Hurricane Harvey – Extraordinary Flooding for Houston and Surrounding Areas

The analysis finds that Harvey brought 1,000-year rains over a 24-hour period over much of the Houston, Galveston, and areas extending east all the way to Louisiana.

In addition, the storm delivered wide-spread 25,000-year rains over a 120-hour period, and isolated locations witnessed 500,000-year rains over a 120-hour period.

1,000 year rainfall is defined as rainfall with only a 0.1% chance of occurring in any one-year. 500,000-year rainfall is defined as rainfall with only a 0.0002% chance of occurring in any given year.

It’s important to note that the probabilities for rainfall this extreme are calculated by extrapolating the distribution curves for precipitation observed over the last century and with the most current statistical approaches available.  This event is so far off the charts that terms like "500,000-year rainfall” should be understood to represent where these events fall on the curve, as opposed to being a understood projection of their frequency. Although extremely rare, rainfall statistics of this nature are routinely computed by companies like MetStat to help design infrastructure.