New Atlas-14 data from the National Oceanographic and Atmospheric Administration (NOAA) sheds some light on the intensity of storms in the last week. How unusual were they?

May 3rd was a 200-year storm. May 7 was a 100-year storm. And we can expect a week that wet about once every 5-10 years. Here’s how to find the probability for any storm, location or interval.

Step One: Find Gage Nearest You

Large variations in rainfall totals exist even within small geographic areas. So finding the gage nearest you is the first step. For instance, in this week’s storms, official gages in New Caney and Humble registered differences of more than 25% for the same event. Harris County Flood Control has a Flood Warning System that shows all gages in Harris County and many in surrounding counties. You can also use your own data if you have a rain gage.

Step Two: Determine Time and Volume

When you’re obtaining the official gage data, try to narrow down the duration as close as possible. That’s because you’re estimating rainfall INTENSITY – a function of both time and volume. A two-inch rain spread out over a day is NOT the same as a two-inch rain in ten minutes.

Step Three: Find Average Recurrence Interval

Once you know how much rain fell in your area in a given amount of time, the next step is to determine the “average recurrence interval (ARI).” That tells you how often you’re likely to experience a storm of that intensity. Here’s where and how to find it.

To determine the ARI for any location, go to this page on the NOAA site.

Example

Once you’re at the NOAA site, click your location on the map. Here, I clicked on the approximate location of Elm Grove in Kingwood.

A chart showing the average recurrence intervals for Kingwood will appear below the map.

On Friday, May 3, Kingwood received 5-6 inches in less than an hour. Scrolling down to the 60-minute line and over to the column that shows 5.49 inches, we can see that that was an estimated 200-year rainfall.

On Tuesday, May 7, the Porter/New Caney area received about 8-10 inches of rain in less than six hours. That was an estimated 100-year rain.

For the seven days starting May 3, gages in the Lake Houston area averaged 10-12 inches. We can expect a week that wet about every 5-10 years.

Probabilities Can Be Mind-Benders

But wait! How can that be? How can you get a 200-year rain and 100-year rain four days apart? And how can we get weeks this wet in 2015, 2016, 2017 and 2019 if it’s supposedly a 5-year event? Simple. You’re dealing in probabilities of independent events. If you toss a coin ten times and it comes up heads each time, the chances of it coming up heads on the 11th toss are still 50%.

It’s the same way with weather. Just because you won the lottery last week doesn’t mean you can’t win it again this week. With that happy thought, I’ll wish you pleasant skies today.

Posted by Bob Rehak on 5/11/2019 with help from Diane Cooper

620 Days since Hurricane Harvey