How Intense Was that Storm

How Intense Was That Storm? And Why Does It Matter?

The sky darkens. Thunder grows louder. Suddenly, the rain comes down so hard you can barely see. Soon, water starts coming over the curb toward your house. Your weather radio crackles with warnings. And when it’s over, the stormwater has stopped just feet from your door. Close call! But then you wonder. How intense was that storm? And does it really matter?

The answers affect disaster planning, flood insurance, mitigation, and more.

Consider the intensity question first.

How Intense Was That Storm?

Was it a 1-, 2-, 5-, 10-, 25-, 50- or 100-year storm?

Determining the intensity requires more than just reading your rain gage. That’s because intensity depends on volume within a certain time period. Five inches of rain in an hour should concern you far more than in a day.

Start by going to the Harris County Flood Warning System. It shows all official gages in the region. Hover over any one. A box with a hyperlink will pop up. For this example, let’s assume you live near the East Fork San Jacinto and FM1485.

Hovering over the gage by New Caney pulled up box with the blue link.

Click on the More Information link. It will lead you to a search screen that lets you specify date(s). You can also specify time ranges from one hour to one year.

In this case, I selected 2 days and adjusted the date until I captured all the significant rainfall.

By experimenting with different time ranges, you can narrow down the start and stop times. This is crucial, because intensity depends on both quantity and duration.

At our sample gage, I learned that rainfall from Tropical Storm Imelda in 2019 happened over a two-day time period.

Now, look up the total (28.68 inches) in the Atlas-14 table below. This table shows the rainfall precipitation frequency estimates for the Lake Houston Area.

atlas 14 rainfall probabilities
If you live somewhere else, download your table from the National Weather Service here.

So, how intense was that storm? The total rain for two days – 28.68 inches – qualifies Imelda as a 500-year storm at that gage. Look down to the 2-Day line and then across to the 500-year column.

If you captured more or less rain in your rain gage at home, use that total instead. At least now you know that you’re working with a 2-day event. (Note: some rain fell outside this range, but not much.)

Check other Gages to See Geographic Rainfall Distribution

While you’re exploring the Flood Warning System, check several different gages, especially those upstream. Rainfall can vary considerably within short distances.

For instance, during Imelda, a gage at the West Fork and SH242 showed only 13 inches of rain during an identical period – less than half the rain just a few miles west. That ranks as a 25-year storm, not a 500!

When people say that “We got X number of 500-year storms in Y years,” it’s important to remember that the 500-year designation applies only to a particular gage, not an entire region.

Do 500-Year Storms Mean 500-Year Floods?

Also remember this. As the example above shows, having a 500-year storm somewhere near you does not automatically mean you will experience a 500-year flood. It depends on where the rain falls in your watershed. If it falls upstream, it could result in a 500-year flood. But if it falls downstream, it will not.

During Imelda, the gage(s) at:

  • Greens Bayou and US59 got 15 inches – between a 25- and 50-year rain.
  • Spring Creek and I-45 got 8 inches – a 5-year rain.
  • Cypress Creek and US290 got 2.52 inches, not even a 1-year rain.
  • Lake Creek at Dobbin got 1.52 inches.
  • Lake Conroe at 1375 got .84 inches.

Use the drop-down list of gages to explore rainfall rates elsewhere.

A few miles in each case separated Armageddon from something far more manageable.

What Can You Do with This Information?

This exercise can help protect you in several ways. For instance:

  • If you didn’t flood during Imelda and you heard it was a 500-year rain, you might assume you didn’t need flood insurance. That could be a financially devastating miscalculation.
  • If you get little rain upstream and still flood, you should investigate why.
  • If you flood frequently on small rains, you need to investigate the causes and mitigation. Engage with HCFCD, your county engineer or floodplain manager. Perhaps the floodplain has been altered by upstream development. Perhaps the stream near you has become clogged. Perhaps you need to elevate your home or seek a buyout.

The more you know, the safer you’ll be.

Posted by Bob Rehak on 1/18/23

1968 Days since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.