The Flood Proof City

The weather forecast for Johnstown, PA on July 19, 1977 called for a 30 percent chance of thundershowers. Forecasters knew that it would take less than four inches of rain in three hours to cause flash flooding, but no flash flood watches were in effect despite the fact that a warm, moist airmass was in place over western Pennsylvania and a complex of thunderstorms was heading into the teeth of the unstable air.

Johnstown is famous for one thing: floods. There was the legendary 1889 flood which killed over 2,200 that was caused by days of heavy rain weakening and eventually breaking a dam in the hills above the town. A wall of water rushed down the narrow valley on that fateful day, causing historic death and devastation.

A warm Spring storm system in 1936 caused rapid snow melt and three days of heavy rain that resulted in another devastating flood that killed 24 people. A massive public works project rebuilt the town and the Corps of Engineers channeled the rivers through the town, increasing the capacity and making the city “flood proof”.

Over forty years later, residents went about their business on that sultry Tuesday evening, impressed by the dazzling lightning show over the Valley but confident that the flood defenses would protect them. Unbelievably, in a city with this kind of flood history, there was no local flood observation and warning system.

The National Weather Service in Pittsburgh had no idea that up to 12 inches of rain was falling in Johnstown. Information from the two rain gauges there was not communicated to forecasters. There was a river gauge, but it had to be interrogated by telephone, and the storm knocked phone systems out. Lightning knocked out police communications. Forecasters did not use radar data then to estimate rainfall amounts.

As torrential rains fell starting about 7 p.m., water rushed down hillsides and swelled streams. For nine hours, thunderstorms reformed over the same areas, dumping more heavy rain. Cars and houses started to be washed into the maelstrom. Bridges buckled in the fast flows. Flood waters rose.

But no warning was forthcoming. By 2 a.m., the NWS was receiving reports that flooding was occurring in Johnstown. At 2:30, a Johnstown radio station called to see if a warning had been issued. Only then was a flash flood warning posted.

By dawn, Johnstown was under six feet of water, with 64% of the town flooded. A total of 76 people died and 10 were not found. A total of 2,700 people were injured in the 500 year flood.

Category: Headlines, Met 101/Weather History