In late June and early July 1972, major flooding caused by the exceptional rainfall associated with Tropical Storm Agnes ravaged the Middle Atlantic States.
Although only a category 1 hurricane when it hit Florida and a tropical depression when it moved into the Northeast, the rainfall produced by Agnes made this storm more than twice as destructive as any previous hurricane in the history of the United States and remains the worst natural disaster ever to strike Pennsylvania.
Agnes originated as a weak tropical disturbance first detected over the Yucatan Peninsula on June 14. The depression intensified rapidly and moved eastward into the Caribbean Sea. It reached tropical storm intensity on June 16 and began to migrate northward toward the Florida Panhandle. Agnes moved into Florida on June 19, with reduced maximum wind speeds.
By the morning of June 20th, the storm had weakened to tropical storm intensity and never again reached hurricane strength.
Tropical Storm Agnes ranks among the weakest hurricanes in intensity, but had a relatively large diameter of 1,000 miles. Also unusual was its long overland path and the large amount of rainfall it brought to the relatively populous and heavily developed coastal regions of the Atlantic States. During the waning stage of its life cycle, the weakened Agnes merged with an extratropical cyclone circulation centered over the northeastern United States. This reinforced storm stagnated over western Pennsylvania for about 24 hours, yielding additional rain over the northeastern section of the Nation.
The rainfall over the eastern United States from Tropical Storm Agnes and other weather systems during June 16-25 produced record floods. Greatest rainfall amounts were measured in Pennsylvania and New York. The greatest 24-hour amount measured in Pennsylvania was 14.8 inches in Schuylkill County in the Mahantango Creek basin. This amount well exceeds the value expected once every 100 years - this was bigger than the 100-year flood. Total precipitation for the storm at several locations from New York to Virginia was in excess of 15 inches.
The flooding caused by the heavy rain ravaged parts of twelve States. Record peak stages (water levels) and discharges (streamflows) were measured on many streams, according to the U.S. Geological Survey, and water levels in many reservoirs in New York and Pennsylvania were at their highest level since construction.
Along the main stem of the Susquehanna River from the New York- Pennsylvania border to its mouth on Chesapeake Bay, this was the greatest flood since at least 1784. Peak flows from Harrisburg downstream to the Bay were in excess of 7.5 million gallons per second (1 million cubic feet per second (cfs)).
In Maryland, Delaware, and Washington, peak streamflow discharges of three six times the previous maximum flows were recorded by the USGS at many gaging stations on streams and rivers. These flow levels have never been approached since, even during the severe winter flooding in January and September 1996 (following the Blizzard of '96 and during Hurricane Fran, respectively).
Total damage from Tropical Storm Agnes was estimated at $3.1 billion*, or more than twice that caused by Camille, the second most destructive hurricane to strike the United States. Most of the damage resulted from flooding in the Middle Atlantic States.
Damage reached $2.12 billion in Pennsylvania alone, making this still the worst natural disaster in the history of the Commonwealth. Damage included destruction of homes and other structures, flooding of public water and waste-water treatment facilities, inundation of industrial and public utility plants, loss of crops, and disruption of normal activities.
*Damage figure in 1972 dollars, equates to $7.5 billion in 1996 dollars
Improvements since 1972
Since the devastating floods of 1972 that took nearly 500 lives, the human toll has declined in the United States, thanks in part to advances in science and technology as well as improved partnerships among local, state, and federal agencies working together to provide advance warnings to citizens. Floods continue to cost the Nation an average of $3 billion in damages and claim about 95 lives every year.
"Recent technological advances in water-level sensing and recording equipment and data-transmission methods have greatly increased the availability of streamflow data to water-resource managers," said Robert Hainly, Assistant District Chief of the U.S. Geological Survey's Water Resources Division, Pennsylvania District. "We now have the ability to provide streamflow and river-level data to decision-makers within minutes after the readings are taken by USGS equipment installed on streams across Pennsylvania. This capability, along with advances in flood- forecasting tools, such as the Doppler radar system used by the National Weather Service, have greatly enhanced the quantity and quality of information available to water-resources and emergency managers in Pennsylvania," Hainly said.
Source: U.S. Geological Survey