The pattern responsible for sparse severe thunderstorm activity and heavy rainfall the past few days will energize and increase the risk to lives and property across the northeastern United States this weekend.
Those sweltering in recent days from blazing sunshine, heat and high humidity will get relief but at a price.
Motorists and those spending time outdoors should be on the lookout for rapidly changing weather conditions. Airline passengers are likely to face delays as storms approach their departure, arrival or connection hubs.
A couple of storm systems will roll eastward from the Midwest this weekend. These storms will tap the high level of moisture in the region as well as a north-south temperature contrast.
Some communities will be hit with severe thunderstorms containing high winds, hail, torrential downpours and frequent lightning strikes.
Some neighborhoods and rural areas can be hit with flash and urban flooding, where streets suddenly become rivers and small streams become raging torrents of water. Those camping along streams should remain vigilant and keep up to date on flash flood advisories as they are issued.
In some cases, a single thunderstorm can lead to flooding. In others, the cumulative effect of multiple rounds of storms can lead to high water.
During Saturday and Saturday night, the area from the Ohio Valley to areas within 100 miles or so of the Mason-Dixon line will be at greatest risk for one or more lines of severe thunderstorms.
"Areas from Cincinnati to Pittsburgh; Morgantown, West Virginia; Philadelphia; Washington, D.C.; Atlantic City, New Jersey; and close to New York City will be at greatest risk for this dangerous and disruptive storms into Saturday evening," according to AccuWeather Senior Meteorologist Kristina Pydynowski.
Storms along this swath could knock down trees and lead to power outages.
"Meanwhile, areas from northern Ohio and southwestern Ontario to northern Pennsylvania, northern New jersey and southern New York state face the greatest risk for torrential rainfall and flash flooding," Pydynowski said.
"Localized damaging winds can also occur in this zone on Saturday and into Saturday evening," Pydynowski added.
During much of the weekend, a wedge of slightly cooler and less humid air may prevent severe weather across northern New York state and much of New England.
However, the second of the two storms may track farther north during the last part of the weekend.
"If the second storm is able to pull enough warmth and humidity farther north, severe weather could occur in New York City, part of the Hudson Valley and southwestern New England on Sunday afternoon and evening," Pydynowski said.
Farther south on Sunday, another round of severe thunderstorms may occur from the Ohio Valley to the central Appalachians and the mid-Atlantic coast.
Some areas from Cincinnati to Pittsburgh, Philadelphia, Washington, D.C., and others could be hit with severe weather two days in a row.
Meanwhile, the best chance of showers and thunderstorms in Boston will be Saturday night and Sunday night with Sunday night as the greatest chance of heavy rain. Much of the time during the daylight hours both days of the weekend may be free of rain in Boston and northern and eastern New England.
Sunday is likely to be the more active of the two weekend days for the Tennessee Valley and the southern Appalachians and Piedmont. Locally severe storms and flash flooding could occur.
Behind the second storm, the atmosphere should become cool enough and dry enough to prevent shower and thunderstorm activity across the Great Lakes, Ohio Valley and western slopes of the central Appalachians on Monday.
However, areas from New England to the mid-Atlantic coast and southern Appalachians could face another day of showers and thunderstorms, some of which could be heavy, gusty and perhaps severe.