Maps and Charts: The Ellicott City rain disaster in two maps and three charts

by Washington Post

Map 1: A remarkable amount of rain fell, and not just over Ellicott City.

(National Weather Service)

Ellicott City received 6.6 inches, which is a ton of rain and most in the region. But it wasn’t the only area deluged.

The above map,  presenting an analysis of rainfall from both radar and ground sensors, shows amounts of at least two inches in sizable chunks of Montgomery, Howard and Baltimore counties. Quite a large area received at least three inches, as portrayed by the orange and darker shades.

Chart 1: The rainfall intensity was incredible.
(National Weather Service)

Of the 6.6 inches of rain that fell in Ellicott City, an amazing 4.56 inches of that total fell in just one hour, from 7:30 to 8:30 p.m.

Consider this mind-boggling comparison: During the paralyzing blizzard of Jan. 22-23, known as Snowzilla, Reagan National Airport received the equivalent of 1.48 inches of rain over 36 hours. Approximately this amount of water (1.44 inches) fell in just 10 minutes (from 7:50 to 8 p.m.) during the Ellicott City flood.

Map 2: This was an extremely rare event.

(National Weather Service)

The amount of rain that fell in Ellicott City over three hours Saturday night was exceptionally unlikely. The National Weather Service’s Hydrometeorological Design Studies Center (HDSC) determined the probability was less than 1 in 1,000.

“This does not mean this extreme rainfall will only occur once in a thousand years,” the National Weather Service stressed. Rather, the Weather Service said, it simply means the chance of this happening in Ellicott City in any given year is less than 0.1 percent.

Chart 2: This was an extremely rare event by many different measures.

(National Weather Service)

This rainfall event produced exceptionally intense rain amounts over a range of time periods. It delivered both unusually strong bursts as well as a historic multi-hour torrent, which the above chart helps illustrate.

The chart is a little difficult to interpret, so bear with me. The red line shows the maximum amount of rain (shown on the left y-axis) which fell over a range of increasing time increments (expressed on the x-axis) during the event. The black lines represent a given likelihood (expressed on the right y-axis) of certain amounts of rain (shown the left y-axis) falling within those time increments.

Notice that the red line showing the observed rainfall remains above the top black line, which represents a likelihood of 1 in 1,000, spanning increments of five minutes to three hours.

Chart 3: The Patapsco River rose unbelievably fast.

(National Weather Service)

(National Weather Service)

The hydrograph for the Patapsco in Ellicott City, which shows the rate of water rise, is breathtaking.

The river rose over 13 feet in just 100 minutes between 7:20 and 9 p.m. At one point (from 8:40 to 8:45 p.m.), it rose over two feet in five minutes.