Even if you spent most of June 2015 in a cave, it was difficult to miss that it was a very wet month. Blair County's resident meteorologist Joe Murgo recently noted that it was the second wettest June on record. The wettest was in 1972 when we were inundated with more than fifteen inches of rain, nearly half of that from Hurricane Agnes. While riverside communities like Johnstown and Harrisburg expect problems during such events, even places at the headwaters of the Susquehanna and Ohio rivers were swamped. It was one of the worst floods in Altoona's history.
Agnes became a tropical depression near the Yucatan Peninsula and went northward through the Gulf of Mexico. Had it stayed on land it would have just been another rain storm by the time it hit the Northeast. But it wandered back into the western Atlantic off the coast of South Carolina and intensified before turning westward into Pennsylvania. Parts of northeastern Pennsylvania were especially hard hit, part of Schuylkill County getting more than fifteen inches of rain, washing away a cemetery in the town of Forty Fort, dumping caskets and body parts in people's yards downstream.
While the remnants of Hurricane Bill brought some heavy rain in this second wettest June of all time, Bill was on dry land for several days before arriving in Pennsylvania. While it still brought a rainy day, Bill was not the reason we had such a wet June. What brought so much precipitation this June was the persistence of the rainy weather. Some parts of Blair County got rain on twenty of June's thirty days. (An average June has six rainy days.) Yet surprisingly, flooding was not as widespread this June as one might have expected for a record-breaking month.
Rain in Pennsylvania typically comes from two sources in the summer, frontal lifting and thermal convection. Frontal lifting (more often associated with steady, widespread rain) occurs when warm air is forced up colder, denser air. Moisture in that air condenses when the air cools as it is lifted into cold air higher up in the atmosphere. Thermal convection brings the pop-up storms we experience when air is very warm and moisture laden (usually when we are under the influence of air from the sultry Gulf of Mexico).
This June had a bit of both of these and we didn't go long between the rainy spells brought about by either rain-producing mechanism. Both kinds of rain storms can bring flooding and that flooding can be as different as the rain storms that make them. The typical heavy showers and thunderstorms of summer usually cause flash flooding, while long periods of steady rain bring larger scale river flooding. Both types of floods are made worse by paving and building over large expanses of land that previously absorbed rain water. While most county conservation districts require and oversee larger scale storm water management efforts, much can be done collectively by homeowners and other small scale property owners.
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