Unusual Storm Will Bring Life-Threatening Flash Flood Threat, Cool Conditions to Mid-Atlantic and Northeast, Including Washington D.C., Philadelphia

Linda Lam
Published: July 28,2017
An unusual weather pattern for late July is taking shape and will bring a winter-like coastal storm, with the threat of life-threatening flash flooding, to the mid-Atlantic and Northeast coasts this weekend and possibly into early next week, putting a damper on weekend beach plans.
A strong upper-level trough, or southward dip in the jet stream, will move into the East this weekend with a cold front that will even push through much of the South, bringing cooler temperatures and lower dew points from the Midwest into the East and South.
(MORE: Storms, Temperature Changes to End July)
As this system approaches the region, a first round of rain and storms will develop Friday in the mid-Atlantic and Northeast. Ample moisture will be in place, which will bring locally heavy rainfall and the risk of flooding, along with a few severe thunderstorms.

Current Radar, Watches and Warnings

Over 3 inches of rain fell in about 45 minutes midday Friday near Silver Spring, Maryland. Water rescues were also reported near Wheaton and Garrett Park in Maryland, with multiple vehicles trapped in high water.
A landslide reported near Farquhar along westbound I-70 in Washington County, Pennsylvania. Multiple vehicles were in flooded roadways in the city of Washington, Pennsylvania.
A stream is overflowing its banks and into low lying areas with several inches of water flowing over KY Highway 30 near Salyersville, Kentucky.
More than 4 inches of rain has fallen in Merrifield, Virginia (5.10 inches) and Onley, Maryland (4.25 inches) already through mid-afternoon Friday.
Friday night into Saturday, an area of low pressure will develop and strengthen off the mid-Atlantic coast, increasing the risk for heavy rainfall.

Coastal Low Setup

The upper-level pattern will allow this coastal low to linger off the Northeast coast, possibly into early next week. This may result in several days of a persistent northeasterly flow with showers and breezy conditions.
However, there remains uncertainty regarding the forecast track, timing and strength of this low-pressure system. There is also the chance that this low will track away from the region this weekend and another wave of low pressure may emerge off the mid-Atlantic coast Sunday into Monday.
This overall pattern, including gusty northeast winds at times, will also keep temperatures on the cool side for late July, with highs 5 to 20 degrees below average for portions of the mid-Atlantic and Northeast.
(MORE: 7 Odd Weather Events Happening Now)
Below, we take a closer look at the forecast into next week.

Forecast Into Next Week

Saturday
This more winter-like low-pressure system is expected to be just off the mid-Atlantic coast Saturday.
Locally heavy rain and flash flooding will be the main concerns, especially in the mid-Atlantic through Saturday morning.
(FORECAST: Washington, D.C. | Philadelphia | New York City | Boston)

Saturday's Forecast

Saturday Evening into Sunday
Showers and storms will begin to taper off during the evening hours Saturday with the best chance of rain in the mid-Atlantic from the Delmarva peninsula southward to coastal North Carolina. Parts of southern New England may see light to moderate showers throughout the evening, especially in southeastern Massachusetts.
On Sunday, this low-pressure system may continue to impact portions of the mid-Atlantic coast. A few showers may continue impact southeastern Massachusetts as well. The exact location will determine where and how much rain and wind occur.
Showers, along with breezy and cool conditions, will persist from the mid-Atlantic coast into southern New England.
(FORECAST: Virginia Beach | Baltimore, Maryland | Atlantic City, New Jersey | Portland, Maine)

Sunday's Forecast

By Monday, this low-pressure system is finally expected to begin to track northeastward, away from the mid-Atlantic coast. Light rain is possible, once again, along with some gusty winds toward the coast. Temperatures will also begin to return closer to average for areas where clouds and rain do not prevail.
Impacts
The heaviest rainfall is expected Friday into Saturday morning, with 3 to 6 inches possible during this time period from West Virginia into Delaware and southern New Jersey. A few spots may see in excess of 6 inches in eastern Maryland or elsewhere on the Delmarva peninsula.

Rainfall Forecast

Flash flood watches have already been issued for portions of the Ohio Valley and mid-Atlantic, including Washington D.C., Baltimore and Philadelphia, due to the risk of heavy rain in a short period of time falling on saturated ground.

Flood Alerts

Light rainfall, however, will continue through Monday overtopping the impressive rainfall totals that are likely during the weekend.
This storm will also bring higher surf and an increase in the rip current risk to the region, which will only add to the overall wet and cool forecast that this may not be a great beach weekend in the region.
Surf up to 6 feet is possible from near shore Long Island, southern Rhode Island and southeastern Massachusetts. Gusts in these regions may also be as high as 40 mph into Sunday.
Thankfully winds will be mainly parallel to the coast, so minor coastal saltwater flooding will only be a concern in parts of coastal Delaware, Long Island (on the northern side) and along the northward facing shores of Cape Cod. Some beach erosion will continue into Sunday from New Jersey to Virginia.
Winds will lighten as we move into Monday morning along the entire coast.
(MAPS: Weekly Planner)
High pressure will likely build into the region midweek and will finally bring improving conditions across the Northeast.

A Welcome Change to End July

The front will continue to push farther south and east into this weekend, and that's where the good news comes into play.

A cold front will usher in lower humidity throughout the Midwest and parts of the South and East late week into this weekend.

Lower humidity will engulf the Ohio Valley on Friday, kicking off a stretch of mainly dry weather that will continue through the weekend and possibly into early next week.
Meanwhile, parts of the East and South will see scattered showers and thunderstorms into Saturday. An isolated severe storm cannot be ruled out, and locally heavy rain is also a concern.
The front will likely continue to scour out moisture through late week and this weekend across the East. Decreased humidity levels are also possible across much of the South by late this weekend.
(MORE: 6 Odd Weather Events Happening Now)
Temperatures will be near or below average in the eastern third of the nation into the weekend.
Relatively mild temperatures may continue into the start of August, as well, due to an overall southward dip in the jet stream across the East. NOAA's Climate Prediction Center is forecasting the highest probability of below-average temperatures to be in parts of the South and Plains in their latest 6- to 10-day outlook which includes the first few days of August.
(MAPS: Forecast Temperatures)
Be sure to check back to weather.com for updates on this upcoming coastal storm.
MORE: Severe Storms in Midwest, Northeast – July 2017

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.