After the worst floods since Allison what comes next?

This post will summarize the remarkable rain event Houston experienced during the last 24 hours and look ahead at what we can expect during the rest of the work week.


It is not an overstatement to say this was the Houston region’s worst flooding event in nearly 15 years, since Tropical Storm Allison deluged the upper Texas coast and dumped in excess of 30 inches of rain over parts of the city.

24-hour rain totals from 1pm Sunday through 1pm Monday. (National Weather Service)
24-hour rain totals from 1pm Sunday through 1pm Monday. (National Weather Service)

As you can see in the map shown above, a large swath of Texas from Lockhart nearly all the way to the center of Houston received in excess of 10 inches. Some isolated areas recorded nearly 20 inches of rain in less than a day.

How rare are such events? The map below, produced by a NOAA laboratory, shows the “return period” for a rain storm of such magnitude for a given area. The area of dark purple just west of Harris County lies over Waller County. It suggests that rains experienced in this area, including along the western Harris County line, were a 200-year storm, which is to say on average you could expect a storm this intense about every 200 years.

Return period for an event of Sunday night/Monday's magnitude for the region. (Multi-Radar/Multi-Sensor (MRMS) Flooded Locations and Simulated Hydrographs )
Return period for an event of Sunday night/Monday’s magnitude for the region. (Multi-Radar/Multi-Sensor (MRMS) Flooded Locations and Simulated Hydrographs )


By the middle of Monday these rains had done their work. According to Jeff Lindner, with the Harris County Flood Control District, 74 out of the agency’s 142 flood gauges were running above or near flood. Thirteen of the regions 22 waterways were out of their banks and flooding nearby neighborhoods. Preliminary estimates of home damage ran into the thousands of structures, but it will take several days to firm up those numbers. Suffice it to say, this was a very significant flooding storm, and delivered on the near-record atmospheric moisture levels that had been building up over Houston during the last few days.

So what’s next?


As expected, the rains have ended for now. With some drier air moving in from the northwest we should see a gradual end to rains this afternoon, and I’m expecting a fairly quiet evening in terms of showers and thunderstorms. Less certain is what comes on Tuesday, Wednesday and Thursday.

We are not done with the potential for heavy rainfall, as we’ll continue to see atmospheric moisture flowing in from the Gulf of Mexico for the next few days, and the atmospheric instability that led to Sunday night’s very heavy rain showers isn’t going away entirely. However I do not foresee the kind of gargantuan, very slow moving system that we just experienced.

As a result we’re likely to see additional showers and thunderstorms over the next three days, with accumulations of perhaps 1 to 4 inches between now and Friday. While it is possible there will be higher accumulations in certain areas—a possibility I will be closely watching out for—at this time I do not expect to see another bout of widespread 4 to 8 inches of rain. Some forecast models pinpoint Wednesday as the time frame in which we are likely to see the most organized rain showers.

The rains will finally come to an end on Friday, as a cold front comes down and dries out the atmosphere. By Friday or Saturday I think most of us will be quite happy to see the Sun again.

Posted at 3:30pm CT Monday

12 thoughts on “After the worst floods since Allison what comes next?”

  1. Do you think it’s okay to venture out this evening? I need to go from 290 and Tidwell to Montrose. Thank you.

  2. Eric,
    The city has obviously made some improvements to Houston’s ability to handle this much water since Allison, particularly in the Med Center. Do you think more can reasonably be done to significantly improve drainage or is this just an epic storm and no matter what they do it won’t make much difference?

    • As long as we are continuing to develop Houston it will offset a lot of the flood control measures being taken. I would also agree that when it comes to truly epic rain storms there is only so much you can do with Houston generally flat topography.

      • A lot of it depends on the developer. The Clear Lake/Ellington area, Kingwood , and The Woodlands were master planned communities where the drainage was designed for heavy rains.

        After Allison, I spent every other Saturday for months sheet rocking flood houses along the Hardy where the developers simply plopped down tract house after tract house and as many as they could fit. No wonder they had two feet of water in them. I can still smell the mold in my mind.

        Places like the Heights (three or four story townhome after three of four story townhome and no master plan) and any suburban development started in the 80s or 90s scare me to death from a flooding standpoint. At least the higher-end builders started getting religion after Allison.

  3. What about April 2009? There was 9-11.5 inches in the west part of town falling on ground saturated from several rain events leading up to it. 2300 homes flooded? I hope this doesn’t turn out as bad. Praying for the rain to STOP.

  4. We got off easy in beautiful downtown Ellington – only 5.3″ in the last 24 hours. Also the easiest commute home in a long time….probably since the last time the TV anchors told everyone to stay home.

    Thoughts and prayers to everyone affected by the flooding.

    • Totals appear to have been variable over short distances. The three local HCOEM gauges on Clear Creek all read around 5 inches, but I have a solid 6.5 inches in my rain gauge in League City, less than two miles south.

  5. Eric, when’s the next update? I’m in Pearland and made out ok. I lived in South Belt during Allison and remember that day 2 is when we got flooded.

  6. Looking at the rain gauges, I think this is one time I’m glad I did *not* evacuate to Katy.

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