The atmosphere unloaded at least half a foot of rain in three hours on Ellicott City Sunday, resulting in a catastrophic flash flood. The National Weather Service has conducted a preliminary analysis, which finds this rainfall intensity had less than one-tenth of a percent chance of happening in a given year, or that it was “1 in 1,000.”
Sunday’s cloudburst marked the second 1 in 1,000 chance rainstorm to deluge Ellicott City in two years. The other event happened July 30, 2016. “Amazing … 2 such rare events over the same area in last 22 months,” the National Weather Service Eastern Region tweeted.
The amount of real estate affected by such extreme rainfall was even more extensive for the 2018 event, compared with that in the 2016 one.
In 2016, the 1 in 1,000 chance rainfall zone, shaded in purple (below), is substantially smaller than the comparable area in 2018 (above).
For this year’s event, the 1 in 1,000 year rainfall area not only affected Ellicott City but also a large surrounding area including Catonsville, where severe flooding occurred. Radar estimates and rain gauges suggested at least 10 inches of rain fell in this area.
Rainfall events determined to have less than a 1 in 1,000 chance of happening in a given year are sometimes reported as “thousand-year rainstorms.” However, such phrasing is controversial, because it doesn’t necessarily mean that amount of rain will occur so infrequently, since past data is limited, and the climate is changing.