 hours, which is also an American record on September 9-10, 1921.
Tragically, 22 people lost their life from Allison in Houston area. Of the 22 killed, 12 people died while driving into flooded streets, 6 died while walking only to be washed away, 3 were electrocuted at their homes, and the most ghoulish of them all, a person used an elevator to reach the garage below the street. Think about 22 families who lost loved ones from Allison and the flood event. Allison did $5 billion in damage and is the most costliest tropical storm in US history. No one thought Allison would be responsible for a horrible disaster. Can another Allison happen again? The answer is yes, it will happen again. It is not a what if, it is just a matter of when.