NOAA’s Weather Prediction Center issued a level 3 out of 4 flash flooding risk for major cities in the Mid-Atlantic and Northeast, including Washington, D.C., Baltimore and Philadelphia.

After days of record-challenging heat, the Northeast is bracing for a dramatic weather shift as rounds of heavy rain threaten to unleash dangerous flash flooding across major cities and travel corridors.

A daily rainfall outlook map issued by the Weather Prediction Center put the risk of “excessive” showers capable of triggering flash floods at 40% or higher for a swath of the mid-Atlantic and Northeast that is home to 37 million people.