Flood watch for entire Philly region, and get ready for 'a tropical rain forest'

For a solid week, nature has capped the region under a brooding, leaky, gray ceiling. The copy-and paste weather forecasts — as in "cloudy with showers," "cloudy with showers," and "cloudy with showers" — now lap well into next week.

Not surprisingly, the National Weather Service has the entire region under a flash-flood watch until 6 p.m. Saturday. Already over 3 inches of rain has fallen in the last week officially in Philadelphia, with 1 to 4 more coming.

Keep the mowers handy. The trees and grasses are lapping it up, said Dave Dombek, a meteorologist with AccuWeather Inc. "The vegetation is going to be like a tropical rain forest," he said.

That may well have an effect on temperatures and humidities once the rain shuts off, assuming it does.

For all that rain, however, the waterways weren't looking particularly scary Friday morning, and the streams that were brimming Thursday, such as the East Branch of the Brandywine Creek, have backed off considerably.

The forecasts call for Delaware River at Burlington and at Washington Avenue in Philadelphia to flirt with or go over flood stages on Saturday.

Significant rains are expected into Saturday, and "there can be a time in there where you could really get some pretty good downpours," said Dombek.

The weather service says a "variety of flooding types are all possible," including poor-drainage and flash-flooding during the strong thunderstorms.

And we notice that the ending times of the soggy siege keep getting pushed back. The forecasts has showers into Wednesday.

So what is going on out there?

The front that set off that wild squall line that incited even the ocean to riot, generating a meteotsunami off Atlantic City, has stalled to the south of the region.

It has assumed a new career as a highway for subtropical moisture. Meanwhile, high pressure in the western Atlantic is helping to direct those soggy impulses from scooting out to sea.

A refreshing air mass from Canada is nosing southward, said Dombek, but it's not going to get here. In fact, it's not going to help at all, as the proximity of such contrasting air could enhance the rainfall.

"It's kind of a squeeze play," Dombek said. "It's not going to change much at all."

The key to the flooding potential, of course, would be in the timing: A well-spaced four-inch total rainfall would be more benign than a two-inch downpour.

Downpours are in play, Dombek said.

"Since this moisture has a tropical component, there's always the possibility that some areas overachieve," he said.

In the aftermath, he said, the wet grounds and vegetation would retard heating since the sun has to use some of its energy for evaporation. But that also would mean an increase in humidity.

At least no tsunamis are in the forecast.