“Ring of fire” thunderstorms that blew up over Philadelphia and radiated east and west on Monday inundated parts of the region with more rain than had fallen in the previous two months and set off flash-flooding that submerged vehicles, closed roads, and led to numerous water rescues.
At the height of the storms, with cloud tops soaring as high as 55,000 to 60,000 feet, hailstones the diameter of half-dollars were reported.
The Frankford and Pennypack Creeks jumped several feet to flood stages in less than two hours, and serious flooding along the Tookany Creek Parkway in Cheltenham Township was one of many areas where water submerged vehicles.
In New Jersey, the rains closed West Central Avenue in Moorestown and downed wires shut a portion of Route 73 in Marlton. Roads were flooded in Ocean City and Ventnor. On the other side of the Delaware River, rain caused lane closures on the Schuylkill Expressway and the Roosevelt Boulevard.
Almost six inches of rain was measured by a trained National Weather Service spotter in Jenkintown, and close to five inches — more than has fallen on Philadelphia in the last two months — was reported in the Northeast, with nickel- and half-dollar-size hail elsewhere.
The deluges have been set off by a slow-motion front approaching from the north interacting with a steamy air mass over the region.
The weather service described the storms as a “ring of fire” that appeared to spread to the east and west of Philadelphia starting around lunchtime.
“The front is not moving, and stuff keeps blowing up,” said Patrick O’Hara, a meteorologist with the National Weather Service in Mount Holly, which was sorting through storm-damage reports into the evening.
Joe Miketta, the office’s warning-coordination meteorologist, noted that some areas not all that far from the deluge zones received relatively little rain. Other towns in Montgomery County that were spared were available to help out Cheltenham with water rescues, said Kenneth C. Hellendall, the township’s emergency management coordinator.
But in the places that got hit, “When it rained it really poured.”
“It was crazy stuff,” said Dave Dombek, a meteorologist with AccuWeather Inc. He noted that hail is more common in May and June, as opposed to July when the upper air warms up along with everything else.
The government’s Storm Prediction Center has most of eastern Pennsylvania and all of New Jersey and Delaware under a severe-thunderstorm watch until 11 p.m.
Earlier the weather service had a “heat advisory” in effect for urbanized areas for heat indexes up to 100, and that was very much related to the thunderstorm threats as the sultry, saturated air supplied moisture for the rains.
And all that heat and humidity provided energy for the updrafts that generate strong thunderstorms, with hail and strong winds possible.
The flood warnings were in effect until 7 p.m.
The storms are associated with a “cold” front sagging ponderously from the north and isn’t expected to clear the region until sometime Tuesday.
This is not the polar vortex.
Daytime high temperatures are forecast to remain around 90 at least through Sunday, as the region logged its first official heat wave of the season, although heat indexes are not forecast to broach the danger levels.
And all that wetness and the possibility of more clouds and showers Tuesday might keep temperatures below 90.
Hellendall said Cheltenham could do without the showers.