Earth Matters: Recalling Hurricane Agnes in Blair
By John Frederick
For those of us old enough to remember, 1972’s Hurricane Agnes is the literal and figurative barometer by which all extreme flood events are measured.
Like all hurricanes, this one formed when conditions enabled an atmospheric wave to begin circulating along the Intertropical Convergence (ITC) Zone. This might sound like complicated science, but like many scientific processes, the development of such storms is understandable if one can grasp a few basic principles.
Big summer storms of all kinds need two ingredients: large amounts of moisture and a mechanism to lift and cool air. The area near the ITC in the Atlantic Ocean provides both and becomes the place where tropical storms and hurricanes are born.
In simple terms, the ITC is where southern and northern hemisphere air meet. This meeting (or convergence), especially in a place that is warm the entire year, causes the air to be lifted. As it is lifted, it cools, and the already juicy air forms clouds and rain.
Since the strongest sunlight — and hence, the warmest weather — moves northward during our summer, hurricanes form in our summer and fall. So it’s rare to have a hurricane in the northern hemisphere much before mid-June. Agnes, then, became the most damaging early season storm in U.S. history.
The actual tropical depression formed over the Yucatan Peninsula on June 14 and was upgraded to a tropical storm two days later. After wandering into the Caribbean Sea, it went directly north toward the western part of the Florida panhandle, landing there on June 19.
As hurricanes go, its winds were not so nightmarish, but the amount of rain was. This was so, in part, because it meandered into the western Atlantic after going through the Southeast, pulling in more moisture from the ocean.
But one more twist and an additional ingredient were added to the recipe for Maryland and Pennsylvania. In the weather coincidence from Hell, a mid-latitude low pressure system came across the country just as Agnes turned westward from the Atlantic.
To make matters worse (as if they weren’t bad enough already), the very heaviest rain fell in the Susquehanna River watershed. A swath from Wilkes-Barre to the Maryland border south of York got at least 10 inches. A large subset of that area north of Harrisburg got more than 14 inches from the storm, according to the detailed report compiled by the US Geological Survey and the National Oceanic and Atmospheric Administration.
One weather station in Schuylkill County recorded 19 inches, nearly half the area’s average annual rainfall in a three day period. All of that ended up in Harrisburg, flooding, among other things, the Governor’s mansion just off Front Street north of downtown.
Reports from the June 23 Altoona Mirror noted that Altoona fared better than other parts of the county, but Mill Run, Frankstown, Williamsburg and Tyrone were overwhelmed. A 15-year-old at the time, I vividly recall my father taking me to see the massive washouts along Mill Run.
Like so many catastrophes, Agnes provided a wakeup call in this part of the country that floodplain management wasn’t just important, but could save lives. When someone complains about floodplain restrictions, they’d be well-served to recall Agnes and realize that not building in flood zones makes sense.
John Frederick lives in Blair County. To read more of his writings, visit www.johnjfrederick.com.