The first hint of the trouble to come popped up on radar at noon Monday — an unexceptional patch of light rain over an area from Clifton Heights to Glenolden, Delaware County.
By the end of the night, the Montgomery County Emergency Center was flooded with more than 1,600 calls and had commissioned 25 water rescues as cosmic downpours inundated the region within just a few hours.
The National Weather Service posted scores of reports of hailstones with the diameter of half-dollars, submerged vehicles, and closed roads from Berks County and Philadelphia’s western suburbs to Ocean City and Ventnor.
“The ferocity of that storm definitely caught us all by surprise,” said George Locke, the borough manager of Jenkintown, which received close to 6 inches of rain, more than the 60-day total for any county in the region through Sunday, according to National Weather Service data.
While the atmosphere on Monday was primed for mayhem, said Dave Bowers, senior meteorologist with AccuWeather Inc., “it was even more violent than I expected.”
The outbreak was as peculiar as it was ferocious. Meteorologists said they could not recall the last time they saw something similar around here.
Rather than move west to east as thunderstorms are wont to do, these propagated outward from those humble Delco origins, forming what the weather service called a “ring of fire” centered near Philadelphia. Weather Service meteorologists described the radar loop as “unreal.”
And the storms were at once symmetrical, chaotic, and capricious. For example, close to 4 inches of rain was measured in the Fox Chase section of Philadelphia. That was 16 times the 0.24 that fell at Philadelphia International Airport, near where all the trouble started.
While the Tookany Creek spilled over in Cheltenham, closing the Tookany Parkway, other municipalities in the county were able to help out with water rescues since nothing much was happening, said Kenneth C. Hellendall, the township’s emergency management coordinator.
“It was definitely crazy,” said Lee Robertson, a weather service meteorologist in Mount Holly.
By late Monday morning the temperatures had reached 90 in Philadelphia and the atmosphere was a broth of water vapor. The “precipitable water” content, that’s the rain potential, was at extreme levels, forecasters said. In the morning, the air was soaking shirts; in the afternoon the rain was doing the soaking.
Meanwhile, a pool of cold air was situated in the upper atmosphere, and this being July, when the air becomes as languid as the rest of us, not much was moving up there.
It was all but inevitable that the steamy air near the surface was going to soar above that cool pool and condense into showers, said Bowers. All the air needed was a little agitation.
Banging cymbals together might have worked better, but “we could have all stood out there with pillows and made a thunderstorm,” he said.
It is uncertain what they were up to in Delco around noon, but once that first outbreak occurred, it was a Sorcerer’s Apprentice situation.
The rains created cool downdrafts, as they always do; these storms were tapping cold air up to 50,000 feet in the atmosphere, which would help explain all the hail reports.
The downdrafts in turn forced the warmer air at the surface to rise and create more storms, which in turn generated more downdrafts and more hail and thunder, meteorologists said.
The storms just kept radiating in a more-or-less circular pattern for well over three hours. With no upper-air winds to nudge them along, once it started raining in a given area, it kept raining.
The Storm Prediction Center in Norman, Okla., had the region placed under a “slight risk” for severe weather.
In its midmorning update, the National Weather Service in Mount Holly said thunderstorms were possible and to “expect heavy rain from the storms.” It began issuing flash-flood warnings shortly after 1 p.m.
But the science of predicting precisely where storms are going to form and identifying their targets remains very much a work in progress.
“It was an easy call to say there was going to be thunderstorms,” said Bowers.