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Tropical Storm Allison: The high-water mark of Houston floods

This week marks 16 years since Tropical Storm Allison set forth an apocalyptic flood on the Houston region.  Although the area has had more than its fair share of floods since then there are many living here now who did not experience this storm; in fact the Houston Metro population has grown by almost 2 million people since 2001.  In addition to changing how the city approached flood control, Allison was also personally significant to me.  Thankfully Allison did not cost me my home or possessions as it did many others, but it was this storm that launched my obsession with the weather.  I have always found this storm to be fascinating and I wrote this piece to share both my memories and the history of this (hopefully) once in a lifetime event.

The 2001 hurricane season got off to a fast start when a relatively benign and ragged looking tropical storm formed on June 5th off the upper Texas coast. Little did anyone know then, that “little” tropical storm named Allison would produce the benchmark of all floods for the Houston area, and cause enough damage to become the first tropical system to have its name retired without reaching hurricane status since naming of storms began in 1947.

On a personal level this storm was the turning point in my relationship with meteorology. Prior to Allison the thought of bad weather gripped me with fear. I was terrified of thunderstorms, to the point that I would not be able to sleep if any kind of rain was forecast during the nighttime. I absolutely hated the raw and totally uncontrollable fury that nature could unleash outside of my window. However, watching the events of Allison unfold in my backyard (at night no less) caused that fear to transition into the fascination that I have today.

Allison was born on June 5th, 2001 just offshore and made a slow landfall just west of Galveston that same evening as a 50 MPH tropical storm. Despite officially being a weak tropical storm, as measured by wind speed, Allison showed it was nothing to be scoffed at – producing as much as 12 inches of rain in some spots around the Houston area and flooding hundreds of homes that night. That in itself would have been enough to make this a memorable storm, but this was only Allison’s introduction.

Following landfall Allison was downgraded to a depression and it drifted for the next 2 days to about 100 miles northeast of Houston. The storm was caught between two areas of high pressure which led to the storm’s slow and wobbly movement; like a spinning top bouncing between two walls.  On Friday June 8th one of those areas of high pressure strengthened while the other weakened causing Allison to loop around and head back to the south – towards the areas it had flooded earlier in the week.

Despite just being 14 years old at the time I vividly remember that Friday. It was one of the first days of my summer break. For some reason I distinctly remember that day as being particularly sunny, hot, and very humid. I spent that day hanging out with friends playing video games and going to the movie theater.  In research I did in the years following the storm I learned that the sun and oppressive heat and humidity during the day on the 8th played a role in the flood that would come about that night.  The sun warmed the atmosphere and sped evaporation of water from the already saturated ground.  This heat and humidity helped to charge up the atmosphere for what ended up being Allison’s knockout punch.

By mid-evening the of June 8th Allison had drifted back over the Houston area and stalled. I remember having a family dinner at my grandmother’ house that evening when the rain started again.  We ended up cutting it short as my parents were worried about getting back to our house in the Meyerland area with this new batch of rain falling.  During the evening Allison’s center had drifted into an already moisture rich atmosphere and moved close enough to the coast to begin drawing in even more moisture off the Gulf of Mexico. After the sun went down a phenomenon known as a core rain event developed. Core rain events can happen after nightfall, usually with relatively weak tropical systems, when the loss of daytime heating causes the low pressure center to contract, and the cooling atmospheric temperatures lead to the production of intense rainfall near the center of the low.

The rain dramatically picked up after dark that night and did not stop until the middle of the next morning. The sensory experience that night was unforgettable especially the sounds of the rain.  If you experienced this storm you know what I am talking about. If you did not the only way I can describe the rain that night would be to tell you to imagine the heaviest rain you have ever seen, the kind of rain that produces a deafening jet engine like roar as it falls on your roof, then imagine it continuing without a break for about 12 hours. On that night as much as 28 inches of rain fell over parts of the Houston area in less than 12 hours.

June 8-9 map marked

This map from the Harris County Flood Control District shows rainfall totals from June 8th to June 9th. Almost all of this rain fell within a 12 hour period. I circled in red some of the highest totals.

As day broke on June 9th incredible images began to appear. Much of America’s 4th largest city was underwater.

 

Allen pkwy before

Allen Parkway leading into Downtown prior to Allison

Allen pkwy after

After Allison

59 before

HWY 59 before Allison (look how new and fresh it is)

59 after

HWY 59 after Allison

June 5-9 marked

Total rainfall measurements for the whole June 5th-9th period that Allison affected the area. Again, I circled some of the more significant totals.

 

The 4 day Allison event dumped as much as 36 inches of rain in some spots (80% of the area’s average annual rainfall), caused 22 deaths and over $5.5 billion in damage, and forever changed the way the city approached flood control.  Personally, this storm is still the most incredible weather event I have ever experienced.  In less than one week I went from being terrified of bad weather to being utterly captivated by it.

Allison has become the shining example of why the sexy statistic of wind speed can be deceiving when gauging the danger posed by a tropical system, especially for inland areas.  Often it is these “smaller” or “weaker” systems that can be the most prolific rain makers since they can also be slow movers.  As hurricane season is beginning once again Allison should serve as a reminder that all tropical systems should be taken seriously, especially given the flood prone nature of the region.

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