A severe weather outbreak unleashed damaging winds that knocked out power to hundreds of thousands of people and caused widespread travel disruptions across the Eastern US. Forecasts warned of damaging hurricane-force wind gusts of up to 80 mph in the strongest storms.

Power outages and flight cancellations grew exponentially as the storms slammed several states and major metro areas with strong wind gusts measured up to 71 mph, including Atlanta, Washington, D.C., Baltimore, Philadelphia, and Charlotte, North Carolina.

More than 1 million customers lost power on Monday from Alabama to Pennsylvania, with most of the outages in North Carolina, Georgia, Pennsylvania, and Maryland, according to PowerOutage.us.

Nearly 1,500 flights were canceled Monday with 7,000 additional delays as storms disrupted flight paths to major hubs like Atlanta, Charlotte, Washington D.C., and Baltimore according to data from FlightAware. Nine simultaneous ground stops, which prevent flights from landing, were in effect at airports, including Atlanta’s Hartsfield-Jackson International, Newark Liberty in New Jersey, LaGuardia in New York and Philadelphia International Airport. By early Monday evening, the storms left a swath of over 300 wind reports from Mississippi to the Carolinas and northward into New York.

Accounting for every level of severe storm risk, around 120 million people in the Eastern US were threatened by damaging weather Monday as a potent storm system, especially for August, crossed the region. A Level 4 of 5 moderate risk of severe thunderstorms was issued Monday morning – the first Level 4 issued in the US since June 29 and the first for Washington D.C., in over a decade.

Widespread damaging and locally destructive wind gusts were the greatest risk from Monday’s storms. Multiple tornado watches were issued Monday, covering around 40 million people across a large area from western Tennessee and Kentucky eastward into portions of North Carolina, Virginia, Maryland, Pennsylvania, New Jersey, and New York. Dozens of tornado warnings were issued from Tennessee to Pennsylvania Monday afternoon with more possible as storms track farther east Monday evening.