Downpours have set off flood warnings in the region, and a flash-flood watch in effect until 1 a.m. Friday.
Those storms are tapping into some of the moisture associated with Tropical Storm Barry, which is soaking the Gulf Coast and whose remnants could affect the region late next week, the National Weather Service says.
The atmosphere, which has felt more like a water-vapor broth, evidently is “primed” for heavy rain, and the weather service says isolated amounts of 3 to 5 inches are possible.
This is not a new phenomenon.
Last month was among the 10 wettest Junes on record in Philadelphia — 7.94 inches officially; the record is 10.54 set in 2013 — and statewide was No. 17 in 124 years of record-keeping in Pennsylvania.
In both states it was a close second to the 12-month period that ended in January. Nationally, it was the wettest of all 12-month periods, NCEI said.
Already officially Philadelphia has had about a month’s worth of rain in the first 10 days of July, including a daily record of 2.27 inches on Saturday.
The concern for Thursday ris the ultra-juiced atmosphere. The “precipitable water” levels, a key indicator of the heavy-rain potential — 2 and 2-plus inches — rank as “anomalously high,” said Brian Hurley, senior meteorologist at the Weather Prediction Center.
“It’s a very good parameter,” he said, for showing the “potential for the atmosphere to perspire — on you.” That would be similar to the moisture levels over Washington on Monday when rain flooded the White House basement.
Thunderstorms are notoriously random, but as a rule of thumb the rain potential in targeted areas is about double the precipitable water value, said Jonathan O’Brien, a meteorologist at the weather service’s Mount Holly office.
All that water vapor needs is something to trigger convection, he added.
“You’re primed, but you still need the spark. You can prime the lawn mower, but you need a spark to get it going," he said.