When Tropical Storm Allison hit in 2001 and dropped almost three feet of rain on central Houston, there were all sorts of terms like the "100-year storm" or the "500-year storm" thrown around. These kinds of weather events do more than just destroy property; they re-define flood planes, adjust (usually upward) insurance rates and force people to re-think initiatives meant to curtail flooding like concreting bayous (yeah, that one worked!).
I'm not one to normally say something will never happen again and I won't do it here either. At some point in the future, Houston will probably be struck by another major weather disaster. In fact, we are in the middle of one of the worst droughts we've ever experienced. But, fortunately,a storm dropping that much rain on Houston in that short a period of time is exceedingly rare. Allison was truly the definition of a once-in-a-lifetime storm for many reasons and we've managed to put together five.
5. More than 38 inches of rain in 24 hours. That's how much rain was measured at the Port of Houston, the highest 24-hour rain total in our city's history. While our fair city is certainly no stranger to flooding, particularly the flash variety, none of us have ever seen anything like this in Houston. The closest was the national record set in Alvin by Tropical Storm Claudette in 1979 (Allison is second), but Houston only had mild rainfall totals during that storm. Allison caused flooding in places that never even considered flooding to be a possibility and dramatically re-defined the boundaries of the flood plane in Harris County.
4. Storms don't often form, almost die and then re-form. For most tropical systems (unless you believe the "science" in Day After Tomorrow), dry land is like kryptonite. Once a storm encounters it, the storm begins to rapidly diminish. It can remain as a rain event for days but doesn't re-strengthen unless it makes it back over water, which Allison did. Then it stalled and down came the rain. To have a storm make landfall near Freeport, dance around east Texas, move back southeast and re-enter the Gulf only to stall out near Galveston is as complicated and rare as it sounds. Storms can move over land and then re-form back over water -- it happens often when storms pass over the Florida peninsula -- but not usually after traveling several hundred miles inland.
3. Allison actually impacted the Houston area twice. As mentioned above, Allison moved onshore near Freeport, dousing us with a solid, if unspectacular, batch of rain. That managed to saturate the ground, making whatever came next more likely to cause flooding. No one could possibly predict that what would come next would be three feet of water from the very same storm that soaked the ground in the first place.
2. The weather conditions that caused the second strike were more than unique. The reason weather forecasters have such a difficult time predicting what will happen even a few days out is because weather patterns in the upper atmosphere are controlled by literally millions of variables that interact in ways we have yet to be able to fully understand. The upper level conditions before, during and after Allison likely never occurred before and certainly not in such a way as to impact the direction and intensity of a storm they way they did with Allison.
1. They retired the name Allison. See what I did there? But, seriously, Allison was such a devastating storm, it became the first ever non-hurricane to have its name retired. It remains the only storm with such a distinction.