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Northeastern U.S. put on alert for serious flash flooding risk

By Renee Duff, Accuweather.com
Some parts of the northeastern United States from eastern Pennsylvania to Maine could receive a month's worth of rain in a matter of hours beginning Sunday, forecasters say. File Photo by Bill Greenblatt/UPI
Some parts of the northeastern United States from eastern Pennsylvania to Maine could receive a month's worth of rain in a matter of hours beginning Sunday, forecasters say. File Photo by Bill Greenblatt/UPI | License Photo

Residents across the northeastern United States were advised Saturday to be aware of flash flooding risks in the coming days as tropical moisture pours into the region.

The incoming swath of moisture could result in damaging consequences from eastern Pennsylvania to Maine -- in some locations possibly producing a month's worth of rain in a matter of hours, according to AccuWeather forecasters.

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A slow-moving weather system is expected to tap into moisture from the Gulf of Mexico and Atlantic Ocean as it slowly turns northeastward Sunday into Tuesday. This path will direct a swath of heavy rainfall across part of the mid-Atlantic and New England.

The heavy rain can be insult to injury for portions of New England where downpours on Friday resulted in the closure of U.S. Route 4 in Killington, Vt., after a mudslide flung debris across the roadway, making it impassable.

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Flash flooding was also reported in the Washington, D.C., and Baltimore metro areas, eastern Pennsylvania and eastern New York state to close out the week.

High amounts of moisture available in the atmosphere will make this upcoming weather system a very efficient producer of torrential downpours, AccuWeather meteorologists say.

"Rainfall rates could reach 2 inches per hour in some locations as the system slowly moves," AccuWeather Senior Meteorologist Adam Douty said. "Infrastructure in the metro areas may not be able to handle rainfall of this magnitude, and as a result, rising water could quickly inundate some locations."

The timing of the heaviest rain on Sunday is likely to be during the afternoon and evening in Washington, D.C. In Philadelphia, the heaviest rain is likely to last from late Sunday afternoon to early Sunday night. Around New York City, the greatest risk of flooding downpours will occur from Sunday evening to early Monday morning.

"The rain from Sunday night can result in significant travel delays for the Monday morning drive, especially if there are road closures due to high water," AccuWeather Meteorologist Alex DaSilva said.

In Boston, downpours are expected to be most intense from later Monday, perhaps for the commute home, to early Tuesday morning.

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"Widespread rainfall totals of 2-4 inches are forecast across the Northeast with an AccuWeather Local StormMax of 10 inches," DaSilva said.

The historical average rainfall for the entire month of July is generally between 3 to 4.5 inches for the major cities across the Northeast. During this event, some locations could pick up that entire amount or even double, and it may fall within the span of hours.

Motorists are likely to face significant slowdowns as a result of poor visibility and possible road closures. The amount of rain forecast will not only heighten the risk of street flooding but also lead to rapid rises along small streams, which can then spill over their banks.

The moisture-laden air will ram into the mountains of upstate New York and New England, resulting in an enhancement of downpours in the higher terrain. Air that hits mountain ranges is forced to rise upward, and that upward motion results in an enhancement of clouds and precipitation.

"We are closely monitoring the risk of mudslides in the Green Mountains of Vermont. The heaviest rain is forecast to fall across the Green Mountains and that, combined with the rain that fell on Friday, can increase the chance of dangerous mudslides in the mountains," DaSilva said.

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There will be another weather hazard for residents and visitors alike to be on alert for as the weekend comes to a close and the second week of July begins.

"Thunderstorms across the mid-Atlantic on Sunday afternoon will be capable of producing hail, damaging wind gusts and isolated tornadoes, in addition to flooding downpours," DaSilva said.

Locations where the ground is saturated from rainfall to end the week will face a higher likelihood of gusty winds knocking over trees and power lines in any of the more intense thunderstorms.

AccuWeather meteorologists say downpours will gradually ease from west to east on Monday and Tuesday as the weather system responsible for the deluge shifts farther to the north and east.

A slight reduction in humidity levels across the interior Northeast early next week will offer a brief reprieve from the stifling conditions of late. Hot weather with high humidity will be quick to rebound as the week progresses, however.

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