A flood watch is up for the entire Philly region, with an ‘atmospheric river’ of rain on the way

Flood watches are in effect for the entire region Sunday afternoon through late Monday afternoon for rainfall totals as high as 4.5 inches, along with wind gusts past 50 mph at the Shore and 40 mph on the mainland, forecasters warned Saturday.

A long “fetch of moisture” will surge from the Gulf of Mexico and up the eastern seaboard, said Bill Deger, senior meteorologist with AccuWeather Inc. Flood watches are posted over areas from Virginia to far-northeastern Maine, including just about the entire Northeast snow country.

“It’s almost like an atmospheric river,” said Deger, invoking a term more common to the West Coast to describe a plume of moisture channeled from a body of water. In this case, that body would be the Gulf, not the Pacific.

» READ MORE: This is expected to be quite a stormy winter. Will it even snow?

“This certainly is not like an event when you’re seeing heavy showers and storms. This is a good, widespread soaking.” Overall, forecasters see two to three inches, with isolated higher amounts.

The rains will pound the headwaters of the Schuylkill and Delaware Rivers, and “there will be some rises,” said Patrick O’Hara, meteorologist at the National Weather Service Office in Mount Holly.

The rivers aren’t forecast to reach flood stage, but the the rains could send some of the flashier streams, such as the Brandywine and Neshaminy Creeks, over their banks.

The downpours will be ignited by an approaching cold front that will wring out the Gulf moisture, with a strong coastal storm developing later, meteorologists said.

The front is forecast to generate winds from the south gusting to 50 mph or perhaps higher along the coast, said Deger and 40 mph inland. Those south winds also will drive up temperatures, which are expected to climb into the 60s Sunday.

The fact that the winds won’t be blowing onshore should reduce the threat of major coastal flooding, and none is forecast, but the new moon will be giving the tides an extra kick, increasing the threat potential for minor flooding, especially on the back bays.

» READ MORE: This storm will do nothing to break the region's snow-deprivation streak

O’Hara said he couldn’t rule out some power outages; however, the fact that the trees have shaken out most of their leaves likely would hold down the numbers.

This, being the season, the winds do raise some “concerns for holiday decorations,” said AccuWeather’s Deger.

The heaviest rains are expected by midafternoon and will continue into Sunday evening, said Deger. The flood potential will be all about how quickly the heaviest rain falls.

The rains are due to shut off Monday afternoon, and they will be followed by a drying December-like cooldown with daily highs in the 40s Tuesday through Friday.

On a positive note, the skies should be clear Wednesday night into Thursday morning for the annual Geminid meteor showers.