Thunderstorms will bring the risk of severe weather to a portion of the mid-Atlantic states into Thursday night.
People on the road or spending time outdoors will need to keep an eye out for rapidly changing weather conditions from Virginia to Maryland, Pennsylvania, Delaware and New Jersey.
The risk area includes a very heavily-populated part of the nation, including Philadelphia, where the Democratic National Convention is entering its final night. Chelsea Clinton will introduce Hillary who will conclude the convention with her acceptance speech.
Due to the potential for flash flooding, Philadelphia Mayor Jim Kenney urged residents and visitors in town for the convention to take precautions ahead of the storms.
"Flash flooding is a very dangerous situation," Kenney said in a statement. "Whether you're driving, walking or riding, we urge you to stay clear of potentially flooded roadways and seek shelter during heavy rains."
There is the potential for damaging winds, frequent lightning strikes and flash flooding in the strongest storms.
In a couple of the strongest storms, a brief tornado cannot be ruled out.
The severe storm threat will extend from Roanoke and Richmond, Virginia, to Washington, D.C., and Baltimore.
Some of the storms can down trees, cause sporadic power outages and result in minor property damage.
The severe weather is anticipated as a storm system moves eastward from the Ohio Valley and heads into a zone of hot and humid air.
Near and north of the violent storms, a general area of heavy rain is in store with a second round of drenching rain likely later this weekend.