Florence may fuel torrential rain, flooding risk in northeastern US early this week

By Alex Sosnowski, AccuWeather senior meteorologist
September 16, 2018, 5:16:09 AM EDT

After triggering catastrophic flooding in the Carolinas, Florence will survive long enough to bring rounds of heavy rain and the risk of flooding to the central Appalachians and northeastern United States early this week.

High pressure is expected to promote largely dry and warm weather from the Mason-Dixon line northward to end the weekend on Sunday.

Showers may reach the Washington, D.C., area before the day is done as Florence's flooding rain expands from the Carolinas to southwestern Virginia and neighboring West Virginia.

The weather pattern is expected to finally shift after this weekend, allowing Florence, as a tropical rainstorm, to accelerate northeastward.

NE Sunday Sep 15

There is the potential for 3 to locally 6 inches of rain from western Virginia and the mountains of West Virginia to central Pennsylvania and Massachusetts.

The greatest amounts are likely on the southwestern portion of this zone, where local amounts could top 8 inches, especially in the mountains.

Areas in the West Virginia, northern Virginia, western Maryland and Pennsylvania mountains are likely the most vulnerable for flash and stream flooding, due to the saturated state of the ground, proximity to tropical moisture and the terrain.

Mudslides can also be triggered in the higher terrain.

Florence rain East 9.16 AM

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However, enough rain may fall to cause flash and urban flooding may not be confined to this swath.

"Localized issues may also occur along the I-95 corridor from Washington, D.C., to Boston and west of the Appalachian Mountains from Huntington, West Virginia, to Erie, Pennsylvania," according to AccuWeather Senior Meteorologist Kristina Pydynowski.

A mere 1-2 inches of rain in six hours or less could be enough to initiate flash flooding from northern West Virginia and northwestern Virginia to southern New York state and western New England.

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"Even in the absence of flooding, downpours can bring yet another round of disruptions to outdoor plans and travel," Pydynowski said. "The downpours will reduce visibility for motorists and heighten the risk for vehicles to hydroplane when traveling at highway speeds."

AccuWeather meteorologists warned about the enhanced potential for tropical storm-induced major flooding months ago in the eastern United States, even as rain was just beginning to pour down during the summer. The soggy summer of 2018 was the wettest summer on record in some cases.

Florence track 9.16 AM

In the past week, rain once again has left the surface soil saturated and pushed water tables to unusually high levels in some areas for the middle of September.

The ground does not dry out as quickly now, when compared to the middle of the summer. As a result, the ground will likely stay wet until the rain from Florence arrives.

Even in the absence of tropical storm intervention, ongoing high levels on streams and water tables may result in periodic flooding problems moving forward during the autumn and early winter in the East.

Despite the saturated ground over much of the Northeast, there are a few pockets in northern New York State, northern Vermont, Maine and southeastern Massachusetts that can tolerate a moderate amount of rain spread over a couple of days.

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