Powerful Pacific System Impacting the West; First Significant Snow for Portions of the East

Power Pacific system will continue to bring significant impacts for Pacific Northwest into northern California the remainder of the week. Dangerous coastal affects, heavy rain, flooding, strong winds, and higher elevation mountain snow continues. Meanwhile, a storm across the east is set to bring the first accumulating snow to many higher elevations of the Catskills into the central Appalachians. Read More >

The Anniversary of the
1977 Johnstown Flood

photo credit: Tribune Democrat

On the evening of July 19th, 1977, Multiple thunderstorms rolled through western Pennsylvania dropping 2 to 12 inches of rain. The heaviest rain fell over the southern half of Cambria County where 10 to 12 inches accumulated.

Multiple thunderstorms that were born in northwestern PA matured over Johnstown, dropping the bulk of their rain over the city. These storms took nearly identical tracks as they crossed Cambria County. Thus, the same areas received rainfall from multiple thunderstorms. This phenomenon is known as "training" - as the storms follow each other like train cars on a track. See the example of Training Thunderstorms in the image below.

Example of training storms

This excessive rainfall produced flash flooding in and around Johnstown. The water caused several smaller dams to break, as well as the Laurel Run Dam - pictured below. The flooding caused 78 deaths and did between 200 and 300 million dollars worth of damage in 1977 - or perhaps over 2 billion dollars in 2017.

Failed Laurel Run Dam, image courtesy damfailures.org
Laurel Run Dam after failure, photo courtesy damfailures.org

In the past 40 years, the National Weather Service has made significant advances in weather forecasting with the help of powerful computing technology.

The national Doppler Radar Network is now in place to accurately track storms and estimate the precipitation they produce. Continuous satellite coverage is now available as frequently as every minute during severe weather. Additionally, forecasters can now disseminate warnings in real-time (instantaneously) via many methods and channels to emergency management officials, broadcast media, cell phones and to the internet.

Certainly, if rainfall of this magnitude were to occur today, there would be flooding and property damage. However, it is likely that advances in forecasting and warning communications/dissemination would help to greatly mitigate the loss of life.

When thunderstorms threaten, you can monitor local broadcast media, or listen to NOAA Weather Radio for immediate notification of watches and warnings. Many cell phones are also set to go off when a Flash Flood Warning is issued for the location where the cell phone is located. Monitoring social media - including the Twitter feed from @NWSStateCollege - can alert you to warnings, as well, but are not as reliable as NOAA Weather Radio and broadcast media (TV & Radio). Weather apps and software may also provide alert features to let you know when a warning has been issued for an area you are interested in.