Above is a map of cooling centers in operation now in Washington, D.C. and Prince George's County, Maryland. Call the center to confirm hours of operation before you go.
As for other counties: Montgomery County, Maryland, says it is not operating cooling centers right now. Fairfax County in Virginia advises citizens to go to libraries, community centers, and government centers if the heat is extreme.
If your community supports 311, you can also call that number for a referral to a cooling center.