(CNN) — Moisture streaming in from the Gulf of Mexico, combined with a stubborn, nearly stationary storm system, will cause slow-moving thunderstorm activity and the possibility of flash flooding across 20 US states Monday.

“Scattered to numerous flash floods (are) possible early this week from southern New England to the southern Appalachians,” the Weather Prediction Center said.

The same storm system triggered a flash flood emergency in western Georgia Sunday afternoon.

What was once thought to be a 1 in 200-year rainfall event, which led to swollen creeks and rivers in the mountainous region, water rescues, and flooded homes and businesses, was, in fact, a 1 in 1000-year rainfall event, according to analysis from CNN meteorologist Haley Brink.

The area flooded yesterday is under the gun again today.

The Weather Prediction Center pointed out a corridor of moisture will continue the convection which has plagued the area for the past couple of days. More thunderstorms could develop, dropping anywhere from 1 to 3 more inches of rain.

And today, the moisture will not be contained in the South.

Flash flooding extreme rains move into major populated areas of the Northeast

Flash flood watches extend all the way into New England, affecting more than 80 million people, including a long list of major cities: Birmingham, Atlanta, Nashville, Louisville, Cincinnati, Pittsburgh, Philadelphia, New York, and Boston.

A flood watch is issued when conditions are favorable for flooding. It does not mean flooding will occur, but it is possible.

The heaviest rainfall is likely in the Northeast, where widespread amounts of 2 to 4 inches are possible. Still, isolated areas could see up to 10 inches, CNN meteorologist Rob Shackleford noted.

“Many pieces to the weather puzzle this morning,” the National Weather Service in State College, Pennsylvania, wrote. “The main takeaway is that flash flooding is possible.”

Parts of the Mid-Atlantic and the Southeast are more in the 1- to 2-inch range with isolated amounts up to 4 inches, Shackleford reported.

“Flash floods are increasingly likely over northeast Pennsylvania, where a Moderate Risk (Level 3 of 4) of excessive rainfall has been issued,” the Weather Prediction Center said Monday morning.

Scattered downpours are possible throughout parts of drought-stricken southern New England.

“Even though this region is currently experiencing severe to extreme drought conditions, several inches of rain falling in a relatively short period of time could still lead to areas of flash flooding,” the Weather Prediction Center cautioned.

Many of the southernmost flood watches will expire by this evening, and the watches in the Northeast are set to expire Tuesday afternoon.

On Tuesday, a low pressure system is expected to develop just off the Mid-Atlantic coastline.

“This will allow for additional chances for heavy rain along the I-95 corridor from Washington D.C. to Providence, Rhode Island, to kick off the shortened workweek,” according to the Weather Prediction Center.

The plume of moisture will be along the coast Tuesday morning, shifting off the coast by the evening. Thus, the greatest rainfall totals will be over New Jersey, the Philadelphia and New York City areas, Long Island, and much of Connecticut, the Weather Prediction Center added.

A slight risk (Level 2 of 4) for excessive rainfall, which could lead to flash flooding, was issued by the Weather Prediction Center for the region Tuesday.

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