Severe thunderstorms, many with high winds, will affect more than 80 million people in the eastern US into Monday night, meteorologists have warned.
The severe weather also forced the Federal Aviation Administration to briefly order ground stops at several airports, including in Washington, New York, Philadelphia and Atlanta, which is the busiest in the US.
Washington is under a rare tornado watch until 9pm on Monday, which spurred an early closure of federal offices in the afternoon.
The storms will pose a significant risk to lives and property and cause significant travel disruptions, AccuWeather reported.
There were more than 300 damaging storm reports from Colorado to Virginia, ABC News reported.
There were also 10 reported tornadoes – eight across Iowa, Illinois, Nebraska and Colorado on Saturday and two in Illinois on Sunday, according to the National Weather Service.
The storms knocked out power to tens of thousands across the country.
“Thunderstorms will continue to build in intensity through the afternoon with the most intense weather conditions likely between 3pm and 9pm,” AccuWeather meteorologist Alex Sosnowski said.
Damaging winds, hail and tornadoes are in the forecast for Georgia to New York state. Major cities including Washington, Philadelphia and Atlanta are in the path of coming storms.
The Washington area forecast was for severe weather at a Level 4 out of 5, a “very rare” ranking, from Monday afternoon into the evening, according to The Washington Post's Capital Weather Gang.
“The pattern this week will feature frequent showers and thunderstorms, typically every other day or so, across much of the East,” Mr Sosnowski said.
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“Even though it may not rain as much or as often as it did in July, conditions may again pose daily challenges for outdoor plans and travel.”
Forecasters have said the stormy, wet weather should keep at bay the intense heat that engulfed the mid-Atlantic and part of the North-East in July, forecasters said.
But the heat could make a comeback.
“Heat can build during the middle to late part of August in the North-east and mid-Atlantic as many kids return to school,” AccuWeather meteorologist Brandon Buckingham said.
“This can be accompanied by high humidity and a risk for thunderstorm activity.”